Method OIV-MA-BS-01 : R2017

Type II method

Reference method for the determination of alcoholic strength by volume of spirit drinks of viti-vinicultural origin: General remarks

(OIV/OENO 379/2009;

OIV-OENO 587-2017:

OIV-OENO 588-2017)

Introduction

The reference method includes two Annexes:

Annex I - Preparation of distillate

Annex II - Measurement of density of distillate by three methods A, B, and C

1.                Scope

The method is suitable for the determination of the real alcoholic strength by volume of spirit drinks of viti-vinicultural origin.

2.                Normative References

ISO 3696:1987 Water for analytical laboratory use - Specifications and test methods.

3.                Terms and Definitions

3.1.      Reference temperature:

The reference temperature for the determination of alcoholic strength by volume, density and specific gravity of spirit drinks is 20 °C.

Note 1: The term 'at t °C' is reserved for all determinations (of density or alcoholic strength by volume) expressed at a temperature other than the reference temperature of 20 °C.

3.2.      Density:

The density is the mass per unit volume in vacuo of spirit drinks at 20 °C. It is expressed in kilograms per cubic metre and its symbol is ρ_{20 °C} or ρ_2.

3.3.      Apparent alcoholic strength:

The apparent alcoholic strength of alcohols and spirituous beverages is equal to the number of litres of ethyl alcohol contained in 100 litres of an aqueous-alcoholic mixture with the same density as that of the alcohol or spirituous beverage. Therefore, the apparent alcoholic strength is directly deduced from the density of the product, without distillation. The apparent alcoholic strength is expressed in % vol.

3.4.      Specific gravity:

The specific gravity is the ratio, expressed as a decimal number, of the density of spirit drinks at 20 °C to the density of water at the same temperature. It is denoted by the symbol d20 °C/20 °C or d20/20, or simply d when there is no possibility of confusion. The characteristic that was measured must be specified on the assay certificate using the above-defined symbols only.

Note 2: It is possible to obtain the specific gravity from the density ρ20 at 20 °C:

ρ₂₀ = 998.203 x d_20/20 or d_20/20 = ρ₂₀ /998.203 where 998.203 is the density of water at 20 °C.

3.5.      Real alcoholic strength by volume:

The real alcoholic strength by volume, or alcohol by volume (ABV), of spirit drinks is equal to the number of litres of ethyl alcohol contained in 100 l of a water-alcohol mixture having the same density as the alcohol or spirit after distillation. The reference values for alcoholic strength by volume (% vol.) at 20 °C versus density at 20 °C for different water-alcohol mixtures that are to be used are those given in the international table adopted by the International Legal Metrology Organisation in its Recommendation no. 22.

Note 3:  For liqueurs and crèmes for which it is very difficult to measure

volume accurately the sample must be weighed and the alcoholic strength is calculated first by mass.

Conversion formula:

Alcoholic strength by volume (%vol)=

where ASM = alcoholic strength by mass,

ρ₂₀ (alcohol) = 789.24 kg/m³

3.6.      Density – Alcoholic Strength Correspondence

The reference values for the alcoholic strength (% vol.) at 20 °C, defined in 3.3 and 3.5, versus density at 20 °C for different aqueous-alcoholic mixtures that are to be used are those given in the international table adopted by the International Organization of Legal Metrology in its recommendation N° 22.

3.7.      Obscuration:

Obscuration is defined as the difference between the real alcoholic strength by volume and the apparent alcoholic strength, expressed in % vol.

4.                Principle

Following distillation the alcoholic strength by volume of the distillate is determined by pycnometry, electronic densimetry, or densimetry using a hydrostatic balance.

5.                Bibliography

1. Commission Regulation (EC) N° 2870/2000 of 19 December 2000 laying down Community reference methods for the analysis of spirits drinks, OJEC of 29 December 2000, L333/20
2. P. Brereton, S. Hasnip, A. Bertrand, R. Wittkowski, C. Guillou, Analytical methods for the determination of spirit drinks, Trends in Analytical Chemistry, Vol. 22, No. 1, 19-25, 2003

Method OIV-MA-BS-02 : R2009

Type II method

Reference method for the determination of alcoholic strength by volume of spirit drinks of viti-vinicultural origin: Preparation of the distillate

(OIV/OENO 379/2009)

1.                Scope

The method is suitable for the preparation of distillates to be used to determine the real alcoholic strength by volume of spirit drinks.

2.                Principle

The spirits are distilled to separate the ethyl alcohol and other volatile compounds from the extractive matter (substances which do not distil).

3.                Reagents and Materials

3.1.      Anti-bumping granules

3.2.      Concentrated antifoam emulsion (for crème liqueurs)

4.                Apparatus and Equipment

Usual laboratory apparatus and in particular the following.

4.1.      Water bath capable of being maintained at 10 °C to 15 °C.

Water bath capable of being maintained at 20 ºC ( 0.2 ºC)

4.2.      Class A volumetric flasks, 100 ml and 200 ml, that have been certified to 0.1 % and 0.15 % respectively.

4.3.      Distillation apparatus:

4.3.1.                    General requirements

The distillation apparatus must meet the following specifications:

• The number of joints must be no more than the strict minimum needed to ensure the system is leak-tight.
• Inclusion of a device designed to prevent priming (entrainment of the boiling liquid by the vapour) and to regularise the distillation rate of alcohol-rich vapours.
• Rapid and complete condensation of the alcohol vapours.
• Collection of the first distillation fractions in an aqueous medium.

