detectability of selenium ton-ard one p.p .m.
Coleman, R. G., Delevaux, Maryse, Econ. Geol. 52, 499-527 (1957). Feldman, C., J . Opt. SOC.Am. 35, 180-4 (1945). (4) Harrison, G. R., Lord, R. C., Loofbourow, J. R., “Practical SpectrosCOUY.” Prentice-Hall. Kex York. 193s. Helz, A. W.,Scribner, B. F., J . Research S a t l . Bur. Standards 38, 439-47 (1947). Rockenbauer, W.,Schroll, E., bsterr. Akad. Wiss., Math.-naturw. Kl., - 4 ~92, . 192-6 (1955). Waring, C . L., Worthing, H. W., Am. Mineralogist 38, 827-33 (1953).
Table II. Sample Weights for Different Selenium Percentages
Mg.
ACKNOWLEDGMENT
of Sample
The authors appreciate the help of their associates of the U. S. Geological Survey, especially Irving May and M. H. Delevaux for the chemical analyses, and R. G. Coleman for providing most of the samples. This study is part of a program being conducted by the U. S. Geological Survey on behalf of the Division of Ran- Materials of the U. S. Atomic Energy Commission.
10 20
40 80
Range % Selenium 1 -10 0 . 1 -1
0.01 -0.1 0.001-0.01
LITERATURE CITED
(1) Borovik, S. A, Doklady Akad. Nauk S . S . S . R .65,315 (1949).
RECEIVEDfor review October 9, 1957. Accepted May 5, 1958.
Poten tiometric Measurement of pCI Application to the Determination of Chloride in Sweat, Urine, and Miscellaneous Solutions MILTON STERN and HARRY SHWACHMAN Division o f laboratories and Research, Children’s Medical Center and Department of Pediatrics, Harvard Medical School, Boston, Mass. TRUMAN S. LlCHT and ANDRE J. deBETHUNE Department of Chemistry, Boston College, Chestnut Hill, Moss.
b A rapid method is described for the potentiometric determination of chloride ion expressed as pCI. A silversilver chloride indicator electrode and a saturated calomel reference electrode are used with conventional p H measuring equipment and technique. With the Beckman Model G p H meter and commercially available electrodes, the pCI of as little as 1 ml. may be determined a t room temperature within 1 minute, with no prior physical measurement or chemical treatment of the sample. In pure solutions, the pCI may be determined with an accuracy of 0.02 to 0.03 pCI unit over a pCI range of 0 to 4 (a concentration range of 1.0 to 10-4M). A variation of 0.01 pCI corresponds to a 2.370 change in the chloride concentration. The method has been applied to the analysis of chloride in sweat, collected as a diagnostic test for cystic fibrosis of the pancreas, in urine, and in miscellaneous solutions, where chloride i s the predominant anion. A discussion of the limitations of this method i s presented.
