692
INDUSTRIAL AND ENGINEERING CHEMISTRY
developed from antimony trichloride. Small errors in the determination of carotene have comparatively little effect on the accuracy of the vitamin A measurement. ACKNOWLEDGMENT
The authors wish to thank James K. Brody and Harold H. Williams of the Research Laboratory of the Children's Fund of Rlichigan for their courtesy in discussing the unpublished procedure which that laboratory is using for the determination of vitamin A and carotene in human foods. Up to that time the authors had been unsuccessful in obtaining a suitable adsorbent. The method herein described was developed later and differs from the procedure developed in the Research Laboratory of the Children's Fund in the adsorbent used as uell as in other respects.
Vol. 17, No. 11
LITERATURE CITED
(1) dssoc. Official Agr, Chem., Official and Tentative Methods of Analysis, 5th ed., p. 369 (1940). (2) Baird, F. D., Ringrose, A. T., and MacMillan, M . J., Poultry Sci., 18,441 (1939). (3) Fraps, G. S., and Kemmerer, A. R., Texas Agr. Expt. Sta., Bult. 557 (1937). (4) Kernchan, G., Science, 90,623 (1939). (5) Koehn, C. J., and Sherman, W. C., J. B i d . Chem., 132, 527 (1944). (6) Little, R. W., IKD. ESG.CHEM.,ANAL.ED.,16, 288 (1944). (7) Oser, B. L., Melnick, D., and Pader, M., Ibid., 15,724 (1943). (8) Strain, H. H., "Chromatographic Adsorption Analysis", p. 25, New York, Interscience Publishers, 1943. (9) Ibid., p. 57. (10) Wall, M. E., Lnd Kelley, E. C., IND.ENG.CHEM.,ANAL. ED., 13, 18 (1943).
Improvements in the Potentiometric Titration of Chlorides ROBERT P. YECK
AND
G.
H. KlSSlN
Amzrican Smelting & Refining Company, Central Research Laboratory, Barber,
A simplified electrode system without a salt bridge is described. The assembly consists of a metallic silver electrode and a reference electrode reversible to chloride ions, mounted with a stirrer in a compact unit. The method makes possible titration of chloride ion, with silver nitrate solutions, to an abrupt potential swing at the equivalence point, detected with an electronic voltmeter, resulting in a simplicity and convenience comparable to conventional redox eledrometric titrations. The use of an electronic voltmeter for the detection of the end point eliminates errors of polarization and makes possible the routine potentiometric titration of chloride in solutions containing heavy metal ions.
I Electrolyte -Ig1 Sample
N. J.
KK03(l K) AgCl(sat.)
1 AgCl 1
Ag
At the beginning of the titration when the chloride ions are at high concentration in the sample they are relatively low at the silver chloride electrode. Additions of silver nitrate decrease the chloride-ion concentration in the sample, causing a corresponding change in the potential of the silver chloride indicator electrode.
THE
potentiometric method for chloride determination has considerable advantage over titrations involving chemical indicators and the gravimetric silver chloride method. The recent paper by Yao (@, proposing the titration of chlorides with silver nitrate by a series of potential measurements, and finally, to a definite potential, is not adaptable to routine work; the equipment required and the time and care necessary in making a series of readings make routine use impractical. The titration of chlorides by use of the polarized electrode systems described by Foulk and Bawden (S), Willard and Fenwick ( 7 ) ,and Clippenger and Foulk ( I ) , while capable of giving excellent results, had the disadvantage of requiring chemical separation of heavy metal ions. Because of the applied electrode potential in this type of circuit, depositable ions must be absent, necessitating chemical separations. Titrations of chlorides R ith adsorption indicators proposed by Fajans ( 2 ) , Kolthoff ( 6 ) , and others required the usual separations and, in addition, close pH control. The older chromate method has these disadvantages plus the personal factor of error in exact reproducibility. ELECTRODE CHARACTERISTICS
For the determination of chlorides, an electrode system not adversely affected by other ions, giving a potential change (Figure 1) abrupt enough for easy reading on an electronic voltmeter (titrimeter) was desirable. Such a system has proved satisfactory for rapid control on a variety of materials without chemical separations and with reasonable accuracy. The electrode system consists of two reversible electrodes: the silver chloride electrode reversible to chloride ions, and the silver electrode reversible to silver ions. The cell may be indicated as follows:
70
0.1 .2 .3
.5 ,6 .7 .8
,.,"ML.TITRANT Figure 1.