The heat source must be used with a suitable heat-diffuser to prevent any pyrogenic reaction involving the extractive matter.

4.3.2.                    As an example, a suitable distillation apparatus would include the following parts:

• Round bottomed flask, 1 litre, with a standardised ground-glass joint.
• Rectifying column at least 20 cm high (a Vigreux column, for example).
• Elbow connector with an approximately 10 cm long straight-rimmed condenser (a West-type condenser) fitted vertically.
• Cooling coil, 40 cm long.
• Drawn out tube, taking the distillate to the bottom of a graduated collecting flask containing a small amount of water.

Note: The apparatus described above is intended for a sample of least 200 ml. However, a smaller sample size (100 ml) can be distilled by using a smaller distillation flask, provided a splash-head or some other device to prevent entrainment is used.

5.                Storage of test samples

Samples are stored at room temperature prior to analysis.

6.                Procedure

6.1.      Distillation apparatus verification

The apparatus used must be capable of the following:

The distillation of 200 ml of a water-alcohol solution with known concentration close to 50 % vol. must not cause a loss of alcohol of more than 0.1 % vol.

6.2.      Spirit drinks with alcoholic strength below 50 % vol.

Measure out 200 ml of the spirit into a volumetric flask.

Record the temperature of this liquid, or maintain at standard temperature (20 ºC).

Pour the sample into the round bottomed flask of the distillation apparatus and rinse the volumetric flask with three aliquots each of approximately 20 ml of distilled water. Add each rinse water aliquot to the contents of the distillation flask.

Note: This 60-ml dilution is sufficient for spirits containing less than 250 g of dry extract per litre. Otherwise, to prevent pyrolysis, the volume of rinse water must be at least 70 ml if the dry extract concentration is 300 g/l, 85 ml for 400 g/l dry extract, and 100 ml for 500 g/l dry extract (some fruit liqueurs or crèmes). Adjust these volumes proportionally for different sample volumes.

Add a few anti-bumping granules (3.1) (and antifoam for crème liqueurs).

Pour 20 ml of distilled water into the original 200 ml volumetric flask that will be used to hold the distillate. This flask must then be placed in a cold water bath (4.1) (10 - 15 °C for aniseed-flavoured spirit drinks).

Distil, avoiding entrainment and charring, occasionally agitating the contents of the flask, until the level of distillate is a few millimetres below the calibration mark of the volumetric flask.

When the temperature of this distillate has been brought down to within 0.5 ºC of the liquid's initial temperature, make up to the mark with distilled water and mix thoroughly.

This distillate is used for the determination of alcoholic strength by volume (Annex II)

6.3.      Spirit drinks with alcoholic strength above 50 % vol.

Measure out 100 ml of the spirit drink into a 100 ml volumetric flask and pour into the round bottomed flask of the distillation apparatus.

Rinse the volumetric flask several times with distilled water and add the washings to the contents of the round-bottomed distillation flask. Use enough water to bring the flask's contents up to approximately 230 ml.

Pour 20 ml of distilled water into a 200 ml volumetric flask that will be used to hold the distillate. This flask must then be placed in a cold water bath (4.1) (10 °C to 15 °C for aniseed-flavoured spirits).

Distil, agitating the contents occasionally, until the level of distillate is a few millimetres below the calibration mark of the 200 ml volumetric flask.

When the temperature of this distillate has been brought down to within 0.5 °C of the liquid's initial temperature, make up to the mark with distilled water and mix thoroughly.

This distillate is used for the determination of alcoholic strength by volume (Annex II)

Note: The alcoholic strength by volume of the spirit drink is twice the alcoholic strength of the distillate.

OIV-MA-BS-03 Reference method for the determination of real alcoholic strength by volume of spirit drinks of viti-vinicultural origin: measurement by pycnometry

Type II method

1. Principle

The alcoholic strength by volume is obtained from the density of the distillate measured by pycnometry.

2. Reagents and Materials

During the analysis, unless otherwise is stated, use only reagents of recognised analytical grade and water of at least grade 3 as defined in ISO 3696:1987.

2.1. Sodium chloride solution (2 % w/v)

To prepare 1 litre, weigh out 20 g sodium chloride and dissolve to 1 litre using water.

3. Apparatus and Equipment

Usual laboratory apparatus and in particular the following.

3.1. Analytical balance capable of reading 0.1 mg.

3.2. Thermometer, with ground glass joint, calibrated in tenths of a degree from 10 to 30 °C. This thermometer must be certified or checked against a certified thermometer.

3.3. Pyrex glass pycnometer of approximately 100 ml capacity fitted with a removable ground-glass thermometer (A.3.2). The pycnometer has a side tube 25 mm in length and 1 mm (maximum) in internal diameter ending in a conical ground joint. Other pycnometers as described in ISO 3507 e.g. 50 ml may be used if appropriate.

3.4. A tare bottle of the same external volume (to within 1 ml) as the pycnometer and with a mass equal to the mass of the pycnometer filled with a liquid of density 1.01 (sodium chloride solution A.2.1).

3.5. Thermally insulated jacket that fits the body of the pycnometer exactly.

Note 1: The method for determining the densities in vacuo of spirits calls for the use of a twin-pan balance, a pycnometer and a tare bottle of the same outside external volume to cancel out the effect of air buoyancy at any given moment. This simple technique may be applied using a single-pan balance provided that the tare bottle is weighed again to monitor changes in air buoyancy over time.

