Colorimetric Determination of Potassium M. F. ADAMS
AND
J. L. ST. JOHN, Division of Chemistry, Agricultural Experiment Station, Pullman, Wash.
THE
advantages of the colorimetric determination of small amounts of potassium in the form of potassium iodoplatinate were recognized by Cameron and Failyer (1). While this method gives good results if proper care is used, the chemical stability and the rate of color development are not entirely satisfactory ( 3 , 6 , 6 ) . I n connection with previous work ( 2 ) a reliable and convenient colorimetric method was desired. It was expected that colorimetric determination by means of the corresponding chloro compound would have the advantage of greater convenience, and this was confirmed by the results described below. For very small amounts of potassium the iodoplatinate method, of course, cannot be dispensed with. For this reason a check of the iodoplatinate method appeared desirable. EXPERIMENTAL DETAILS
Potassium chloroplatinate, obtained and purified during analyses of plant material, was used for a stock solution, the concentration of which was determined gravimetrically. The colorimetric determinations were carried out by means of a Coleman double monochromator spectrophotometer. For the chloroplatinate the 30-mp slit was generally used. For the iodolatinate only the 5-mp slit was used. According to the manuracturer, the effective thickness of the round cells used is 16 mni. CHLOROPLATINATE
The practical advantages resulting from the application of the Beer-Lambert law are so important that a substance which does
not obey this law Kill be adopted for colorimetric analyses only in exceptional cases. The rewlts obtained for the transmittance of chloroplatinate solutions as a function of the concentration are represented in Figure 1 by open circles. The c u n e through these points deviate3 Plightly but xmiistakablg from a straight line. It is obvious t o aswnie thnt the deviation from the BeerLambert law is due t o hydrolysis of the complex ion, as observed previously for the iodoplatinnte by Schlesinger and Tapley (4). Actually the deviations disappeared when the chloroplatinate Kas dissolved in a 0.1. A' solution of potassium chloride. Sometimes it is advisable to use a solution of the colored substance rather than the solvent in the reference cell of the spectrophotometer. I n this way one can extend the concentration range, and this arrangement furnishes a morc sensitive test for the validity of the Beer-Lambert law. If, and only i f , this law holds the measured transmission valiies depend only on the difference, C - Co, of the two solutious and not on the conccntration Co, of the reference solution. Some readings obtained with a reference solution of 1.255 millimoles per liter in 0.1 S potassium chloride are included in Figure 1. The transmittance of three solutions as a function of the wave length is indicated in Figure 2. The extinction coefficients are: 0.18, 0.10, and 0.07 liter X millimole-l X cm.-* for 410, 440, and 470 mp, respectivc,Iy. The wave length 410 mp is recom-
?oo l
C-Co
Figure 1.
MILLIMOLES PER LITER
0 Solvent water.
e
Figure 2.
Transmittance of Potassium Chloroplatinate Solutions at 410 mp
Sotvent' 0 Solvent:
oIk
6.1 N
- - o ~d; CO CP
-
0.t N KCI o cSolvenI. 1.255 millim~~les per liter
00
KCI.
Transmittance of Potassium Chloroplatinate Solutions at Various Wave Lengths
(3 C
C
1.255 millimoles per liter
435
0
2.09 millimoles per liter 6.27 millimoles per liter
INDUSTRIAL AND ENGINEERING CHEMISTRY
436
Table I. Concentration rn.M/liter 0 0126 0 0252 0 0377 0.0503 0 0627
Vol. 17, No. 7
Transmission of Potassium lodoplatinate Immediately
6 Days 8 Days Per cent at 490 m p
67.7 44.2 28.0
64.5 40.6 25.8
64.0 40.3 25.6
17.6 10.1
16.1 10.0
16.0
30 Days
9.9
62.2 38.0 23.5 13.0 10.0
with samples containing 0.2 mg. of potassium or more. The iodoplatinate method is about one hundred times more sensitive, but slow changes of color have been observed. Appreciable errors can be eliminated only by careful calibration under well-defined conditions. ACKNOWLEDGMENT
The authors wish to thank Otto Redlich, research associate, State College of Washington, for advice. LITERATURE CITED
(1) Cameron, F. K., and Failyer, G. H., J . Am. Chem. SOC.,25, 1063
(1903).
WAVELENGTH
Figure
3.
mp
Transmittance of Potassium lodoplatinate Solutions at Various Wave Lengths Solvent. 9
(2) S t . John, J. L., and Midgley, M. C., IND. ENQ.CHEY.,ANAL.ED., 14, 301 (1942). (3) Salit, P. W., J . B i d . Chem., 136, 191 (1940). (4) Schlesinger, H. I., and Tapley, M. W., J . Am. Chem. SOC.,46, 276 (1924). ( 5 ) Shohl, A. T., and Bennett, H., J . B i d . Chem., 78,613 (1928). (6) Tenery, R. M., and Anderson, C. E., Ibid., 135, 659 (1940). PRESENTED before the Division of Agricultural and Food Chemistry a t t h e CHEMICAL SOCIETY, Pittsburgh, Pa. 106th Meeting of the AMERICAN
N KI
mended for concentrations up to 2 millimoles per liter, 470 mp u p to 6 millimoles per liter. The color develops immediately and has been found stable over a period of 2 months, The method requires somewhat less time t h a n the gravimetric method and furnishes better results for samples furnishing a weighed precipitate of less than 30 mg. of potassium chloroplatinate.
A DAY OF ANALYTICAL CHE-MISTRY 2
IODOPLATINATE
For the colorimetric determination of potassium as iodoplatinate purified samples of potassium chloroplatinate were dissolved i n 2 N potassium iodide solution. Figure 3 indicates the transmittance as a function of wave length and concentration. The values for wave lengths below 440 mp are not definitely reproducible, probably because in this range traces of free iodine cause appreciable deviations. Highest sensitivity is obtained a t 490 mp, which is the wave length chosen by previous authors. Immediately after preparation and a t room temperature a p pTeciable deviatiom from Beer-Lambert law have been found; these deviations practically disappear in the course of several days. A few results obtained a t 490 mp are reported in Table I. According to these figures the change of the apparent concentration may amount to as much as 0.003 millimole per liter. The results obtained after 8 days are well represented by the BeerLambert law with an extinction coefficient of 10 liters X millimole-’ X cm.-’ Thus the iodoplatinate is approximately one hundred times more sensitive than the chloroplatinate method. SUMMARY
The photometric determination of potassium as chloroplatinate is reliable and convenient, A precision of 2% can be attained
R . K D ~ r n m i t t. 113 N. Inqdlls
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Ann Arbor,
Mach.