T
HE most reliable test for the diagnosis of cystic fibrosis of the pancreas is the determination of the elevated sodium and chloride content of thermally induced body sweat (6,
1506
ANALYTICAL CHEMISTRY
9, 68, 29). Because the sodium and chloride concentration in sweat are approximately the same, the measurement of chloride will suffice. The search for a rapid, simple, inexpensive method to determine sweat chloride prompted this investigation. The potentiometric determination of chloride has been studied with a concentration cell employing two silver-silver chloride electrodes ( S , I I , 24). A simpler and more direct potentiometric measurement of chloride has been the object of recent investigations (5, 14, 16). Chanin (b) used a cell with a silver-silver chloride indicator electrode and a mercury-mercurous sulfate reference electrode. He reported such limitations as an analytical range of 0.1 to 1.0 meq. per liter, the need for buffering the unknown to eliminate drifting, and a daily change in the voltage developed by known chloride concentrations. Helmkamp et al. (14) analyzed chlorinated insecticide residues with a silver-silver chloride indicator electrode and a saturated calomel reference electrode a t 40” C. with a prototype Beckman Nodel GS pH meter. I n the present method, a silver-silrer chloride indicator electrode is substituted for the glass electrode of a common laboratory p H meter. This provides for the rapid potentiometric meas-
urement of chloride a t room :temperature. This technique measures pC1 of a solution b y a method analogous to that used in measuring pH. pC1 is readily converted to chloride ion concentration. EXPERIMENTAL
Reagents and Apparatus. Potassium silver cyanide solution. Dissolve 4.0 grams of potassium cyanide in 25 ml. of distilled water. Slowly add 10% silver nitrate solution until a slight precipitate persists on mixing. Make u p t o 100 ml. with distilled water. This solution is stable indefinitely. Potassium chloride solutions, 0.1000N. 0.01O O X . Sodium chloride solutions in concentrations from 0.001N to 1.0X: see Table I. Beckman Model G p H meter. Two Beckman (KO. 281) datinuni button electrodes. Beckman (No. 270) reference saturated calomel electrode (S.C.E.). Two (Eveready No. 6) 1.5-volt dry cells. PREPARATION OF ELECTRODE. Soak a platinum electrode (Beckman S o . 281) in 1 to 1 nitric acid for 5 minutes, then rinse with distilled water. Connect the two dry cells in series, attach another platinum electrode to positive terminal, and the Beckman No. 281 electrode to negative terminal. Place both elec,
A
trodes in potassium silver cyanide solution and plate for 4 minutes. Observe the formation of a grayish white silver deposit on the negative electrode, and gas bubbles at the positive electrode. Rinse both with distilled water. Reverse polarity, immerse in 0.100N potassium chloride solution, plate for 20 seconds, reverse polarity again, and plate for 5 seconds; repeat cycle twice. The silver chloride coating should be light purple. Rinse the anodized silver chloride electrode with distilled water and store in 0.100A7 potassium chloride. MEASURENEXT OF PCL. Replace the glass electrode in the p H meter with a silver-silver chloride electrode. Retain the saturated calomel electrode as a reference electrode. Use the range 8 to 4 of the p H scale for reading pC1. Let 8 correspond to pC10; then 7 corresponds to pC1 1; 6 to pC1 2 , etc. The general relationship adopted is pC1 = 8 minus the scale reading. Standardize the meter at room temperature n ith O.lO0N potassium chloride or sodium chloride. Rinse the electrodes and cup, and adjust the temperature compensator. Balance the instrument with the zero adjustor to read pC1 1.11 (scale reading 6.89). Confirm standardization by measuring a 0.01OOX potassium chloride or sodium chloride solution which should read pC1 2.04 (scale reading 5.96). or bv measuring- the e.m.f. ?Table I). T o determine pC1 of unknowns, rinse the electrodes and CUD three times with distilled water, and once with the solution to be measured. As little as 1 nil. of solution can be used. Immerse the electrodes and measure the $1. Analyze cerebrospinal fluid as obtained, but acidify urine with concentrated sulfuric acid, 1 drop to 2 ml. The pC1 may be converted to chloride ion concentration by use of a curve (Figure 1). T o minimize the reading error, any portion of this curve can be enlarged from the values given in Tables I, 11,and 111. COLLECTIOKOF SWEAT SAMPLE. Collect sweat samples by the method of Shwachman et a[. (21, 28, 29) using a gauze pad of k n o m weight for the collection, and a plastic bag to enhance sweating. Keigh the pad after the collection and dilute the sweat collected with 20 ml. of distilled water (21). This diluted sample is analyzed for chloride by both the pC1 method and the mercurimetric titration method of Schales and Schales ( 2 7 ) . RESULTS
KNOWN SOLUTIOKS O F
S O D I U M CHLOObserred pC1 and e.m.f. data are presented in Table I for known solutions of sodium chloride in the range of 1.0 to 10-4N at 27' f 2" C. Values represent averages of 10 to 20 measurements. The pC1 was observed, using this procedure, with the scale adjusted to a theoretical pC1 of 1.11 using 0.1M potassium chloride. The e.m.f., E, was measured on the millivolt scale of the Beckman Model G RIDE.
