Determination of Water in Alcohols By Means of High-Frequency Oscillators PHILIP W. WEST, PASCHOAL SENISE', AND T. S. BURKHALTERl Coates Chemical Laboratories, Louisiana State University, Baton Rouge, La. This study:was undertaken to determine the applicability of high-frequency oscillators for the direct measurement of the water content of monohydric and polyhydric alcohols. Calibration curves for various alcohol-water systems are presented. Particularly favorable results have been obtained in the study of the system ethyl alcohol-water, and the proposed method is recommended for routine control purposes in alcohol manufacture. The method requires only a few seconds of the operator's time for a single measurement, and the results fall within the ordinary limits of accuracy required.
I
K A previous paper (a) a high-frequency oscillator utilizing the heterodyne principle, designed and constructed for use
as a chemical analyzer, was described and its adaptability to most types of conductometric titrations as well as to concentration measurements on many organic binary systems was shown. A simple control method for the analysis of the ternary system water-benzene-methyl ethyl ketone was also developed (4). Monaghan, Moseley, Burkhalter, and Nance have applied instruments of this type to the location of bands in chromatographic separations ( 1 , 2 ) . In the present investigation measurements were carried out on binary systems of various alcohols with water, using the abovementioned oscillator and a commercial instrument having similar characteristics. The calibration curves obtained showed that, in addition to speed and simplicity of the technique employed, the method can be use with a satisfactory degree of accuracy over any range of composition. Calibration curves were determined for binary systems of a number of aliphatic monohydric and some polyhydric alcohols with Fater.
I GLYCEROL
APPARATUS
Measurements were performed using both the heterodyne-type high-frequency oscillator described by West, Burkhalter, and Broussard ( S , 4 )and the Model V oscillometer recently made commercially available by the E. H. Sargent Co. The Model V chemical oscillometer depends on a precision capacitor and several calibrated range capacitors to compensate for changes produced by the sample. An F.M. discriminator circuit and a vacuum tube voltmeter constitute the frequency reference and detection systems of the instrument. I n operation, the instrument is tuned to a reference frequency, as indicated by a zero reading of a meter, and measurements are then made by restoring the zero reading by means of appropriate combinations of the range and measuring capacitors. From the dial readings and a knowledge of the cell constants, it is possible to compute the capacitance change produced by a sample and to calculate its dielectric constant. For routine analytical determinations, plots of dial readings versus concentration may be readily determined for use as working curves. REAGENTS
Reagent grade alcohols were used. Methyl, ethyl, n-propyl, isopropyl, tert-butyl, and allyl alcohol were dehydrated by refluxing several hours over calcium oxide, leaving overnight, and then distilling over calcium oxide. Middle fractions were always utilized. Isoamyl alcohol was first distilled, then left overnight over calcium oxide, decanted, and 1 Present address, Departmento de Quimica d a Faculdade de Filosofia, Ciencias e Letras da Universidade de SLo Paulo, S i 0 Paulo, Brazil. 2 Present address, Chemistry Department. North Texas Stat,e College, Denton, Tex.
0
PROPYLENE GLYCOL
A
ETHYLENE GLYCOL
0
METHYL ALCOHOL ETHYL ALCOHOL
3000
m
0 ALLYL ALCOHOL
* N.PROPYL ALCOHOL D ISOPROPYL ALCOHOL
2600
X TERTBUTYL ALCCHX
2200 WATER W T % Figure 1. Binary Systems of Water-Miscible Alcohols with Water
redistilled. n-Butyl, isobutyl, sec-butyl, n-amyl, tert-amyl, and n-hexyl alcohols were used after careful fractional distillation. Ethylene and propylene glycols were dried over anhydrous sodium sulfate during a few days and distilled after decantation. The following boiling ranges were recorded for the fractions collected of the various alcohols: Methyl, 64.2-64.5; ; ethyl, 77.5-78.0' ; n;propyl, 95.0-96.0 '; isopropyl, 80.5-81.0 ; n-butyl, 116.5-117.0 . isob$yl, 106.5107.0". sec-butyl, 98.0-98.5 ; tert-butyl, 81.'0-82.0 ; n-amyl, 136.0-i37.Oo; isoamyl, 129.8-130.2"; !erf-amyl, 101.0-101.8"; nhexyl, 156.0-157.0'. allyl, 96.0-96.5 ; ethylene glycol, 196.5197.0"; propylene &ycol, 186.5-188.8' C. Glycerol was subjected to prolonged warming a t 90' to 100' C., and cooled by passing a stream of dry air through it for a few hours. It was stored in a desiccator in thin layers over concm1250
V O L U M E 24, NO. 8, A U G U S T 1 9 5 2
1251
trated sulfuric acid. Dehydration of ethyl alcohol and glycerol was controlled by titration of the samples with Karl Fischer reagent.
