Conductance Method for Checking Accuracy of Water Analyses J. R. ROSSUJI California W a t e r Sercice C o m p a n y , Sun Jose, Calif. and accurate methods are available for determining all K .WID the major constituent,s forming ions in natural waters ( I !
4 ) except sodium. If sodium is determined, the accuracy of the analyses may be established by showing that the sum of concentrations of the cations is equal to that, of the anions. Considerable time can be saved by computing sodium by difference, but if this is done, there is need for a rapid, precise, niet,hod for vcrifying the total ion concentration. The elcct'rical conductivity method of Gustafson and Behrmari ( 2 ) appeared promising, but lacked the desired precision. This mclthod tacitly assumes that Iiohlrausch's law holds for finite concentrations, t'hat the specific conductance is a linear function of conccntration over small ranges, and that calcium, magnesium, and sodium have equal equivalent conductances. '1s the principal salts in natural waters aie strong electrolytes anti concentrations in the neighborhood of only 0.001 n' are involved, the first assumption may be expected to hold with reasonable accuracy. I t can be shown that less than ly0error is introduced by the second assumption over the concentration range 0.8 to 1.2 milliequivalent pcr liter by substituting empirical constants in the equation:
S,
x = -K= x C
o-b.\/c
where h is the equivalent conductance, & is the equivalent conductance at infinite dilution, K is the specific conductance, c is the concentration, and b is a constant. A perusal of a table of equivalent conductances a t infinite dilution indicates that considerable error may be introduced by the third assumption.
Table I. Constituent Chloride Sulfate Carbonate Bicarbonatc Sitrate Calcium AZagnesium Sodium
Factors
1Iicromhos per RIilliequivalent per Liter a t 2Z0 C . 75.9 73.9 84.6 43.6 71.0 52.0 46.6 49.6
1Iicromlios per P.P.11. a t 25O C. 2.14 1.54 2.82 0.713 1.15 2.60 3.82 2.16
Accordingly, a table of factors ivas devised, which, when multiplied by the concentration of the corresponding ion, gives the specific conductance contrihuted by that ion, provided that the total concentration is such as to have a specific conductance of approximately 100 micromhos at 25 C. As these factors are essentially the equivalent conductance at 0.001 thyy \\-ere computed in a manner analogous to that used for computing the equivalent conductance of the individual ions at infinite dilution. Gustafson and Berhman's data were used freely in these computations, along with other data found in testbooks on physical chemistry. Some of the factors have been slightly niodified as experience on actual analyscs has indicated. As finally arceptcd, these factors are shown in Tnhle I. ,-\A
pipetted, into a volumetric flask, made up to the mark with cool, boiled, distilled water having a known specific conductance of not over 2 micromhos at 25' C., and thoroughly mixed. The resistance is measured after carefullv adjuqting the temperature to exactlv 25" C. The diluted specific conductance is obtained from the follon ing formula:
where Kd is the diluted specific conductance in micromhos, A i3 the cell constant, D is the dilution factor, Rd is the measured resistance in ohms, and K , is the specific conductivity of the distilled w-ater in micromhoi. DISCUSSION
The procedure requires less than 15 minutes, including calculations. The author prefers to adjust the temperature to cxactll- 26" C. rather than to assume that the conductance increaws linearly with temperature and make the indicated correction. The cool, boiled, di5tilleti water used for dilution is conveniently kcpt in a 10-liter Pyres bottle equipped with a plug of Absorbite to protect it from carbon dioxide in the air. Standard work sheets for n-atcr analyses in this laboratory are arrzngcd to facilitate the necessar?. calculations and comparisons. It has been the author's practice to redetermine constituents when the specific conductance as computed from chemical analysis is more than 1.5% greater or 2% less than the diluted specific conductance. Of a typical group of 100 consecutive analyses 9 2 5 , fell within this range. The average deviation of the computed from the measured specific conductance n-as 0.78%. Less importance is attached to negative deviations, because the presence of minor constituents ivould increase the measured conductance. Inasmuch as sodium is determined by difference, small amounts of potassium or ammonium, reported as sodium, would increase the measured conductance because these ions have a much higher equivalent conductance than sodium. The method is not valid for waters having a specific conductance of less than DO micromhos, but the author has used it successfully on brines with a specific conductance as high as 26,000 micromhos. I t is not applicable to waters having unusually high or low pH values, yince the equivalent conductance of both hydrogen and hydroxyl ions is comparatively high. Essentially all the author's analyses are of water having a pH of between 7 and 9, and he has made no investigation of the results that can be obtained outside this range. Thc niethod provides an accurate check on the negative ions, but when sodium is estimated by difference, comparat,ively large errors in the positive ions \ d l cause on!y small deviations in a computed specific conductance. This procedure has been used for 2 years on over 800 water samples that represent a variety of both ground and surface n-ater supplies throughout the state of California. iCKNOW LEDGMENT
The assistance of Primo .\. T-illarruz in making man\- of the analyse.. is gratefully acknowledged. LITERATURE CITED
Apparatus. A Lreds & Sortlirup alternating current resistance bridge is used in conjunction with a dip type conductance instrument capable of measurcell having a constant of 0.1. ing specific conductance over the range of from 50 to S0,OOO micromhos with an error not exceeding 1 % may be used. Procedure. The conductance of the water sample is measured in the usual manner, and from this value, a dilution is chosen such that the specific conductance of the diluted sample will be betvieen (30 and 120 niicrunlhos. The proper volume of sample is then
(1) Am. Public Health Assoc., "Standard Methods for Examination
of Water and Sewage," 9th ed., 1946. ( 2 ) Gustafson, H., and Behrnian, A. S., ISD.ESG.C H E x r . , AKAL.ED.. -_ 11, 6 2 s (1939). (3) NOH, C. A . . I b i d . , 17, 426 (1945). (4) Sheen, R. T., Kahler, 11. L., and Ross, E. M.,I b i d . , 7, 262 (1935). -
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R E C E I V E February D 9, 1948.
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