Alcohol Refractometer

You would want to speak to the manufacturer directly but I suspect they are still reading must but are calibrated in alcohol potential. You cannot depend on the refractive index for a solution of must and alcohol. There are many ways to measure alcohol, from the little vinometer for dry wines, to distilling up to chromatography are all used depending on the precision required.

I guess you could calibrate a refractometer to measure alcohol but you would still need to distill it to remove the dissolved solids as I see it.

Distilling followed by use of a proof and tralle hydrometer is a relatively inexpensive way to measure precisely.

Joe

Folowing is a summary of alcohol analyses I prepared some years ago when I was a guest speaker at a conference. I hope it comes through in a readable form

Bob Molony

MOLAB Ltd. Consulting Analytical Chemist & Food Technologist. 14 Goldie St., St. Heliers, Auckland, New Zealand. eMail snipped-for-privacy@ww.co.nz

Phone/Fax (09) 5755478 , Mobile (027) 4523984.

ESTIMATION of ALCOHOL CONTENT in BEVERAGES.

There are number of ways in which the amount of alcohol in a beverage may be estimated. The actual method used depends both on the product and the reason for determining the alcohol content. The methods of estimation fall into two categories. (A) The separation of the alcohol from the product by distillation followed by the estimation of the alcohol in the distillate by a chemical or physical measurement.The only primary standard procedures belong to this category. GLC of the whole product (not headspace chromatography ) is a special case as a distillation actually occurs in the machine. (B) The measurement of some physical parameter the value of which is related in some way to the alcohol content of the product. In general any physical parameter which can be used in this way is influenced not just by alcohol but by other materials present in solution. Note in particular the vapour pressure (and boiling point) of alcohol is affected by the soluble solids present. These procedures are calibrated by the use of samples which have been previously analysed by a primary standard procedure.

(A)(1) Standard distillation followed by determination of alcohol in distillate. (a) Specific Gravity (aa) Specific Gravity Bottle ** The traditional standard procedure. For reasonable accuracy a water bath with a temperature stability of 0.2 degrees C or better is required (ab) Hydrometer ** Special small hydrometers are available with integral thermometer for temperature correction. These hydrometers are very expensive and also very fragile

(ac) Electronic Densitometer An expensive instrument suited to multiple determinations. However the same intrument can be used in other applications to defray cost. (b) Refractive Index ** Accurate procedure but requires the use of an immersion refractometer and very accurate temperature control (c) Ebuliometer Distillation first avoids the use of emperical correction for residual extract (d) Chemical oxidation ** The chemical oxidation procedure as per JAOAC

52,85(1969) may be carried out directly on an aliquot of distillate.The dichromate is less susceptible to some interfering factors than the specific gravity procedure.

(2) Steam Distillation and Oxidation (a) AOAC Method ** Steam distillation followed by Dichromate oxidation as per JAOAC 52,85(1969). Typical accuracy 1 part in 200 of the alcohol present. Critical factors (1) Preparation of the dichromate reagent. (2) Water used in preparation of reagents. (b) Combi method Dichromate oxidation procedure. There are a number of objections to the use of this procedure for accurate work.These are technical questions related to variations in the stochiometry of the reactions used. Other critical factors as for (2)(a)

(A)(3) GLC (a) Direct (a) Without Internal Standard Accuracy is low due to the problems of accurately measuring very small volumes. A standard solution of alcohol must be used for comparison

(b) With internal Standard n-Butyl alcohol is usually used as the standard Procedure is capable of a reproducability of 2% however standard solutions are not stable and must be made up regularly. Alcohol and Butyl Alcohol are volatile and absorb water readily. This can make accurate measurement of these materials unreliable.

(B)(1) Ebuliometer The ebuliometer or boiling point procedure at one time was an official UK customs procedure. With care results can be obtained consistantly to the nearest 0.1 % alcohol by volume. Individual on site calibration with known samples is not required as Temperature vs boiling point varys little between instruments and a variety of suitable corrections for soluble solids are available. Critical factors are (1) Correction for any extract present. Tables are available. (2) Adequate cool water in the condenser. (3) Use of a barometer to monitor for the presence of barometric pressure change between calibration and determinations.

(B)(2) Headspace Gas Chromatography Vapour pressure of alcohol component is affected by non volatile components in sample. Equipment must be calibrated against similar samples of a known composition. The calibration samples would normally be analysed by the distillation and specific gravity or by distillation (steam or otherwise) and dichromate oxidation. Carbon dioxide also has the potential to alter vapour pressure of the alcohol present an therefore has the potential to cause inaccuracies. Standard solutions are unstable and must be made up regularly. Alcohol and Butyl Alcohol are volatile and absorb water readily. This can make accurate measurement unreliable

(B)(3) Headspace gas alcohol meter (a) Fuel cell Vapour pressure of alcohol is affected by presence of non volatile components. Equipment must be calibrated against similar samples of known composition. The standards must be analysed for alcohol by another procedure.

(B)(4) Loss of gravity during fermentation The traditional method for determining alcohol levels during fermentations. Can be quite accurate provided that no additions are made to the product in the time between the gravity determinations. Care must be taken when degassing sample in order to prevent alcohol loss.

(B)(5) Williams Field test A rapid field test of very limited accuracy used for higher alcohol levels in the US. Not suitable for beer or wine.

(B)(6) NIR , Near Infrared Spectrophotometry. A modern instrument which provides very rapid results. The results are affected by other components in solution and the calibration must be carried out using samples of similar composition which have been analysed by other procedures.

** Suitable primary standard procedures.