that atmospheric conditions do not affect significantly the reproducibility of the measurements, provided the samples are kept a t the same temperature as the tank circuit coil.
Heterodyne Oscillator. A reference point was arbitrarily chosen by adjusting the instrument so as to have a beat frequency change of 5760 cycles per second with distilled water (as compared to an empty cell), and this point was frequently checked in the course of the measurements. Samples were always thermostated a t 37" C. (temperature of the tank circuit coil). In previous papers ( 3 , 4 ) it has been shown
Figure 1 shows the results of measurements on binary systems of some a ater-miscible monohydric and polyhydric alcohols with water. In the study of systems with partially water-miscible alcohols it was observed that in some cases if small amounts of mater were added to the pure alcohol a lower value for the beat frequency change was read on the meter, so that the curve of composition versus beat frequency change of the corresponding binary system would show a minimum, as appears in the graphs of Figure 2. In such cases, for some ranges of concentration, two mixtures having different composition may induce a beat frequency change to the same extent. Consequently, for analytical purposes, it is necessary to add a known amount of water to the sample being analyzed and take a second reading to determine rn hether the first reading was on the ascending or descending portion of the curve. In Figure 3 results obtained with a few other partially miscible alcohols are shown. Reproducibility of results was studied with the binary system ethyl alcohol-water. The observations reported were made a t three different concentrations: 1.1, 5.1, and 17.3Y0 of water by weight. Results obtiined are given in Table I. Six readings were taken for each measurement recorded and standardization against distilled mater was made every third reading. The composition versus beat frequency curve of the system ethyl alcohol-water approaches linearity in the range from 0.0 to 16.070 water by weight, so that if a straight line is assumed to be representative for the system in the mentioned range, the probable error of a single measurement in terms of percentage of water
EXPERIMENTAL
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3400
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3200
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W -I 0
F
0
3000
W W 2
a
I 0
Ll.
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a W
2800
2400 SECBUTYL ALCOHOL
0
ISOAMYL ALCOHOL
* N . H E X Y L ALCOHOL
21,000
2200
5
10
15
20
25
20,000
WATER, WT %
Figure 2. Binary S y s t e m s of Partially Water-Miscible Alcohols w i t h Water
lqooo 3400 W
UJ \
fn -I W
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's 18,000 f: 9 W
ci
a
3000
0 2-
0 W
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>. 0
z 3 W
2200
0 W
E *
' P
I800
X N.BUTYL
ALCOHOL
N.AMYL
ALCOHOL
0
TERT. AMYL ALCOHCX
Y
14,000
W 4:
ai
1400 5
IO
15
20
25
l3pOOL
' IO
WATER ,WT %
Figure 3. Binary S y s t e m s of Partially Water-Miscible Alcohols w i t h Water
'
'
20 30
'
40
'
50
'
60
'
m
80
WATER,WT X S y s t e m Ethyl Alcohol-Wgter I
Figure 4.
Measurements made with Sargent oscillometer
I
so
A N A L Y T I C A L CHEMISTRY
1252
by weight, as calculated from the data of Table I, is found t o be +0.22 for a sample containing 1.1% water.