4. Procedure

Preliminary remarks:

• The following procedure is described for the use of 100-ml pycnometer for determination of the alcoholic strength; this gives the best accuracy. However, it is also possible to use a smaller pycnometer, for example 50 ml.

4.1. Calibration of pycnometer

The pycnometer is calibrated by determining the following parameters:

• tare of the empty pycnometer,
• volume of the pycnometer at 20 °C,
• mass of the water-filled pycnometer at 20 °C.
  1.     Calibration using a single-pan balance

Determine:

• the mass of the clean, dry pycnometer (P),
• the mass of the water-filled pycnometer at t °C (P1)
• the mass of the tare bottle (T0).
  1.                      Weigh the clean, dry pycnometer (P).
  2.                      Fill the pycnometer carefully with distilled water at ambient temperature and fit the thermometer.

Carefully wipe the pycnometer dry and place it in the thermally-insulated jacket. Agitate by inverting the container until the thermometer's temperature reading is constant

Set the pycnometer flush with the upper rim of the side tube. Read the temperature t °C carefully and if necessary correct for any inaccuracies in the temperature scale.

Weigh the water-filled pycnometer (P1).

4.1.1.3.                     Weigh the tare bottle (T0).

4.1.1.4.                     Calculation

• Tare of the empty pycnometer = P - m
• where m is the mass of air in the pycnometer.
• m = 0.0012 x (P1 - P)

Note 2: 0.0012 is the density of dry air at 20 °C at a pressure of 760 mm Hg

• Volume of the pycnometer at 20 °C:

  = [P1 - (P - m)] x F_t

 where F_t is the factor for temperature t °C taken from Table I below.

  must be known to the nearest 0.001 ml.

• Mass of water in the pycnometer at 20 °C :

  = x 0.998203

 where 0.998203 is the density of water at 20 °C.

Note 3: If necessary, the value 0.99715 of the density in air can be used and the alcoholic strength calculated with reference to the corresponding density in HM Customs and Excise tables in air.

4.1.2.    Calibration method using a twin-pan balance:

4.1.2.1.                     Place the tare bottle on the left-hand pan and the clean, dry pycnometer with its collecting stopper on the right-hand pan. Balance them by placing weights on the pycnometer side: p grams. ( p )

4.1.2.2.                     Fill the pycnometer carefully with distilled water at ambient temperature and fit the thermometer; carefully wipe the pycnometer dry and place it in the thermally insulated jacket; agitate by inverting the container until the thermometer's temperature reading is constant.

Accurately adjust the level to the upper rim of the side tube. Clean the side tube, fit the collecting stopper; read the temperature t °C carefully and if necessary correct for any inaccuracies in the temperature scale.

Weigh the water-filled pycnometer, with p' the weight in grams making up the equilibrium. (p' )

4.1.2.3.                     Calculation

• Tare of the empty pycnometer = p + m

 where m is the mass of air in the pycnometer.

 m = 0.0012 x (p - p')

• Volume of the pycnometer at 20 °C:

  = (p + m - p') x F_t

 where F_t is the factor for temperature t °C taken from Table I below.

 V_{20 °C} must be known to the nearest 0.001 ml.

• Mass of water in the pycnometer at 20 °C:

  = x 0.998203

 where 0.998203 is the density of water at 20 °C.

4.2. Determination of alcoholic strength of test sample

4.2.1.    Using a single-pan balance.

4.2.1.1.                     Weigh the tare bottle, weight T1

4.2.1.2.                     Weigh the pycnometer with the prepared distillate (see Annex I), P2 is its weight at t °C.

4.2.1.3.                     Calculation

• dT = T1 - T0
• Mass of empty pycnometer at moment of measuring

 = P - m + dT

• Mass of the liquid in the pycnometer at t °C

 = P2 - (P - m + dT)

• Density at t °C in g/ml
• ρ_{t °C}= [P₂ –(P - m + dT)]/V_{20 °C}

 Express the density at t °C in kilograms per m³ by multiplying ρ_{t °C} by 1000, the value being known as ρ_t.

• Correct ρ_t to 20 using the table of densities ρT for water-alcohol mixtures in the Manual of Analysis Methods for Wines of the OIV.

In the table find the horizontal line corresponding to temperature T in whole degrees immediately below t °C, the smallest density above ρ_t. Use the table difference found below that density to calculate the density ρ_t of the spirit at that temperature T in whole degrees.

• Using the whole temperature line, calculate the difference between density ρ' in the table immediately above ρ_t and the calculated density ρ_t.  Divide that difference by the table difference found to the right of density ρ'. The quotient provides the decimal portion of the alcoholic strength while the integer of the alcoholic strength is found at the top of the column in which density ρ' is found (Dt, the alcoholic strength).

Note 4:  Alternatively keep the pycnometer in a water bath maintained at 20 °C (± 0.2 °C) when making up to the mark.

4.2.1.4.                     Result

Using the density ρ₂₀ calculate the real alcoholic strength using the alcoholic strength tables identified below:

The table giving the value of the alcoholic strength by volume (% vol.) at 20 °C as a function of the density at 20 °C of water-alcohol mixtures is the international table adopted by the International Legal Metrology Organisation in its Recommendation no. 22.

