Determination of Water in Soils by an Indirect Conductivity Method C. KINNEY HANCOCK and CHARLES M. HUDGINS, JR. Texas Engineering Experiment Station, Texas A. & M . College System, College Station, Tex. Simple, rapid methods are needed for the determination of water in materials that may contain electrolytes. Using methyl and ethyl alcohols, studies of alcohol-acetone-water-sodium chloride systems show that the conductivity is nearly linear with the water content over the range of 0 to IO70 water. With ethyl alcohol, linearity can be approached more nearly by increasing the acetone concentration, but a parallel decrease in sensitivity occurs. The optimum acetone concentration is about 30%. The procedure requires about 10 minutes and may be useful for the determination of water in aqueous alcohols containing electrolytes as the use of a large excess of sodium chloride tends to mask the effect of other electrolytes. Encouraging results have been obtained in the determination of water in soils.
UUEROUS direct electrical methods have been proposed for the determination of moirture in soils and other materials, but they are subject to the error introduced by the presence of electrolyte. As a result, a study \vas directed toward indirect electrical methods which would mask the effect of contaminating electrolyte. According to Komarov's method (5)for determining moisture in gases, the moisture is absorbed in an organic solvent, an electrolyte-e.g., oxalic acid-is added, and the water is determined by measuring the electiical conductivity of the resulting solution. Venkatanarasinlhachar ( 7 ) found that the specific conductivity of aqueous ethyl alcohol saturated with sodium chloride varied linearly with the moisture content over the range of 0 to 6.77y0water; however, this method is tedious, requiring several hours per determination. Boeke ( 1 ) has described a method that is applicable to the determination of water in liquids or solids. The water is extracted from the sample with a solution of 10% oxalic acid in acetone and the increase in electrical conductivity is a function of the amount of water taken up. This method is also time-consuming, requiring about 2 hours per determination, I n a re-examination of Lannung's ( 5 ) conductivity data for water-acetone-salt solutions, Mysels (6) has concluded that the conductivity of a saturated solution of a salt such as cesium fluoride in aqueous acetone could be used to determine small amounts of water in acetone. Recently, Burton ( 2 ) has reported a high frequency (9.45 megacycles) electrical method, using external electrodes, for determination of moisture in sodium chloride and ammonium nitrate, in which the moist salt is stirred for 5 minutes with an appropriate 1,4-dioxane-methanol mixture and the current through the extract is measured. A good linear relation between current (microamperes) and moisture content was found for the range of 0 to 2.1% moisture. Based mainly on Venkatanarasimhachar's ( 7 ) conductivity method and Burton's ( 2 ) idea of using a mixed organic solvent the study reported here was concentrated on a method involving extraction of the sample with a mixed organic solvent, in the presence of an excess of sodium chloride to mask the effect of contaminating electrolyte, and then measurement of the conductivity of the sodium chloride-saturated extract. APPARATUS
I n most of the studies using ethyl alcohol, a portable Model RC-1 conductivity bridgc (Industrial Instruments, Inc., Cedar Grove, N. J.) was used. This bridge operates on 115-volt, 60cycle alternating current circuits. The Wheatstone bridge is