Table 1. pCI and E.M.F. Values for NaCl Solutions from Cell: S.C.E., NaCI, ASCI, Ag, a t 25' C.
Concn. NaCl, Meq./L. 1000 500 200
_ E.M.F., ___ Mv. _ PC1 Obsd. Theor. Obsd. Theor. 0.22 0.17 -2 -13 0.48 0.46 t 6 4 0.84 0.83 27 26 100 1.11 1.11 43 43 50 1.41 1.39 60 59 1 . 7 8 1.76 82 81 20 10 2.07 2.04 98 98 5 2.34 2.33 114 115 2 2 . 7 3 2.72 137 138 1 3.02 3.01 153 155 0 . 5 3.29 3 . 3 1 173 173 0 . 2 3 . 6 8 3.71 196 196 0 . 1 3.96 4.00 214 214
Table II.
Concn., Mole/L.
+
Theoretical pCI of Sodium and Potassium Chloride Solutions
Density, G./J41.
1
1.0385
0.5 0.2 0.1 0.05 0.02 0.01 0.005 0.002 0.001 0.0005 0.0002 0.0001
1.0183
1.0063 1.0022 1.0002 0,9990 0.9986 0.9984 0.9983 0.9982 0.9982 0.9982 0.9982
Molality, Mean Ion Activity Moles/Kg. H20 Coeffic,ient Sodium Chloride 0.66 1.0204 0.68 0,5055 0 .730 0.2011 0.780 0.1004 0.823 0.05014 0.875 0.02004 0.904 0.01002 0.929 0.005010 0.953 0,002004 0.966 0.001002 0.979 0.0005009 0 . 984a 0.0002004 0.989a 0.0001002
Potassium Chloride 1.0029 0.1005 0.01002 0.9987 0.01 Calculated from Debye-Huckel limiting equation, 0.1
a
p H meter, or on a Leeds & Xorthrup student potentiometer, and required no adjustment. Theoretical pC1 and E values were calculated from Equations 3 and 1, respectively (see Discussion and Table 11). Except for the first and last points, the deviation between observed and theoretical values does not exceed 0.03 pC1 unit ( 2 mv.). The average of the magnitude of the deviations is just under 0.02 pC1 unit (1 mv.). The excellent agreement between the two e.m.f. columns indicates the validity of the Eo constant used in Equation 1. The agreement of the tmo pC1 columns validates the use' of the curve of pC1 us. chloride ion concentration plotted (Figure 1) from the theoretical points calculated in Tables I1 and 111.
Table 111. Theoretical pCI of Dilute Solutions Corrected for Silver Chloride Solub iI it y [K(4gC1) = 1.62 X I)%(
Total Concn. of Chloride Chloride Including Concn. That Being Due to Measured, Solubility 10-6 of AgC1, Mole/L. 10-8 hlole/L. 100 101.6 50 53.1 20 26.2 10 18.7 5 15.5 2 13.8 1 13.2 0 12.7 a With activity coefficient of pC1 value becomes 3.998.
pC1 3.9945 4.275 4.582 4.729 4.811 4.862 4.879 4.896 0.989, this
0.769 0,901
PC1 0.172 0.464 0.833 1,106 1.384 1.756 2.043 2.332 2.719 3.014 3.311 3.705 4.004 1.112 2.044
Below 10-"111 chloride, deviations arise from the chloride contribution of silver chloride in solution. These deviations can be calculated from the known solubility product of silver chloride. and the sensitivity of the pC1 method can be extended theoretically to a low^ limit of about 2 X 10-6M chloridci. or about one sixth of the solubility of silver chloride in water (see discussion and Table 111). SWEATSBJIPLES. The chloride content of sweat samples collected from 9 children with cystic fibrosis of the pancreas (C.F.) and 14 controls was determined by both the pC1 and titration methods (Table IV). Observed pC1 \vas converted t o chloride ion concentration from the theoretical curve. and then multiplied by the dilution factor (21) to give the sweat chloride listed in the third column. The results are rounded off to three significant figures. For the 23 samples analyzed, VOL. 30, NO. 9, SEPTEMBER 1958
1507
Determinations of Chloride in Sweat
Table IV.