4.2.2.    Method using a single-pan balance

4.2.2.1.                     Weigh the pycnometer with the distillate prepared (see part I), p" is mass at t °C.

4.2.2.2.                     Calculation

• Mass of the liquid in the pycnometer at t °C

 = p + m - p"

• Density at t °C in g/ml

ρ_{t °C} = (p + m - p")/V_{20 °C}

• Express the density at t °C in kilograms per m³ and carry out the temperature correction in order to calculate the alcoholic strength at 20 °C, as indicated above for use of the single-pan balance.

5. Method performance characteristics (Precision)

 

5.1. Statistical results of the interlaboratory test

The following data were obtained from an international method performance study carried out on a variety of spirit drinks to internationally agreed procedures.

Year of interlaboratory test	1997
Number of laboratories	20
Number of samples	6

Samples	A	B	C
Number of laboratories retained after eliminating outliers	19	20	17
Number of outliers (Laboratories)	1	-	2
Number of accepted results	38	40	34
Mean value (	23.77	40.04	40.29
	26.51*
Repeatability standard deviation (sr) % vol.	0.106	0.176	0.072
Repeatability relative standard deviation (RSDr)  (%)	0.42	0.44	0.18
Repeatability limit ( r )  % vol.	0.30	0.49	0.20
Reproducibility standard deviation (sR) % vol.	0.131	0.236	0.154
Reproducibility relative standard deviation (RSDR)  (%)	0.52	0.59	0.38
Reproducibility limit ( R )   % vol.	0.37	0.66	0.43

Sample types

A Fruit liqueur; split level*

B Brandy; blind duplicates

C Whisky; blind duplicates

Samples	D	E	F
Number of laboratories retained after eliminating outliers	19	19	17
Number of outliers (Laboratories)	1	1	3
Number of accepted results	38	38	34
Mean value (	39.20	42.24	57.03
	42.93*	45.73*	63.03*
Repeatability standard deviation (sr) % vol.	0.103	0.171	0.190
Repeatability relative standard deviation (RSDr)  (%)	0.25	0.39	0.32
Repeatability limit ( r )  % vol.	0.29	0.48	0.53
Reproducibility standard deviation (sR) % vol.	0.233	0.238	0.322
Reproducibility relative standard deviation (RSDR)  (%)	0.57	0.54	0.53
Reproducibility limit ( R )   % vol.	0.65	0.67	0.90

Sample types

D grappa;  split level*

E aquavit;  split level*

F rum; split level*

OIV-MA-BS-04 Reference method for the determination of real alcoholic strength by volume of spirit drinks of viti-vinicultural origin: measurement by electronic densimetry (based on the resonant frequency oscillation of a sample in an oscillating cell)

Type II method

1. Principle

The liquid's density is determined by electronic measurement of the oscillations of a vibrating U-tube. To perform this measurement, the sample is added to an oscillating system, whose specific oscillation frequency is thus modified by the added mass.

2. Reagents and Materials

During the analysis, unless otherwise is stated, use only reagents of recognised analytical grade and water of at least grade 3 as defined in ISO 3696:1987.

2.1. Acetone (CAS 666-52-4) or absolute alcohol

2.2. Dry air.

3. Apparatus and Equipment

Usual laboratory apparatus and in particular the following.

3.1. Digital display densimeter

Electronic densimeter for performing such measurements must be capable of expressing density in g/ml to 5 decimal places.

Note 1: The densimeter should be placed on a perfectly stable stand that is insulated from all vibrations.

3.2. Temperature regulation

The densimeter's performance is valid only if the measuring cell is connected to a built-in temperature regulator that can achieve the same temperature stability of 0.02 ºC or better.

Note 2:  The precise setting and monitoring of the temperature in the measuring cell are very important, for an error of 0.1 °C can lead to a variation in density of the order of 0.0001 g/mL.

3.3. Sample injection syringes, auto sampler, or other equivalent system.

4. Procedure

4.1. Calibration of the densimeter

The apparatus must be calibrated according to the instrument manufacturer's instructions when it is first put into service. It must be recalibrated regularly and checked against a certified reference standard or an internal laboratory reference solution based on a certified reference standard.

4.2. Determination of sample density

4.2.1.    If required prior to measurement clean and dry the cell with acetone or absolute alcohol and dry air. Rinse the cell with the sample.

4.2.2.    Inject the sample into the cell (using a syringe, autosampler, or other equivalent system) so that the cell is completely filled. During the filling operation make sure that all air bubbles are completely eliminated. The sample must be homogeneous and must not contain any solid particles. Any suspended matter should be removed by filtration prior to analysis.

4.2.3.    Once the reading has stabilised, record the density ₂₀ or the alcoholic strength displayed by the densimeter.

4.3. Result:

When the density ₂₀ is used, calculate the real alcoholic strength using the alcoholic strength tables identified below:

The table giving the value of the alcoholic strength by volume (% vol.) at 20 °C as a function of the density at 20 °C of water-alcohol mixtures is the international table adopted by the International Legal Metrology Organisation in its Recommendation No. 22 (Table IVa).

 

5. Method performance characteristics (Precision)

5.1. Statistical results of the interlaboratory test

The following data were obtained from an international method performance study carried out on a variety of spirit drinks to internationally agreed procedures.