supplied with 60-cycle alternating current,, balance being indicated by a cathode ray tube null indicator. I n some of the later studies Kith methanol, a portable, batteryoperated Node1 RC-12C-1 conductivity bridge (Industrial Instruments, Inc.) was used. The circuit diagram for this instrument is similar to that for t,he Model RC-1 bridge, the main difference being that the Wheat,stone bridge operat.es on 1000cycle alternating current from a vacuum tube oscillator. A& glass, dip-type conduct'ivity cell with cell constant of 0.858 cni.-l, determined a t 35.0" C. x i t h 0.01S potassium chloride ( d ) , Tvas used. Studies were made a t 35.0" 2= 0.1' C. by using a llodel C-2 constant t.emperature xater bath (Precision Scientific Co., Chicago, Ill.). Constant temperature was maintained by using a Merc-to-Merc adjustable thermoregulator (Precision Scientific ('o.), which was adjust.ed to 35.0" C. by using an Xnschiitz thermometer previously calibrated by the Sat,ional Bureau of Standards. six-bladed impeller (20" pitch to the blades) \vas made from a brass disk (0.2 mm. thick, 29 mm. in diameter) and was screwed to the lower end of a brass shaft (20 cm. long, 5 mm. in diameter), so that the leading edges of t'he impeller blades were tilted downward. This stirring impeller was driven by a Sargent cone drive motor. Using a light aluminum gear train n-ith an over-all gear reduction ratio of 20 to 1, the stirring speed in revolutions per minute xas determined accurately by counting the revolutions per minute of the slowest gear and multiplying by 20. The st.irrer shaft ext'ended through a cork st,opper Tvith 4.5cni. smaller diameter by means of a bearing made from glass tubing ( 5 mm. in inside diameter). The cork st,opper fitted the mouth of a 180-ml. electrolytic beaker (inside dimensions: upper diameter, 4.5 cm. j height,, 11 cm.), and was equipped with a 6-mm. glass rod and a short thermometer (10' to 40" C.), which n-ere located on opposite sides of the stirrer shaft, close to the lvalls of the beaker, and extended nearly down to the impeller. The rod and thermometer served as baffles to reduce whirlpooling and to increase turbulence. a &
MATERIALS
The absolute ethyl alcohol was U. S. P. grade (U. S. Industrial Chemicals Co., New Orleans, La.). The acetone, absolute methanol, and sodium chloride all met XCS specifications (J. T. Baker Chemical Co., Phillipsburg, N. J.). I n preparing aqueous alcohol or aqueous alcohol-acetone solutions, the water contents of the solvents were assumed to be: ethyl alcohol, 0.2%; acetone, 0.2%; methanol, 0.1%. These percentages of water are in close agreement with the specifications and are consistent with the experimental results. PROCEDURE
The fundamental basis for the use of alcohol-acetone-watersodium chloride systems to determine water in various ma,terials is that the conductivity of the system will be proportional to t h e sodium chloride concentration, which, in turn, will be proportional to the water concentration. The general procedure was to weigh into a 180-ml. electrolytic beaker enough aqueous alcohol and acetone t,o make a total of 100.0 grams of ternary mixture of the desired compositionfor example, mixtures of 25.0 grams of acetone and 75.0 grams. of aqueous ethyl alcohol svere used to obtain the upper curve of the top half of Figure 1. The abscissa of a point on the curve represents the percentage of water in the aqueous ethyl alcohol.. The solvents were mixed thoroughly, brought to 35.0' 0.1" C. in a constant temperature water bath, and then stirred with ovendried (105' C.) sodium chloride. The excess of sodium chloride settled in a, short time. The stirring apparatus was then replaced with a glass dip-type conductivity cell, which was equipped with a cork stopper to fit the mouth of the beaker. The cell m-as lowered into the liquid until all of the air bubbles were just displaced through the upper ports, and resistance readings were made in triplicate. The cell was raised slightly, then readjusted to the original position, and a fourth resistance reading was made. -4s the deviation among these four readings was never significant, the average was used.
*
V O L U M E 2 6 , N O . 11, N O V E M B E R 1 9 5 4 Table I. Sodium Chloride,
Effect of Stirring Conditions
Sodium Chloride, Sieve Size Passing 40 Passing 40 Passing 40 Passing 40 Passing 100 Passing 100 Passing 60, retained on 100
G.
3.00 3.00 3.00 10.00 3.00 10,oo 10.00
Stirring Time, Rlin.
Stirring Speed, R.P.M.
