Determination of Alcohol in Pharmaceutical
Liquids Use of the Chain Hydrometer and Alcohol-Water Temperature Charts S. JIerrell Co., Cincinnati, Ohio
T
HE chain hydrometer in its present form was iiivent,ed and (lex-eloped by C. IT. Foulk, of Ohio State Univereity, and described by him in 1923 ( 3 ) . The instrument n-ill give accurate density readings through four decimal places in ordinary industrial use and through ais places in careful scientific n-ork. Its theory, operation, and construction have been admirably described and illustrated by Foulk (3). Other details are available ( I , 4, 5, 7 ) ; and there is also an interesting paper by Koch and Smith (6). Up to the present time the chain hytlroiiieter has been specifically applied to the standardization of hydrochloric acid for analytical use (5, '7) and tlie estimation of tiisdved solids in boiler vater (1). As far as the author has been able to ascertain, the chain hydrometer, until now, has never been applied to the measurement of the specific gravity of the alcoholic distillate in a pharriiaceutical alcohol determination. I pycnometer is usually employed for this purpose; in fact, tlie Cnited States Pharmacopeia S specified t,he use of a pycnometer ( 9 ) ,but this requirement v a s omitted from the United States Pharniacopceia XI (11). Because pycnometer det'erminations are time-consuming a nen' in.;trument that is both accurate and rapid is desirable. The cliain hydrometer is sucli an instrument.
in veight (0.044 grain per ern.), and easily availalile was chosen for the n (irk. The forniula given by k'oulk and Brooks (3) concerning the movement of the bulb in a chain hydrometer is Ad =
\There Ad
=
D = d = T' = IC
=
1 Present address, Kettering Laboratory of .Ipplied Physiology, Unirersity of Cincinnati, Cincinnati, Ohio.
10
20 X
F
density change in liquid \rhich cauaeh a vertical displacement of float of b measured in ern. density of material of chain (about 10.7 v-ith most gold-plated chains) density of liquid volume of float measured in cc. weight in g r a m of 1 cin. of chain
Substituting the above data i n lliis foriiiula, \\-ah found to be about 5 cc. This volmne niatle the bulh fit conveniently in the graduated tube of the cliain hytlronieters designed for this work. In Figure 1, A i\ an ordinary 50-cc. graduated cylinder, or better, a cylinder that has not, been gradutited. B is agraduated tube, 20 cni. long and 1.8 cm. in inside diameter, bearing 130 Inarks 0.73 mm. apart. Cis a ruhber stopper. The bulb is ballasted with carbon tetrachloride, the meniscus of vi-hich serves as a reference mark for determining positions on the graduations of tube B. The gold chain is fastened t o the graduated tube and to the bulb with platinum mire, which is uealed into the glass. The length of the chain and the amount of carbon tetrachloride ballast in the bulb are carefully adjusted, so that with one hydrometer the reading is about 100 in water and 0 in 25 per cent alcohol at approximately 25' C., and with the other about 100 in 21 per cent alcohol and 0 in 44 per cent nlcoliol. The overlapping between i he ranges of the two hydrometers is desirable for obvious reasons.