 

Year of interlaboratory test	1997
Number of laboratories	16
Number of samples	6

Samples	A	B	C
Number of laboratories retained after eliminating outliers	11	13	15
Number of outliers (Laboratories)	2	3	1
Number of accepted results	22	26	30
Mean value	23.81	40.12	40.35
	26.52*
Repeatability standard deviation (sr)   % vol.	0.044	0.046	0.027
Repeatability relative standard deviation (RSDr)  (%)	0.17	0.12	0.07
Repeatability limit ( r )  % vol.	0.12	0.13	0.08
Reproducibility standard deviation (sR) % vol.	0.054	0.069	0.083
Reproducibility relative standard deviation (RSDR)  (%)	0.21	0.17	0.21
Reproducibility limit ( R )   % vol.	0.15	0.19	0.23

Sample types

A Fruit liqueur ; split level*

B Brandy ; blind duplicates

C Whisky ; blind duplicates

Samples	D	E	F
Number of laboratories retained after eliminating outliers	16	14	13
Number of outliers (Laboratories)	-	1	2
Number of accepted results	32	28	26
Mean value	39.27	42.39	56.99
	43.10*	45.91*	63.31*
Repeatability standard deviation (sr)   % vol.	0.079	0.172	0.144
Repeatability relative standard deviation (RSDr)  (%)	0.19	0.39	0.24
Repeatability limit ( r )  % vol.	0.22	0.48	0.40
Reproducibility standard deviation (sR) % vol.	0.141	0.197	0.205
Reproducibility relative standard deviation (RSDR)  (%)	0.34	0.45	0.34
Reproducibility limit ( R )   % vol.	0.40	0.55	0.58

Sample types

D Grappa; split level*

E Aquavit; split level*

F Rum; split level*

OIV-MA-BS-05 Reference method for the determination of real alcoholic strength by volume of spirit drinks of viti-vinicultural origin: Measurement by densimetry using hydrostatic balance

Type II method

1. Principle

The alcoholic strength of spirits can be measured by densimetry using a hydrostatic balance based on Archimedes' principle according to which a body immersed in a liquid receives a vertical upward thrust from the liquid equal to the weight of liquid displaced.

 

2. Reagents and Materials

During the analysis, unless otherwise is stated, use only reagents of recognised analytical grade and water of at least grade 3 as defined in ISO 3696:1987.

2.1. Float cleaning solution (sodium hydroxide, 30 % w/v)

To prepare 100 ml weigh 30 g sodium hydroxide and make up to volume using 96 % volume ethanol.

 

3. Apparatus and Equipment

Usual laboratory apparatus and in particular the following.

3.1. Single-pan hydrostatic balance with a sensitivity of 1 mg.

3.2. Float with a volume of at least 20 ml, specially adapted to the balance, suspended with a thread of diameter not exceeding 0.1 mm.

3.3. Measuring cylinder bearing a level mark. The float must be capable of being contained completely within the volume of the cylinder located below the mark; the surface of the liquid may only be penetrated by the supporting thread. The measuring cylinder must have an internal diameter at least 6 mm larger than that of the float.

3.4. Thermometer (or temperature-measuring probe) graduated in degrees and tenths of a degree from 10 to 40 °C, calibrated to 0.05 °C.

Weights, calibrated by a recognised certifying body.

Note 1:  Use of a twin-pan balance is also possible; the principle is described in the Manual of Analysis Methods for Wines of the OIV.

 

4. Procedure

The float and measuring cylinder must be cleaned between each measurement with distilled water, dried with soft laboratory paper which does not shed fibres and rinsed with the solution whose density is to be determined. Measurements must be made as soon as the apparatus has reached stability so as to restrict alcohol loss by evaporation.

4.1. Calibration of the balance

Although balances usually have an internal calibration system, the hydrostatic balance must be capable of calibration with weights checked by an official certifying body.

4.2. Calibration of the float

4.2.1.    Fill the measuring cylinder to the mark with double-distilled water (or water of equivalent purity, e.g. microfiltered water with a conductivity of 18.2 M/cm) at a temperature between 15 °C and 25 °C but preferably at 20 °C.

4.2.2.    Immerse the float and the thermometer, stir, read off the density of the liquid from the apparatus and, if necessary, correct the reading so that it is equal to that of the water at measurement temperature.

4.3. Control using a water-alcohol solution

4.3.1.    Fill the measuring cylinder to the mark with a water-alcohol mixture of known strength at a temperature between 15 °C and 25 °C but preferably at 20 °C.

4.3.2.    Immerse the float and the thermometer, stir, read off the density of the liquid (or the alcoholic strength if this is possible) from the apparatus. The alcoholic strength thus established should be equal to the previously determined alcoholic strength.

Note 2:  This solution of known alcoholic strength can also be used to calibrate the float instead of double-distilled water.

4.4. Measurement of the density of a distillate (or of its alcoholic strength if the apparatus allows)

4.4.1.    Pour the test sample into the measuring cylinder up to the graduation mark.

4.4.2.    Immerse the float and the thermometer, stir, read off the density of the liquid (or the alcoholic strength if this is possible) from the apparatus. Note the temperature if the density is measured at t °C (ρ_t).

4.4.3.    Correct _t to 20 using the table of densities T for water-alcohol mixtures in the Manual of Analysis Methods for Wines of the OIV.

4.5. Cleaning of float and measuring cylinder

4.5.1.    Immerse the float in the float cleaning solution in the measuring cylinder.

4.5.2.    Allow to soak for one hour spinning the float periodically.

4.5.3.    Rinse with copious amounts of tap water followed by distilled water.

4.5.4.    Dry with soft laboratory paper which does not shed fibres.

Carry out this procedure when the float is first used and then regularly as required.