Table 11. Precision of Measurements in System Ethyl Alcohol-Water (Oscillometer)
Oscillometer. The small sample cell provided by the manufacturer was used. A special brass color was built to fit around the cell in order to ensure its uniform position in the holder. This was found necessary to achieve optimum reproducibility of readings. The instrument was set t o zero position against air (no cell in the holder). The cell with the sample was then introduced in the holder and the meter pointer brought again t o zero position by throwing the range switches and turning the multiple-turn, fineadjustment dial. Six milliliters of solution were used in all measurements. The binary systems ethyl alcohol-water and isoamyl alcohol-water were studied.
s
99oc
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 13,571
14,596
5.70 7.42 1.66 5.00 1.12
5.50 6.04 1.35 4.07 0.91
0.05
0.04
c
a
c mean
Prob. error Prob. error mean Prob. error in terms yo water
-1
i3
NO.
Av. Av. dev.
u)
z"
Run
98oc
970C
,
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,
2
4
6
cj
,
W
8
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WATER, WT O k
Figure
'3t ;j 3600
5. System Isoamyl Alcohol- Wa ter
0
>
Measurements made with Sargent oscillometer
0
3200
(3
z Results are shown in Figures 4 and 5. The calibration curve for the system ethyl alcohol-water is practically linear in the range from 0.0 to 15.0% water by weight. The curve obtained for the system isoamyl alcohol-water exhibits a minimum. Both cases are thus comparable with the curves obtained in the study of the same systems with the heterodyne oscillator. Readings were taken a t 27.0' (room temperature). A study of the effect of temperature showed that the readings are affected by the variation of the temperature of the samples. A deviation
Q
g 2800 >-
0
z
2400 0
w
LL2200
1
m
NUMBER OF CARBON ATOMS
K
t w
1
2
3
4
5
6
7
Figure 6. Beat Frequency us. Number of Carbon Atoms Table I.
Precision of Measurements of System Ethyl Alcohol-Water
Run NO.
s
9 10 11 12 13 14 15 16 17 18 19 20
Av. Av. dev. d
c mean
Prob. error Prob. error mean Prob. error in terms % water
Per Cent Water h y Weight 5.1 17.3 Beat Frequency Change 3669 3800 4130 3655 3791 4126 3662 3780 4122 3663 3781 4122 ..~. 3663 3783 4123 4125 3785 3673 3781 4122 3676 4124 3786 3677 4126 3778 3657 4126 3797 3686 4127 3797 3659 3790 4130 3677 4128 3797 3687 3787 4125 3680 4128 3777 3662 3800 3677 4122 3682 3802 4133 3682 3795 4128 3678 3800 4123 3675 3800 4130 3672 3790 4126 1.1
8.60 9.39 2.10 6.33 1.42
7.35 8.31 1.87 5.60 1.26
2.06 3.30 0.71 2.18 0.48
0.22
0.19
0.09
of approximately 30 divisions per degree was found to occur in experiments performed with distilled water. A study of the reproducibility of results was carried out for two different concentrations of the system ethyl alcohol-water as shown in Table 11. Twelve readings were taken for each measurement reported and a check of the zero position was made every six readings. The largest average deviation observed in a group of twelve reading was =k5 divisions. The data obtained indicate that a single determination in a sample containing 1.0% water should yield results correct t o =k0.05 in terms of percentage of water by weight. LITERATURE CITED
(1) Monaghan, P. H., Moseley, P. B., Burkhalter, T. S., and Nance, 0. A., ANAL.CHEY., 24, 193 (1952). (2) Nanoe, 0. -4., Burkhalter, T. S., and Monaghan, P. H., ANAL. CHEY.,24, 214 (1952). (3) West, P. W., Burkhalter, T. S., and Broussard, L., Ibid., 22, 469
(1950). (4) West, P. W., Robiohaux, T., and Burkhalter, T. S., Ibid., 23, 1625 (1951). RECEIVED for review March 31, 1951. Accepted June 2, 1952.