(Comparison of chloride concentration from pC1 and from mercurimetric titration methods 1 From Observed pC1 Titration, Subject Obsd. Dilution C1 concn., C1 Concn., % No. PC1 factor meq./l. Meq./L. Difference 1567 1568 1571
1.60 1.78 1.73 1.65 2.25 2.60 1.68 2.46 1.78
1591 1597
3.53 3.34 3.35 2.18 2.53 3.48 2.33 3.60 3.52 3.17 3.25 2.80 3.23 2.56
1561 1566 1573 1577
1588
1595
Table V.
Patients with Cystic Fibrosis 4.50 133 4.40 84 6.37 136 4.77 122 20.6 125 38 3 103 156 6 44 33 0 123 4 36 79 Control Subjects 61.9 18.7 12.0 25 5 83 7 38.5 4 27 31 .O 49.4 15.7 16.3 48.0 49.9 9.94 17.8 69.8 9.6 31 .O 12.6 18.0 9.1 15.4 16.2 9.70 20.8 34.4 20.2 6.97 ~~
Chloride Determinations in Urine
(Comparison of pC1 and titration method for 12 samples) From Observed $1 TitraSubC1 tion C1 Differject Obsd. concn., Concn., ence, No. pC1 meq./l. Meq./L. % -4.9 1 0.89 175 184.3 -5.2 2 1.02 127 134.0 -4.8 3 0.90 170 178.6 -1.2 4 0.94 154 155.8 5 0.97 144 160.6 -10.3 -8.0 6 0.82 209 222.3 +5.2 7 0.88 179 170.1 8 0.84 200 201.4 -0.7 +0.6 9 0.89 175 173.9 17.3 10 0.82 209 194.8 11 0.80 218 206.2 +5.7 12 1.03 123 135.9 -9.5
PC1 (Sakonnet 0.47 4.0
Redistilled water
4.95
Standard pH solution of pH = 2.08
1.10
&
Lake water (Crystal 3.47 Lake, R’ewton, Mass.) Cerebrospinal fluid 1.03
1508
0
18.2 11.3 37.7 29.3 42.6 15.3 49 2 17.5 9.3 11.5 9.0 18.1 25.3 18.1
19.0 11.3 -0.7
+0.8 1-5.9 t8.6 +6.5 13.6 -3.8 4-2.7 16.2 T2.1 +5.8 116.0 +6.5 +1.4 tl.7 +3.2 19.6 11.0 -10.5 -17.8 111.6
an average difference of 6.0yc was obtained between the two methods. URINESAMPLES. Table V shows the results of chloride analyses on freshly collected, acidified urines from healthy individuals. Thirty-five samples were analyzed and 12 typical results are presented, The average agreement for the entire group between the two methods of chloride analysis is 5.6%. MISCELLANEOUS SAMPLES.The pcl’s of a miscellaneous group of chloride solutions in the range l O + X t o 0.50M are presented in Table VI. This group includes distilled, tap, and sea water, and a chloride solution of known acidity. A comparison n-ith independent data is given for each solution. DISCUSSION
Theoretical pC1 Values. The inter-
pCI of Miscellaneous Solutions
Table VI.
Sea water Bay, R. I.) Tap water (Boston)
122 83 137 121 118 95 147 117 82
ANALYTICAL CHEMISTRY
Chloride from pC1, Meq./L. 490
Chloride Known, Meq./L. 480
Source of Known Value Mer curimet ric tit r ation (27) 0.10 0.09 U. S. Geological Survey (30) and F. (3.5 p.p.m.)(3.2p.p.m.) O’Halloran ( 2 5 ) 0.05