4.6. Result

Using the density ₂₀ calculate the real alcoholic strength using the alcoholic strength tables identified below.

The table giving the value of the alcoholic strength by volume (% vol.) at 20 °C as a function of the density at 20 °C of water-alcohol mixtures is the international table adopted by the International Legal Metrology Organisation in its Recommendation no. 22.

5. Method performance characteristics (Precision)

5.1. Statistical results of the interlaboratory test

The following data were obtained from an international method performance study carried out on a variety of spirit drinks to internationally agreed procedures.

Year of interlaboratory test	1997
Number of laboratories	12
Number of samples	6

Samples	A	B	C
Number of laboratories retained after eliminating outliers	12	10	11
Number of outliers (Laboratories)	-	2	1
Number of accepted results	24	20	22
Mean value (	23.80	40.09	40.29
	26.51*
Repeatability standard deviation (sr) % vol.	0.048	0.065	0.042
Repeatability relative standard deviation (RSDr)  (%)	0.19	0.16	0.10
Repeatability limit ( r )  % vol.	0.13	0.18	0.12
Reproducibility standard deviation (sR) % vol.	0.060	0.076	0.073
Reproducibility relative standard deviation (RSDR)  (%)	0.24	0.19	0.18
Reproducibility limit ( R )   % vol.	0.17	0.21	0.20

Sample types

A Fruit liqueur; split level*

B Brandy; blind duplicates

C Whisky; blind duplicates

Samples	D	E	F
Number of laboratories retained after eliminating outliers	12	11	9
Number of outliers (Laboratories)	-	1	2
Number of accepted results	24	22	18
Mean value (	39.26	42.38	57.16
	43.09*	45.89*	63.44*
Repeatability standard deviation (sr) % vol.	0.099	0.094	0.106
Repeatability relative standard deviation (RSDr)  (%)	0.24	0.21	0.18
Repeatability limit ( r )  % vol.	0.28	0.26	0.30
Reproducibility standard deviation (sR) % vol.	0.118	0.103	0.125
Reproducibility relative standard deviation (RSDR)  (%)	0.29	0.23	0.21
Reproducibility limit ( R )   % vol.	0.33	0.29	0.35

Sample types

D Grappa; split level*

E Aquavit; split level*

F Rum; split level*

Method OIV-MA-BS-06 : R2009

Type II method

Density of alcohols and alcoholic beverages method for determining electronic densimetry (principle based on measuring the period of oscillation)

(OENO 6/94;

OIV/OENO 382A/2009)

1.                Introduction

This method for determining the density of neutral alcohols and alcoholic beverages is based on the change in oscillation frequency in relation to the change in mass based on calibration with two fluids of known density.

Electronic densimeters with digital displays are commercially available to perform this determination.

2.                Object and scope of application

The purpose of this document is to describe a method for determining the density of alcohols and alcoholic beverages at atmospheric pressure.

The application of the method is restricted to products with a vapour pressure of less than 800 hectoPascal (600 mmHg) and a viscosity of less than approximately 15,000 m2/s (I m/s = 1 cSt) at the test temperature.

With reference to the currently applicable regulations, the test temperature is set to: 20°C.

3.                Density

The density of a liquid at a given temperature is equal to its mass divided by its volume:

It is expressed in kilograms per cubic meter (kg/m³) at a temperature of 20 degrees Celsius (°C) for alcohols and alcoholic beverages.

Note: electronic densimeters display results expressed in grams per cubic centimetre which may be converted into kilograms per cubic meter.

4.                Principle

4.1.      A liquid sample of a few millilitres is introduced into a vibrating measuring tube.

Measuring the period of oscillation of the tube containing the sample determines the density of the sample at the test temperature, using a previously calibrated apparatus.

4.2.      Principle of vibrating measuring tube.

Figure 1: Vibrating measuring tube

Electronic densimeters operate according to the vibrating measuring tube principle (fig. 1): the fluid is introduced into a U-shaped tube and subjected to electromagnetic excitation (fig. 2 and fig. 3).

Figure 2: Reactive forces exerted by the fluid

The induced period of oscillation is thus proportional to the total mass subject to vibration and can be used to determine the density of the sample based on the following equation:

Figure 3: Resulting torsion

• where T = induced period of vibration
• M = mass of the empty tube
• V = volume of the oscillated sample
• C = spring constant
• p = density of the sample

4.3.      Detailed principle.

The density of the liquids is determined by the electronic measurement of the oscillations of a vibrating U-shaped tube.

To carry out this measurement, the sample is introduced into an oscillating system, whose specific frequency is thus modified by the mass of the substance introduced.

The system comprises an undamped U-shaped vibration tube, subject to electronic excitation. The two straight sections of the U-shaped tube act like a spring mechanism. The oscillatory movements occur perpendicular to the U. The filling capacity, V, is delimited by the two fastening points. If the oscillator contains the volume established as V, the latter vibrates within and with the tube. It is accepted that the mass is proportional to the density. Since filling the tube beyond the fastening point does not affect the measurement, it is possible to perform a continuous flow measurement.