5,oo
900 600 600 600 600 900 9 00
5.00 15.00 5.00 5.00
15.00 18.00
Resistance, Ohms 94.0,94.0 94.2,94.2 94.0,94.0 92.2,92.4 92.7,92.9 91.5,91.8 Y2.4,92.8
I
15
-
35/ 40
05
I
I
1739 No. 40 sieve) for 5 minutes a t 600 r.p.m. In Figure 1 the percentage of water in the ethyl alcohol is plotted against the conductivity of the ethyl alcohol-water-acetone-sodium chloride system. The number on each curve denotes the percentage of acetone in the acetone-aqueous ethyl alcohol mixture. These results indicate that an optimum between linearity and sensitivity is approached by using a mixture of 70% of aqueous ethyl alcohol and 30% of acet'one. Conductivity of Methanol-Water-Acetone-Sodium Chloride Systems. Using the Model RC-12C-1 meter and mixtures of aqueous methanol and acetone, conductivities were determined after stirring with 10.00 grams of sodium chloride (passing KO. 60 sieve and retained on No. 100 sieve) for 10 minutes a t 900 r.p.m. I n Figure 2, the percentage of water in the methanol is plotted against the conductivity of the methanol-wateracetone-sodium chloride system. The number on each curve denotes the percentage of acetone in the acetone-aqueous methanol mixture. These results indicate that the addition of acetone decreases the sensitivity without significant improvement of linearity. -4percentage of acetone in the range of 0 to 30% might produce the desired opt,imum between sensitivity and linearity. 150
I
% ACETONE
/
1.5
0
1.0 CI
-0 x
' 0.5
O
LA-- -
I
I
2 4 6 0 % WATER IN E T H Y L ALCOHOL
I IO
Figure 1. Conductivity of Ethy 1.4lcohol-AcetoneWaterSodium Chloride Systems
The variable factors in this procedure were: stirring time, stirring speed, weight of sodium chloride, and particle size of sodium chloride Arbitrary values of these variables are indicated below.
3.0
RESULTS
Figure 2.
Effect of Stirring Conditions. Using the Model RC-1 conductivity meter, a series of duplicate determinations was made on 100.0-gram samples of methanol (0.1yo water). Various arbitrary values of the four variables mentioned in the general procedure nere taken. The conditions and results are shown in Table I. Inspection of Table I indicates that satisfactorily reproducible results may be ohtained under a variety of conditions, provided those conditions are well defined. Resistance readings decrease with increase of stirring time or speed, but little is to be gained by exceeding 5 or 10 minutes of stirring a t 600 r.p.m. The resistance readings decrease with decreasing particle size of the sodium chloride, hut, at the same time, there is an objectionable decrease in reproducibility. The rate of settling of the sodium chloride after stirring decreases with decreasing particle size and this prolongs the over-all procedure. It is indicated that 3 to 5 grams of sodium chloride is sufficient for the purpose. Conductivity of Ethyl Alcohol-Water-Acetone-Sodium Chloride Systems. Using the Model RC-1 meter and mixtures of aqueous ethyl alcohol and acetone, conductivities were determined after stirring with 3.00 grams of sodium chloride (passing
1
0
I
I
I
2
I
4
6
8
%
WATER IN
METHYL
I
10
ALCOHOL
Conductivity of Methanol-Acetone-WaterSodium Chloride Systems
Effect of Bridge Frequency. Even though no completely comparable results are available, the results of two series of experiments carried out on mixtures of iO.0 grams of aqueous methanol and 30.0 grams of acetone will serve to show the effect of bridge frequency. One series of measurements was made a t 60 cycles using the Model RC-1 meter after stirring with 3.00 grams of 40-mesh sodium chloride for 5 minutes a t 600 r.p.m.; the other was made a t 1000 cycles using the Model RC-12C-1 meter after stirring with 10.00 grams of sodium chloride (passing S o . 60 sieve and retained on No. 100 sieve) for 10 minutes a t 900 r.p.m. These data are shown in Table 11. Considering the differences in stirring conditions and the results shown in Table I, the values in the third column of Table I1 should be expected to be lower than corresponding values in the second column. As a result, it is indicated that no serious error was introduced by using a 60-cycle bridge. Determination of Water in Air-Dried Soils. A 20.00-gram sample of 40-mesh, air-dried soil was weighed into a 180-ml. electrolytic beaker and 100.0 grams of a 70.0% absolute ethyl
ANALYTICAL CHEMISTRY
1740
Table 11.
Effect of Bridge Frequency Resistance, Ohms
Water in ,Methanol,
a
%
60 cy c1es a
2.2 4.2 6.2 8.2 10.2
129.0 123.3 114.6 108.2 102.9
1000 cycles b 129.0 121.4 113.0 107.6 101.9
3.00 grams of 40-mesh SaC1, 5 minutes a t 600 r.p.m.
b 10.00 grams of 60- t o 100-mesh NaCl, 10 minutes a t 900 r . p . m .
