INDUSTRIAL AND ENGINEERIKG CHEMISTRY
388
obtained by standardizing the iodine solution against pure arsenic trioxide, to which the impurities found t o be present in the sample to be analyzed had been added intentionally. The fourth column gives values obtained by adding together the nonvolatile, arsenic pentoxide, and sulfur trioxide contents and subtracting this value from 100. These results show"c1early that the values obtained by direct titration are about 0.5 per cent too high. Good results were obtained, however, by standardizing the 0.1 N iodine against pure arsenic trioxide to which impurities had been added intentionally, or by subtracting the nonvolatile residue plus arsenic pentoxide plus sulfur trioxide from 100 per cent. TABLE11. ANALYSISOP ARSENICTRIOXIDE (98 to 99 per cent pure 4siOd 1 2 3 4 5 6
%
%
98.42 98.26 1.00
97.93 97.79
!::!:i:~
Probable purity Direct titration value Artificial standard 100 - (nonvolatile .hOs) Nonvolatile AsiOs
+
7 Sb10a 8 Se
E:;
1.10
0.056
0.044
%
%
",:E
!::ti
9s. 30 0.87
9s. 60 98.59 0.069
0.027
0.25
98.46
:::::
Table 11 gives obtained in the analysis of samples of 98 to gg per centpurity, H~~~again, the values obtained by direct titration are about 0.45 per cent t'oo high on an average, but the results are good if the iodine solution is
VOL. 10, NO, 7
standardized against arsenic trioxide to which the same impurities have been added. TABLE 111. ANALYSISOF ARSENICTRIOXIDE (90 t o 9 1 per cent pure AszOs) 1 2
4
AsaOs
5 6 7 8 9
As205 803 sulfide S SbiOs
+
Se
+
%
%
%
%
%
89.89 90.49
90.35 91.61
91.15 92.42
89.65 90.25
90.57 89.94
90.29 7.22 0.40 2.09 1.00 0.55
90.31 6.41 0.15 3.13 0.97 0.41
In Table 111, the results obtained in the analysis of samples of arsenic trioxide which were 90 to 91 per cent pure are given, Here the results obtained by direct titration are erratic. In two cases the results are 1.25 per cent too high but in one case the value is 0.63 per cent too low. These samples contain appreciable quantities of selenium dioxide, antimony trioxide, and sulfur trioxide sulfides. They were hard to titrate and the end points were not easy to find. I n samples of this character, the only thing t o do is to determine the total arsenic (by distilling as arsenic trichloride and titrating the neutralized distillate with iodine) and to make a separate determination of arsenic pentoxide. The arsenic trioxide Can then be found by difference.
+
RECEIVED February 6, 1938.
Chemical Determination of Quartz (Free Silica) in Dusts AND W. THURBER F.ILES, Bureau of Vital Statistics, Baltimore City Health Department, Baltimore, Md.
EMANUEL KAPLAN, Division of Chemistry, Bureau of Laboratories,
I
N THE Knopf method ( I ) for the determination of quartz in the presence of silicates, the silicates are dissolved in hydrofluosilicic acid, leaving behind a residue containing quartz. The acid, however, decomposes quartz also with a resulting average rate of loss of 0.7 per cent per day of the original weight of quartz present in the sample. Knopf (I) notes that "by using the above factor of error it is possible to compute a t the end of an analysis the maximum possible loss in weight of quartz originally present, thus obtaining a maximum figure of quartz.'' Similarly, in a more recent method (2) which proposes the use of fluoboric acid in place of hydrofluosilicic acid, Line and Aradine state that "the free silica content of the residue must be corrected for the amount of free silica dissolved during the time required to decompose the silicate. The correction factor is 0.34 per cent per day." Neither of the above methods indicates the manner in which its respective correction factors are to be applied. Accordingly, the following mathematical treatment is presented for the interest of individuals routinely using these methods of analysis. The problem involves an application of the frequently encountered compound interest law, also called the law of organic growth, or the snowball law, which may be expressed for rates of decrease by the differential equation dy/ds = -ky. This expression is the first derivative of the exponential equation y = yOe-k', or in the logarithmic form log y
= log yo
- k x log e
I n the present problem = yo = k = z =
y
log e
mg. of quartz in residue mg. of quartz originally present rate of loss (0.7 per cent per day in the Knopf method) time of action in days =
0.43429
For example, in a determination of quartz in the presence of refractory silicates by the Knopf method, 500 mg. of dust required 10 days of treatment with hydrofluosilicic acid, leaving a residue corresponding to 50 mg. of quartz. The amount of quartz originally present was calculated in the following manner : log yo = log y $- 122 log e = log 50 (0.007 X 10 X 0.43429) = 1.72937 yo = 53.63 mg. of quartz originally present, corresponding to 10.7 per cent of quartz in the original sample of dust taken for analysis.
+
Literature Cited (1) Knopf, A, U. S. Pub. Health Service, Pub. Health Repts., 48, No. 8, 183-90 (1933). (2) Line, W. R., and Aradine, P. W., IXD.ENG.CHEM., Anal. Ed., 9,60-3 (1937). RECEIVED April 20, 1938,