By maintaining a constant temperature over the entire system, the density will be calculated based on the period assuming a hollow container having mass M suspended by a spring, with a spring constant c. The hollow container will be filled with a volume of liquid of density p. The natural frequency of this vibratory system is:

F=1/2

• A = (c/4)²  V
• et B=M/V
• which gives p = A T2 - B
• (p = rho).

Constants A and B are the oscillator's spring constants, i.e. the mass of the empty tube and the tube's filling capacity. A and B are therefore system constants specific to each oscillator. They can be deduced from the measurement of two periods when the oscillator is filled with substances of known densities.

5.                Apparatus

5.1.      Digital display densimeter.

This appliance comprises the following elements:

• a glass measuring cell containing the measuring tube, a constant temperature chamber to be connected to an external circulating thermostatic bath, and a thermowell. The chamber can also be thermostated using an integrated device with a semiconductor element which uses the PELTIER effect,
• a system for oscillating the measuring tube and for measuring the period of oscillation,
• a clock,
• a calculator and digital display.

5.2.      Temperature control.

The densimeter's performance standards can only be met if the measuring cell is connected to a thermostatic bath, ensuring a temperature stability better than ± 0.02 °C, or if the densimeter has an integrated thermal control device which can achieve the same temperature stability.

5.3.      Sample injection syringes.

At least 2 ml polypropylene or glass syringes with tips that fit to the cell inlet. An adapter with PTFE cones is required in order to avoid the deterioration of the tip of the measuring cell.

The electronic densimeter can also be equipped with the appropriate autosampler for the apparatus.

5.4.      Temperature measurement.

Temperature measurement is carried out on the cell with a temperature probe whose sensing device (a platinum resistance probe compliant with class A of standard NF C 42.330), which is mounted with 4 wires, is introduced into the thermowell provided for this purpose in the cell. The probe is combined with an electronic temperature transmitter whose readout has a resolution of 0.01 °C. The probe and transmitter must first have been be calibrated in an approved calibration centre in order to ensure temperature measurement with an uncertainty less than or equal to ± 0.05°C.

The probe and transmitter must be periodically checked.

6.                Products

6.1.      Reference fluids.

During tests, the fluids must maintain their density characteristics; therefore, they must not be made of mixtures of products with different vapour pressures; their molecular composition and purity must be known. Their viscosity must be lower than 2 mm²/s.

Reference fluids must be chosen so that the densities encompass those of the products to be measured. The difference in density between the 2 reference fluids at the same temperature must be higher than 0.01 g/m³.

The reference fluid densities determined at a temperature of 20 °C with an uncertainty of less than ± 0.05 °C must be known with an uncertainty of less than ± 0.05 kg/m³.

When measuring the density of alcohols and alcoholic beverages, the following must be used, under the conditions previously described:

• hydro-alcoholic solutions whose density is exclusively determined using the pycnometric method (reference method).
• recently prepared, degassed double distilled water, or water of equivalent analytical purity,
• dry air.

6.2.      Cleaning products.

• chromic acid,
• organic solvents: ethanol 96 vol%, pure acetone.

6.3.      Drying.

• Pure acetone, dry air

7.                Period measurements in ambient air

Prior to the commissioning and calibration of the densimeter, it is essential to ensure the reproducibility of the measurement in the ambient air so that this measurement can be used to quickly check the cleanliness of the cell and consistency of the densimeter before every density measurement.

It should be possible to perform measurements of periods in the ambient air with a reproducibility of ± 10^-5 in relative values over the period for the same barometric pressure and the same temperature.

With some densimeters, the resonant period in the ambient air varies depending on the position of the temperature probe in the thermowell. For these densimeters, either the measuring cell must be replaced, or the temperature probe must be permanently attached, or its position in the thermowell must be accurately determined in order to achieve the reproducibility conditions described above.

NOTE: The use of polluted or excessively humid air may negatively influence the measurements. When these characteristics are combined in the test room, it is advisable to make the drying air flow through a purifier/dryer.

8.                Apparatus calibration

8.1.      General.

The apparatus must be calibrated upon initial commissioning. It must be recalibrated if a deviation in the air measurement is observed (see section 9.20) and, in any case, every three months.

8.2.      During initial calibration, it is necessary to calculate the values of constants A and B, determined by measuring the periods of oscillation (T1 and T2) respectively obtained using two reference fluids.

8.2.1.                    Place the display selector on the period measurement (T) position. Rinse the cell with acetone. Dry it with dry air generated by the pump that is integrated to the densimeter. When the reading is almost stable, stop the air supply, wait for thermal equilibrium and record the period of oscillation (Ta) obtained with an ambient air temperature of 20 °C. This process helps to check the cleanliness of the cell and stability of the apparatus at every calibration or determination of sample density.

8.2.2.                    Calibration measurement using the first reference fluid. Use a syringe to fill the cell through its bottom port with the standard liquid until it comes flush with the top port. Leave the syringe in place. While performing this, check the filling quality by visually checking for any air bubbles, even tiny ones. When thermal equilibrium has been reached, record the reading for the period of oscillation (T1). If the thermal control is compliant with the required accuracy, the value of T1 must not vary by more than ± 20 nanoseconds (2 resolution points).

8.2.3.                    Calibration measurement using the second reference fluid. Empty the cell with the syringe by drawing from the bottom port. Rinse the cell with acetone. Dry it with dry air generated by the pump that is integrated to the densimeter. To do so, connect the air outlet to the top port of the cell, start the pump and allow it to operate until the reading for T2 is almost constant; stop the pump, and when thermal equilibrium is reached, record the reading for the period of oscillation (T2) which corresponds to the air. If the reading for T2 matches the value obtained during previous tests carried out with a properly cleaned cell and at the same temperature, the cell can be considered clean and dry.