ions, the pH of the soil may contribute to some of the discrepancies noted. Another source of difficulty in this study was the slow settling of some soil suspensions after stirring. When the conductivity was determined after short standing, some particles were still in suspension between the electrodes and some settled out on the upper surfaces of the horizontal electrodes during the measurement of resistance. The settling out of particles on the electrodes could be eliminated or greatly reduced by using a different type of conductivity cell having vertical electrodes. DISCUSSION
The majority of the studies described in this article were carried out during the summer of 1953. The operating temperature of 35.0' C. was chosen because it was about the lowest temperature that could be maintained \\-ithout refrigeration. A41thoughBurton ( 3 ) obtained good results with methanol1,i-dioxane mixtures as the water-extracting solvent, alcoholacetone mixtures were used in this study because acetone of high purity is more readily available and more economical than I,+ dioxane. I n the application of the results of this study to the determination of water in other materials, it is believed that small amounts of water in the water-extracting solvents present no difficulty, provided that the percentage is constant. Calibration with samples of known water content will correct for the initial presence of Lyater in the extracting solvents. %WATER
IN
SOIL
Figure 3. Conductivity of Ethyl AlcoholAcetone Water Soil Sodium Chloride Systems
-
-
-
alcohol-30.070 acetone mixture were added. The mixture was stirred with 3.00 grams of 40-mesh sodium chloride for 5 minutes a t 600 r.p.m. and then the conductivity was determined with the Model RC-1 meter. A 5-gram sample of air-dried soil was dried to constant weight a t 105" C. in an electric oven and the percentage of moisture was calculated from the lose in weight and the weight of the air-dried sample. The conductivities and percentages of moisture, as determined, are plotted in Figure 3. The majority of the points lie fairly close to a straight line, but some of them depart considerably from the line. Statistical treatment of the data yields the following values: correlation coefficient. 0.983; standard error of the correlation coefficient, 0.045. It is suspected that, because of the high conductances and mobilities of the hydrogen and hydroxide
ACKNOWLEDGMENT
The authors are grateful to John R. Eccles and Ralph E. Zerwekh, Jr., for aid in conducting the experimental wcrk which was performed in the laboratories of the Department of Chemistry, A. and M. College of Texas. LITERATU-RE CITED
(1) Boeke, J., Philips Tech. Rea., 9, KO.1, 1 3 (1947). (2) Burton, 31. B., Jr., A'1.S. thesis, A . & AI. College of Texas,
January 1953. (3) K o n i a r o v , V. A , Russ. Patent 51,904 (Oct. 31, 1 9 3 7 ) . (41 L a n g e , S . A, "Handbook of Chemistry," 7th ed., p. 1416. Sandnskv. Ohio. Handbook Publishers. Inc.. 1949. .. (5) L a n n u n g , 2. p h y s i k . Chem., A161, 2 6 9 ( 1 9 3 i ) . (6) Nysels, K. J., J . Phys. and Colloid Chem., 51, 708 (1947). (7) Venkatanarasimhachar, S . , Proc. Indian Acad. Sci., 16A, 332 (1942). ~~~~
~
A:,
RECEIVED for review February G , 1954. Accepted September 1, 1954. Presented before the Analytical Chemistry Section a t the $MERICAN CHEMICAL SOCIETYRegional Conclave, New Orleans, La., December 10, 19.53.
Photometric Titrations ROBERT F. GODDU' and DAVID N. HUME Laboratory for Nuclear Science and Department o f Chemistry, Massachusetts
The principles of the photometric titration method are discussed and its advantages and disadvantages evaluated. Apparatus for the performance of photometric titration is described, and some of the fundamental factors limiting the accuracy are considered. Previous work in the field is reviewed.
0
F T H E various readily applied methods for the physico-
chemical determinations of titration equivalence point, that of quantitative measurement of monochromatic-light absorption has been one of the least exploited. Photometric titration has a number of attractive features which, if better known, would make it the method of choice in many applications.
institute o f
Technology, Cambridge'39,-Mass,
The idea of using monochromatic-light absorption measurements to determine titration end points is by no means new. Tingle (92), as far back as 1918, used a pocket spectroscope to isolate the color desired to detect end points visually. The advent of photoelectric colorimeters and spectrophotometers greatly increased the potential scope of the method, and many workers have pointed out the possibility of doing photometric titrations, although relatively few have actually investigated them. Muller and Partridge (66) deserve the credit for the pioneer work in the United States, developing their own apparatus and making several useful applications. The literature, up to the end of 1953, is summarized at the end of this paper. 1
Present address, Hercules Powder Co., Wilmington 99,Del.
