September, 1924
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Tables I and 11, in which they have been compared with the values of Ramsay and Young. All determinations were carried out in duplicate and agreed within 1 mm. The vapor pressures of the various special systems were
935
then measured. The data appear in Tables I11 and VI, and have also been plotted in the form of pressure-temperature curves in the accompanying charts. All determinations were carried out in duplicate.
Note on the Hicks Method of Determining Potassium’” By R. C. Wells, R. K. Bailey, and J. G. Fairchild GEOLOGICAL SORVEV, D E P A R T M E N T OF THE INTERIOR, WASHINGTON, D. C.
HE modified chloroplatinate method of determining potassium in salts and brines described by Hicks3in 1913 has been used since that time for rapid survey work with generally satisfactory results. Occasionally, however, determinations on the same sample by different analysts would differ considerably, especially with percentages of t h e or.der of 1 per cent and under. This led the writers to make many experiments on the effe‘cts of slight modifications of procedure, the conclusions from which may be of general interest. It is unnecessary to repeat the full description of t h e method here. The purpose of this note is to emphasize certain precautions rather than to criticize the method. The direction to wash with alcohol “of a t least 80 per cent strength” suggests that 95 per cent or even absolute alcohol may be used, and the writers made a good many determinations with alcohol of these strengths. Blank tests and check determinations with known mixtures, however, show that a small excess of platinum is generally obtained with the stronger alcohol, which may range from a few tenths of a milligram to 4.0 mb. according to the care used. This was in the presence of about 0.5 gram of pure sodium chloride as the principal salt. I n most of the tests the filter papers were not drained by suction, but in other respects the Hicks procedure was carefully followed. The results led to the conclusion that each analyat should without fail make blank tests with salts similar to his unknown mixtures rather than rely too much on his ability to follow an outlined procedure. Working as uniformly as possible, corrections based on blank tests have been made occasionally when the results seemed of sufficient importance. Many tests, however, led to the belief that it is unwise t o count very strongly on the significance of the tentha of a milligram of platinum and the corresponding quantities of potassium. Geologists should also understand this and not attempt to make fine distinctions between reported percentages of potassiumin salt beds, brines, etc., when the method of analysis does not warrant it. This applies especially to very small percentages. The relative magnification of errors that fall entirely on a minor constituent, as in this case, suggests that for strictly accurate results with small percentages of potassium the great excess of other salts should be removed before an estimation of potassium is attempted. The persistent positive error referred to above seems to be caused in part by the enclosure of some platinum compound i n the sodium chloride. Dittmar and McArthur4 found a similar behavior with sodium sulfate. The results are also affected, however, by the strength of $he wash alcohol. Sufficient washing with alcohol of 80
T
1 2
a 4
Received Mav 10. 1924. Published by permission of the Director, U. S. Geological Survey.
’ ~ m sJOURNAL, 6 , 6 S O (1913). Trans. Roy. SOC.Edznbuugh, 33, 595 (1888).
per cent by weight will dissolve out several tenths of a gram of sodium chloride, and of course with it any enclosed excess of reagent. Further, although potassium chloroplatinate is almost insoluble in absolute alcohol, it becomes increasingly soluble in weaker alcohol. Thus, with weaker alcohol any positive error is both reduced and compensated by a negative error. The extent of compensation, however, will depend on the proportions of the different salts, the volume of wash alcohol, and to an even greater extent on the “personal equation” of the analyst. According to the writers’ experience 90 or 95 per cent alcohol should give better results with small percentages of potassium than either 80 per cent or absolute alcohol, but they feel that, in general, blank tests are a much better insurance against errors than reliance on definite specifications. The improvement resulting from a double evaporation with intervening decantation of the excess of reagent does not warrant the extra expenditure of time. Adding a t least 5 cc. of hydrochloric acid before beginning the evaporation is recommended, however. Also, it cannot be too strongly emphasized that the mixture of salts must be very thoroughly ground during the leaching with alcohol, and in washing chlorides out of the platinum a silver nitrate test should be required, as the platinum moss appears to retain a little chloride tenaciously. I n Table I are given some representative results that illustrate these tests. TABLEI (Figures are in grams) CONDITIONS OF NaCl Expt. EXPERIMGNT taken 13W Absolute alcohol 0.5000 58W 5 cc. HCl, absolute alcoho1,suction 0.5000 33W Absolute alcohol 0.5000 16W Absolute alcohol 0,2000 31W 5 cc. HCl, absolute alcohol 0,5000 60F 5 cc. HC1, absolute alcohol 0,5000 56W 5 cc. HCl, absolute alcohol suction 0,5000 27B 95 percent alcohol 0,5000 42F 95 per cent alcohol 0,2000 95 per cent alcohol 1.5000 33B 55W 5 cc. HC1, 80 Der c e n t alcohol, suction 0.6000 57W Same as preceding 0.5000 39aF 74 er cent alcohol &O per cent by volume) 0.5000 41F 74 per cent alcohol (80 per cent by volume) 0.6000 17W 74 per cent alcohol (80 per cent by volume) 0 2000 36aF 67 per cent alcohol 0,5000 24w Two evaporations, absolute alcohol 0 5000 36W T w o evaporations, absolute alcohol 0.5000
Theoretical
Error in Pt + O , 0009
KC1 taken None
Pt found 0.0009
None 0.0025 0.0500
0.0004 0.0038 0.0676
None 0.0033 0.0665
+0.0004 +0.0005
0.0025
0.0037
0.0033
$0.0004
0.0060. 0.0080
0.0079
+0.0001
0.0100 0.0160 0.0450 0.0112
0.0131 0.0221 0.0609 0.0166
0.0131 0.0209 0.0589 0.0147
None $0.0012
None
0.0002
None
+0.0002
0.0100
0.0108
0.0131
-0.0023
0.0140
0,0171
0,0183
-0,0012
0,0350
0.0442
0.0458
-0.0016
0.0500 0,0020
0 0662 0,0010
0 0655 0.0026
+0.0007 0.0016
0,0037
0.0057
0.0049
$0. 0008
0,0250
0.0324
0.0328
- 0.0004
Pi None
+0.0021
+0.0020
+0.0019
-