Description and Use of Hydrometers Since the distillate obtained in the usual methods of alcohol determination is clear and colorless, no difficulty is encountered in measuring its specific gravity directly by the chain hydrometer. The methods in common use in this country furnish a distillate of 50 cc., anti therefore hydronieters were designed to take tjiat volume of sample and to cover a specific grayity range (referred to 11-ater in air a t 15.56") from 1.0000 to 0.9399 or, in terms of equivalent alcohol percentages by volume, from 0 to 47 per cent' at 15.56" C . In the percentage range desired, a difference in density of 0.0001 is equivalent to about 0.06 per cent of alcohol. For routine industrial work, alcohol percentages need not be defined closer than 0.2 per cent-in fact, it is probable that uiiless extreme precautions are taken the Cnited States Phariiiacopceia S I method n-ill not' yield consistent results nearer tlie truth than 0.5 per cent. Sunierous interfering substances present in most pharmaceutical preparations will usually make the error greater than this figure. The position of a chain hydrometer bulb in a clear liquid can easily be read directly to one division and estimated to one-half dixision against a graduated wale in Tvhicli tlie divisions are 0.75 iiim. apart. Therefore chain hydrometers were desired in which a specific gravity difference uf 0.00015 ~\-ouldcause a change of approximately 0.375 nini. in tlie position of the bulb. I t was found that two instruments were necessary to cover the required specific gravity range, in order to keep tlie tube length short enough t o fit in the ordinary 50-cc. graduated cylinder which held the sample. -4 je\\-eler's gold-plated chain nhicli n-as strong, fairly light
( D - d) X b X
'
I
/
I
ternlined n-itli :I pycnoiiieter. The hydrometer readings, teniperatures, and specific gravity data xere correlated to ohtain
I-OL. 10,so. 9
INDUSTRIAL AND ENGINEERING CHEXIISTRY
542
A1 cohol-Water Temperature Charts Tables published in the United States Pharmacopeia X and XI (IO,12) and based upon work done by the n'ational Bureau of Standards (8) can be correlated and the results plotted to give the accompanying charts (Figures 2 and 3). Larger charts, nhich can be read directly to 0.1 per cent of alcohol, can easily be made by plotting the data given in the United States Pharmacopeia X (10). The charts are used as follows : A plcnonieter IS calibrated 111th vater at 15.56" C. An alcohol-\\ ater mixture is T\ eighed at room temperature and its weight is divided by the neight of xater obtained in the calibration of the pycnometer. The resulting specific gravity is found on the ordinate of the chart and the horizontal line is followed until the curved line corresponding to the temperature of the alcohol-water mixture is intersected. The abscissa at this point indicates the alcohol percentage by volume at the standard temperature, 15.56' C.
-4fter a chain hydrometer is calibrated, the calibration figures can be put on the chart in place of the specific gravity figures. Then any hydrometer reading at any temperature can be converted to alcohol percentage a t the official temperature without calculation. Similarly. the weights of a pycnometer contaiiiiiig \-ai ious alcohol-n ater mixtures can be put on the chart in place of the specific gravity figures and tlie same elimination of calculations results as long as the weight and capacity of that particular pycnometer do not change. The 0 930
0.932
0.934 0 936 0.938
0.940 0 942 0 944
0 946 0 948
0.950 0 952
alcoholic distillate does not have to be brought to any definite temperature in taking its specific gravity when one of these alcohol-water temperature charts is used; this fact, together with the elimination of calculations, results in a marked saving of time over the usual method of alcohol determination.
Procedure Distill the sample in the usual way, following the method in tlie United States Pharmacopeia XI. When interfering volatile substances are present, the method using heptane and a special alcohol receiver ( 2 ) is recommended. Bring the clear alcoholic distillate to a volume of 50 cc. with distilled water and transfer it to a dry 50-cc. cylinder. Mix it thoroughly by inverting and righting the cylinder several times (this is important). Allow air bubbles to rise to the top, insert the chain hydrometer, and let the bulb reach equilibrium. When it has come to rest, read its position in the graduated t'ube and take the temperature of the liquid to the nearest 0.3" C. (The temperature can be conveniently taken by allowing a small thermometer to hang in the graduated tube just above the bulb while the bulb is reaching equilibrium.) Then, by reference to the calibration data or to an alcohol-water temperature chart upon which the hydrometer calibration data have been recorded, the correct alcohol percentage at the officialtemperature can be obtained. ..idvantages of the Hydrometer The chain hydrometer offers inany advantages over otlier methods of measuring specific gravities in alcohol determinations. It consumes little manipulative time; placing the hydrometer in the liquid and taking the reading are operation.? which can be done in seconds, not minutes. This factor is decidedlv in its favor n hen the instrument is ;ompared with the pycnometer or the Westphal balance. When the alcohol-water temperature charts described above are used in connection with the chain hydrometer, results are obtained more rapidly than by any other method. The ordinary Westphal balance does not give as consistently accurate results as the chain hydrometers designed for this work, and the hydrometer with a projecting stem is, of course, out of the question for third or fourth decimal place work. The pycnometer and the very sensitive Restphal balance are the only common instruments which approach the chain hydrometer in sensitivity and accuracy for alcohol deterniinations, but their time-consuming characteristics make them unsuitable for routine woik.