-
Potential Change
Until the equivalence point is reached, the potential of the silver electrode is determined by the concentration of silver ions resulting from the solubility of silver chloride, which is approximately constant. At the equivalence point the chloride-ion concentrations in the sample and at the silver chloride electrode are approximately the same, controlled by the solubility of silver chloride, but the silver-ion concentration now changes because of the slight excess of silver nitrate. This results in an abrupt change in the potential of the silver electrode indicated by a meter swing of the titrimeter similar to that encountered in redox titrations. ELECTRODE DESCRIPTION
The reference electrode described here offers maximum simplicity of operation and maintenance, elimination of the salt
A N A L Y T I C A L EDITION
November, 1945
l
693 I
Teble I. Effect of Heavy M e t a l Ions Copper Sulfate Solution Zinc Sulfate Solution Chloride Chloride Chloride Chloride added found added found Gram Gram Gram Gram 0.0005 0.0010 0.0015 0,0020 0.0030 0.0040 0.0050
0.0042 0.0047 0.0057 0.0077 0.0087
0.0005 0.0010 0.0014 0.0018 0.0029 0.0040 0,00.50
0,0040 0,0045 0.0056
O.OOi5 0 0086
bridge, and reasenable precision (Figure 2). The compact design h a k e s it easily adapted to a stirrer assembly (Figure 3). The silver chloride electrode assembly is made of Pyrex tubiilg, I2 cm. long and 7 mm. in outside diameter, with a smaller bore side arm located near the top. The bottom is tapered slightly and fitted with a ground-glass sleeve of the same taper. -4very small hole is bored or blown into the t,apered section, under the ground-glass sleeve. The bottom of the tube is sealed. The silver chloride-coated silver wire or ribbon extends to near the bottom of the tube and is sealed in a t the top with a wire lead extending through the seal. The silver chloride deposit is f0rmc.d by electrolysis, at low current density, of a dilute chloride solution with the .?ilver wire or ribbon used as anode. Platinum may be used for the cathodr:. The electrode is kept about half full of 0.1 -V potassium chloride solution or 1.0 N potassium nitrate containing a small amount of silver chloride to maintain saturation. Tests proved that no detectable amount of chloride leakage xould occur in using i: properly constructed electrode. HoiT-ever, Furman pointed out ( 4 ) that the use of 0.1 N potassium chloride might, raise objections because of the possibility of accidental contamination oi the sample, and suggested the use of potassium nitrate solution, snturated with silver chloride. This resulted in a slightly greater potential break of equal abruptness. Therefore the author. would recommend use of 1.0 N potassium nitrate saturated wit11 silver chloride rather than t,he potassium chloride. The side arm is used for filling the electrode and is providcd with a tight-fitting rubber plug. Continual use of the electrode over long periods may result in diffusion of sample solutions into the electrode electrolyt)e, making flushing and refilling desirable. When flushing is desired, the plug is removed from the side arm, and the sleeve removed from the lower end, resulting in release of a small quantity of the electrode solution. The cap and sidr:win plug may then be replaced, and the electrode thoroughly c.insed. When not in use the electrode should be kept immersed in fresh distilled water. The silver electrode is merely a ribbon or wire of high-purity silver, kepr clean by frequent light abrasion with fine Carborundum paper.
D
S o special precautions are required to protect the electrodes from light. The electrodes have been standing, immersed in distilled water, for 6 month? in daylight, but not direct sunlight, without deterioration. TITRIMETER O P E R A T I O N
figure 2.