Carry out calibration using the second standard by repeating the steps in paragraph 8.22 and record the reading for the period of oscillation (T2) corresponding to the second reference fluid.

8.2.4.                    Based on the T1 and T2 values measured and the known values for the densities of both reference fluids, calculate constants A and B using the following equations:

• A = T1² - T2² / p1 – p2
• B= T2² – Ap2

where

• T1 is the observed period of oscillation with the cell containing the first reference fluid (in ms).
• T2 is the observed period of oscillation with the cell containing the second reference fluid (in ms).
• p1 is the density of the first reference fluid at the test temperature (in g/cm³),
• p2 is the density of the second reference fluid at the test temperature (in g/cm³).

Depending on the procedure for the densimeter being used:

8.2.5.                    Enter constants A and B using the digital display at the top of the apparatus. In order to make sure they have been correctly memorised, display the values by placing the selector on the "A" and "B" positions.

8.2.6.                    Place the selector on the measurement position. The densimeter should immediately display the densities of the samples introduced into the measuring cell.

NOTE: Some electronic densimeter models automatically calculate the calibration constants.

8.3.      Checking the calibration.

In order to validate the operation, measure a reference solution whose density value is within the calibration range used.

Reference substances, verified by a metrology agency, are commercially available.

The calibration is validated if the result of the measurement of the reference substance density complies with the accuracy class of the electronic densimeter being used.

9.                Procedure

9.1.      Preparation of test apparatus.

• Place the densimeter on a perfectly stable support, isolated from any vibrations.
• Connect the densimeter to the circulating constant temperature bath using flexible rubber pipes or insulating tubes. Fill the water bath according to the manufacturer's instructions and add a product to prevent the formation of algae.

Set the bath temperature to reach and maintain the requisite test temperature on the densimeter.

• Accurately setting and controlling the temperature in the measuring cell are very important parameters, as a 0.1 °C error can result in a variation in density in the order of 0.1 kg/m³.
• The following rules must be observed:
• the measuring cell must be maintained at a constant temperature for 6 hours before the test.
• the maximum temperature variation measured by the temperature probe of the measuring cell must not exceed ± 0.02 °C.
• the pipe flow, length and insulation between the thermostatic bath and the cell are to be adjusted to ensure the stability of the cell temperature.

9.2.      Checking period measurements in the ambient air.

• Clean, rinse and dry the cell.
• Carry out a measurement in the ambient air. Check that the period measured does not deviate by more than i0- in relative values from the period determined under the conditions described in section 7. If deviation occurs, clean the cell again with a lukewarm chromatic acid solution (warning: this product can cause serious burns), which is the most effective cleaning agent. If the deviation persists after several cleaning operations, repeat the calibration process.

9.3.      Density measurement.

• Filter the sample first, if necessary.
• Illuminate the cell.

If only a small quantity of sample is available, use the syringe to introduce the quantity needed so that the liquid to be measured reaches the top port of the clean and dry cell. While filling the cell, make sure all air bubbles are completely removed; the sample must be homogeneous and must not contain any solid particulates.

• Leave the syringe in place on the bottom port of the cell.

NOTE: Introducing dark coloured samples into the cell does not help to establish the absence of air bubbles or solid particulates with certainty.

Switch off the lamp immediately after introducing the sample, as the heat it produces influences the measurement temperature.

9.4.      Calculation and expression of results.

9.4.1.                    Densimeter with an integrated calculator.

After a few minutes, the density value stabilises, indicating that the equilibrium temperature of the measurement has been reached. If the measurement temperature does not differ by more than ± 0.01 °C from a temperature of 20 °C, record the reading.

If needed, convert the obtained result into kg/m³.

9.4.2.                    Densimeter without a calculator.

Allow the reading for the period of oscillation (T) to stabilise within one unit from the fourth decimal place. If the measurement temperature does not differ by more than ± 0.01 °C from a temperature of 20 °C, record the reading.

Calculate the density of the sample in kg/m3 using the following formula:

where

• T is the period of oscillation for the sample measured (in ms).
• A and B are the constants defined during the calibration prescribed in paragraph 8.

9.4.3.                    Viscosity correction.

If the liquid whose density is being measured has a viscosity higher than 2 mm²/s, correction is required to take the viscosity into account, using the formula provided by the manufacturer of the densimeter.

10.         Test report

The test report must indicate:

• the method used,
• the result and mode of expression of results,
• the specific details and any unforeseen events recorded during the measurement,
• operations not included in the method.

APPENDIX A: TABLE 1 AIR DENSITY

Air density, expressed in g/cm³, varies with the pressure P expressed in mbar and the temperature expressed in °C.

A t °C and p Ton-, calculate the density using the following formula:

Values are given for contents of 0.03 vol% of CO₂ in the air; values change by ± 1/19000 with each variation of ± 0.0001 of the CO₂ volume.

Composition of dry air at ground level:

	N2	O2	A	CO2	Ne	He	Kr	X	H2
Volume in %	78.09	20.95	0.93	0,03	0.0018	0.0005	0.041	0.068	0.045
Mass in %	75.52	23.15	1.28	0.05	0.0013	0.047	0.043	0.044	0.084