0 954 0 956
0.958 0
0 960
0962
0 964 0.966 FIGURE2. . ~ L C O H O L - ~ ' A T E R TEMPER~TURE CHART
0 968 0'g70Z4
26
28
30
32 34 36 38 40 42
Alcohol
-
Percent
by
44
46 Volume
48
50 52
SEPTEMBER 15, 1938
ANALYTICAL EDITION
543
0 962 0.964
FIGURE 3 . *$LCOKOL-\v.kTER TEMPERATTRE CHART
0.966
0 960 0.9 70 0
0.972 0.974
0.976
liquid drained out through the stopcock. This could be done by an ordinary workman without fear of damage to the delicat,e instrument, since he would not have to handle it at all. blany other industrial applications of this valuable instrument will come t o mind on brief consideration.
0.978 0.980 0.982 0.984
0.986
Summary
0.988
0.990
0.992 0.994
0.996
0.998 1.000
Alcohol
-
Percent
by
Volume
The chain hydrometers used here have surpassed the most optimistic expectations with respect to their continued accuracy over a long period of time. They have held their original calibrations for over 4 years of daily use. Literally thousands of alcohol determinations have been carried out n ith their help and they have shown no indication of change in accuracy or sensitivity during that time. The chain hydrometer also has other important applications in chemical manufacture. Certain processes require careful density control and it often saves time to design special hydrometers for liquids which must undergo frequent density measurements. I n one instance, a particularly disagreeable lachrymator was used in a manufacturing process. Density was an important factor in the control of this liquid, and making pycnometer weighings was an unpleasant task with such a substance d special chain hydrometer solved the problem, for the entire manipulation was then carried out in a fume hood. The greater sensitivity and accuracy of the chain hydrometer give it distinct advantages in the control of alcohol recovery in pharmaceutical manufacture and in the fermentation industries. A properly designed chain hydrometer, securely fastened to a wall in the plant and with a stopcock a t the bottom of the outer tube which contains the sample, could take care of the work which is less accurately done a t present with ordinary projecting stem hydrometers. The sample could be poured in a t the top, the bulb alloi\ed t o come to rest, the reading and temperature taken, and the
The chain hydrometer, a sensitive and accurate instrument for the measurement of the density of liquids, has been successfully applied to the determination of alcohol in pharmaceutical liquids. It has significant advantages over other instruments for this purpose, mainly because it saves tiniewithout sacrifice of accuracy. Other industrial applications are suggested. Alcohol-water temperature charts are described and illustrated, by means of which chain hydrometer readings or pycnometer weights at room temperature car1 be converted directly to alcoliol percentages a t the officialtemperature.
Acknowledgment Appreciation is extended t o C. W. Foulk for the initial inspiration and many helpful contributions he has made t,o this n-ork, and t o T. H. Rider for his 1-aluahle comments and suggestions. The patent rights on the chain hydrometer are owned by the Kauffmann-Lattimer Company, Columbus! Ohio, which has generously given the author permission to make and use the hydrometers described in this paper.
Literature Cited (1) Bambach, Karl, master's thesis, Ohio State University, 1932. (2) Bambach, Karl, and Rider, T. H., J . Am. Pharrn. dssoc., 25, 9s:' (1936). (3) Foulk, C. W., J.Optical SOC. Am., 7 , 327 (19233;. (4) Grady, R . I., doctor's dissertation, Ohio State Ciniversity, 1923. (5) Gran, J. E., doctor's dissertation, Ohio State University, 1932. (6) Koch, W. W., and Smith, G. F., ISD. EKG.C m x , Anal. Ed., 2, 41 (1930). (7) Sinnett, R. IT.,"Abstract of Doctors' Dissertations," KO.'71, Ohio State University Press, 1937. (8) U. S. Bur. Standards, Bull. 9 , No. 3,424 (1913). (9) U. S.Pharmacopaeia X, p. 437, 1926. (10) Ibid., pp. 528, 531. (11) U. S. Pharmacopeia XI, p. 435, 1936. (12) Ibid., p. 604. RECEIVEDJune 6, 1938. This is t h e third paper in a series on t h e "Determination of Alcohol in Pharmaceutical Liquids." F o r the first t w o , see J . Am. Pharm. Assoc., 25, 313, 982 (1936).