Reference Electrode
Seal with lead extending through Silver chloride coated silver wire or ribbon C. Tapered, ground-glass join1 with small hole, sealed on bottom D. Filling tube for potassium chloride solution, with tight-fitting stopper E. Ground-glass sleeve, open on top and bottom to fit C A. B.
The electronic voltmeter used in this u-ork was built by one of the authors (Yeck) from the design of Garman and Droz ( 5 ) . This is an excellent, simple circuit, used for numerous determinations in this laboratory. How-
Figure 3.
Stirre; Assembly
A, Air stirrer
Rubber block cut from largo rubber stoDDer kith holes to Ilt over atirrer and to hold electrodes C. Silver wire or ribbon electrode D . Silver chloride electrode
B.
ever, a n y of t h e c o m m e r cially available t i t r im e t e r s should be suitable. Several tests should be made to determine behavior of electrodes with various types of titrimeters. Standards and samples must be titrated a t the same sensitivity and same degree of meter deflection. A deflection of 50 to 75% of scale i q satisfactory; greater sensitivity is unnecessary. The e l e c t r o d e s a r e sufficiently sensitive t o give rapid, m o m e n t a r y deflections with small additions of titrant, becoming slower in return as the equivalence point is reached, enabling the operator to judge the chloride content of sample and thereby avoid overtitration. GENERAL M E T H O D
The sample should be in dilute nitric. sulfuric. or acetic acid. I n the case of nitric acid, the concentration should be very I , I W (pH 1to a), to avoid solution of the silver electrode, but for all routine purposes the sulfuric or acetic acid need not be carefully :idjusted; up to 10% by volume has been used successfully. Some impairment of the magnitude of the potential break \vi11 occur at higher acid concentrations, however. Iodides and bromides must be absent. Heavy metal ions need not be removed. Silver reducing substances must be absent. Many chloride determinations are made on solutions or water-soluble .substances, in which case slight sulfuric acidification is all that is necessary to prepare for titration. A volume of about 150 ml. is duitable for t,itration, which should be carried out a t room temperature. Standards are prepared by adding known amounts of chloride ion t o chloride-free material as nearly identical Lyith samples as possible. The amount added for the standard should he close to the quantity in the sample. High chloride content material should not be titrated n-ith silver nit'rate solution st'andardized against n Fmall quantity of chloride, or vice versa. EFFECT O F H E A V Y M E T A L I O N S IN DIRECT TITRATIONS
Tests mere made to determine the effect of metal ions in direct titrations preliminary to use of the method on similar materials (Table I). The test solutions consisted of copper s'ulfate acidified with sulfuric acid and containing 50 grams per liter of copper, and zinc sulfate containing 50 grams per liter of zinc. I n both cases 50 ml. of solution plus 50 ml. of distilled water were acidified t o less than 1% ' with sulfuric acid and titrated directly. The magnitude of the potential break has not been investigated for the use of silver nitrate solutionsof greater dilution than 0.03 N , since this was satisfactory for the practical applications de ired. S o interference was encountered from the relatively large amounts of heavy metal ions and impurities present. LITERATURE CITED
'1)
Clippenger and Foulk, IND.ESG. CHEM.,Ax.4~. ED., 11, 216 (1939).
Fajans and von Beckerath, Z . p h y s i k . Chem., 97,478 (1921). Foulk and Bawden, J. Am. Chem. Soc., 48, 2015 (1926). Furman, N. H., private conversation. (5) Garman and Dros, IND.ESG. CHEM.,i l s . 4 ~ ED., . 11, 398 (1939). (6) Kolthoff, Lauer, and Sunde, J . Am. Chem. Soc., 51, 3273 (1929). (7) Willard and Fenwick, Ibid., 45, 715 (1923).
(2) (3) '4)
( 8 ) Yao, Yu-Lin. Trans. Electrochem. Soc., 85, 213 (1944).
