Measuring Alcohol - Part V
-This article is written by Mr. Gary Spedding, Ph.D., Alcohol Beverage Chemist-
Historically, alcohol measurements were grounded in physical measurements of mass and volume through density or mass per unit volume intensive properties. Through density and specific gravity relationships, instruments and devices such as density bottles, hydrometers, densitometers, refractometers and pycnometers were used to establish a recognized and officially accepted body of work. This extensive research effort culminated in the derivation of algorithms and tables which define the relationships between density values and specific gravity readings and alcohol by weight and by volume.
Welcome to the final installment on this series of articles dedicated to the science of alcohol concentration determination. There are many options available, some better suited than others, depending on the degree of accuracy needed and on the equipment available.For more information on the author or on the services offered by his certified beverage laboratory, please visit:
Brewing and Distilling Analytical Services, LL C.
Lexington, KY. USA.
www.alcbevtesting.com
info@alcbevtesting.com
A quick final note on alcohol calculations
For those instruments and methods that lead the chemist to obtain the alcohol by weight (ABW) the alcohol by volume (ABV) can be calculated if the specific gravity is also precisely known:
ABV = ABW x SG sample
SG ethyl alcohol 20°C/20°C
Where, SG: specific gravity, e.g., for the alcohol beverage sample or pure ethanol respectively.
This simplifies to:
ABV = ABW x SG sample
0.7907
Variant equations are available but alcohol should be reported both % by weight (wt.) and by volume (vol.) to two decimal places. For reporting purposes most alcohol beverage producers are allowed a certain tolerance in readings expressed as some degree +/- a specified amount of alcohol by volume (for beer, for example, that is +/- 0.3% ABV).
A final note on the use of a Coupled Density Meter and NIR Instrument (an Alcolyzer) in resolving issues on simple density measurements
Avoiding the need to analyze distillates the coupled Density meter/NIR instruments measure the alcohol via the ethanol absorption profile. This profile is then compared to and adjusted by the density meter which is described as one of the most accurate concentration meters for binary solutions. Several algorithms can be used for this adjustment. The alcohol by volume is thus obtained which is essentially the equivalent as if obtained from the density of a volumetric distillate. The concentration of ethanol, as percent by weight, is computed as the product of the alcohol by volume, as determined/calculated by the NIR alcolyzer, and the density of pure ethanol divided by the density of the sample, as determined by the density meter (ABW = ABV*0.78924/Sample Density). An apparent redundancy (not detailed here) in determining the ABW with such coupled instruments is actually quite useful as it can eliminate any issues (in this author’s opinion and experience) with the density measurement if that is affected in any way with co-distilled components (BDAS, LLC laboratory testing – personal observations and personal communications with the scientists at Anton Paar-USA). In simple terms it means that the coupled density meter/Alcolyzer combination can, in fact, ultimately determine the correct alcohol by volume assuming nothing interferes with the NIR signal in measuring the actual alcohol in the sample. (That is another consideration to take into account in measuring complex samples).
As both density meter and NIR units communicate with each other, a complex series of calculations is performed in order to solve for the correct values of all parameters (Roman Benes of Anton Paar, personal communication). Observations in the BDAS, LLC laboratory (with what we term direct measurements of samples) show this to be true in measuring the alcohol by volume (in mildly acidic solutions – containing low levels of acetic acid for example) within a tolerance of +/-ca. 0.2% ABV. Further work may be necessary to obtain the finite limits on certain complex-matrix alcohol containing beverages but is in accord with descriptions of earlier published official AOAC methods (for wines containing acetic acid for example). [See AOAC methods: 935.21 and 920.57.]
Concluding remarks
From above it is hopefully clear that, for many types of alcoholic beverage, a number of officially accepted and highly accurate methods/techniques/instrumentation can be used to faithfully, accurately and precisely measure alcohol content in suitably prepared samples within quite tight and allowable regulatory tolerances. For one particularly complex type of alcoholic beverage, not yet classified for testing, a collaborative laboratory project – four independent laboratories/four distinct methods (GC, HPLC, NMR and Density Meter/Near-Infrared Alcohol detection) gave almost identical results (within ca. +/-0.2% ABV) for each method, based on the comparative data on three selected samples. It should be noted that in general all the methods discussed are stated to be capable of accuracy measurements and detection limits of 0.1-0.2% ABV.
References
A detailed set of references is available upon request. This article was adapted in part (and extended in scope regarding the methods) from “Alcohol and its Measurement.” Gary Spedding. In: Brewing Materials and Processes: A Practical Approach to Beer Excellence (ed. C. W. Bamforth). Elsevier, Burlington, MA. 2016. [In preparation]. A detailed set of references may be found there. Also recommended reading is “Foundations of Spectroscopy” (Oxford Chemistry Primers). 1st Ed. Simon Duckett and Bruce Gilbert. Oxford University Press. ISBN-10-0198503350. 2000.
As spectroscopy featured as a key aspect of alcohol measurement the reader may also wish to consult: “Molecular Spectroscopy” (Oxford Chemistry Primers). John M. Brown. Oxford University Press. ISBN-978-0-19-855785-2. 1998; “Introduction to Organic Spectroscopy” (Oxford Chemistry Primers). Laurence M. Harwood and Timothy D. W. Claridge. Oxford University Press. ISBN-978-0-19-855755-5. 1997; “Light Spectroscopy” D.A. Harris. BIOS Scientific Publishers, Ltd. ISBN: 1-872748-34-1. 1996, and “Nuclear Magnetic Resonance” (Oxford Chemistry Primers). P. J. Hore. Oxford University Press. ISBN-978-0-19-855682-4. 1995.
Tables of data
OIML.org. The Alcoholometric Tables. https://www.oiml.org/en/files/pdf_r/r022-e75.pdf/view (last accessed, Dec, 2015)
Percentages by volume at 15.56°C (60°F) of ethyl alcohol corresponding to apparent specific gravity at various temperatures. Table 913.02. In: AOAC 16th Ed. Intl. Off. Methods of Analysis Manual. 1995.
Acknowledgments
I wish to thank Amber Weygandt, and Matt Linske (from BDAS, LLC) and Gordon Burns (ETS Laboratories, St. Helena, CA), Patrick Heist and Brian King (Ferm Solutions, Inc., Danville, KY), John Edwards (Process NMR Associates, Danbury, CT), Ray Marsili (Marsili Consulting Group, Rockford, IL) and Alex White (Anton-Paar, USA, Ashland, VA) for attention to editorial details and for pointing out areas of confusion or ambiguity. While they assisted with some details and corrections any errors or omissions remain my responsibility.
ADDENDUM
UV-visible spectroscopy may also provide a useful tool for measuring alcohol concentration. There are instruments on the market to measure both proof and color in whiskey for example (Applied Analytics). Ethanol and water both have unique structural features in their respective absorption spectra (as also noted for the other techniques discussed above). These have stronger or weaker prominence based upon their concentrations in measured alcohol solutions. In such systems the absorbance occurring over the 800-1000 nm region of the visible spectrum is measured. A peak of absorption for ethanol is noted at ca. 907 nm and a broad peak, centered at ca. 988 nm, is noted for water. While the alcohol peak is of very low amplitude it can be used to determine alcohol concentrations as tested vs. known proof samples. Recently UV/visible spectra have been generated in the BDAS, LLC laboratory but data analysis to see if meaningful results from such approaches is only just now underway. It may not be suitable for very low alcohol containing samples but is a powerful technique in its own right and regaining popularity for measuring the “fingerprint” and authenticity, consistency, dilution and adulteration of alcoholic beverages. This topic was discussed in our earlier White Paper: “Scanning UV-Visible Spectroscopy and Beverage Quality, Consistency and Authentication: Preliminary Fingerprinting Application in the Analysis of a Wide Variety of Alcoholic Beverages – A Brief Application Note” Gary Spedding. BDAS, LLC WPSP#1. 2015.