TF 
cobaltidnitrite is added and all the potassium is precipitated as 
potassiumsodiumeobaltidnitrite, which with GirBerT we call cobalt- 
yellow. As we said, the precipitate is centrifugated until the volume 
remains constant and then read off. When the volume of cobalt- 
yellow supplied by a known K-solution has been determined, the 
amount of the unknown K-solution can be found. The imperative 
condition is satisfied that there should be a proportionality between 
the volume of cobalt-yellow and the amount of K, and that the 
result is also found to be independent of the rapidity of centri- 
fugation. 
2. Difficulties in the quantitative determination of SO, 
The fact that SO, is precipitated by BaCl, is known to every 
one who has studied chemistry for half a year. The reaction is a 
classical instance of precipitate-formation. After the experiences met 
with in the case of K, it would be expected that slight volumes of 
SO, in a solution containing a sulphate, could be determined volu- 
metrically without much difficulty by simply measuring the volume 
of the resulting BaSO,. The results of the experiments, however, 
were entirely unsatisfactory. The volume was found to be influenced 
by: the temperature, the volume of the fluid, the amount of the 
excess of BaCl,, the quantity of HCl and the presence of nume- 
rous substances which are met with in the sulphate-solution, nay 
even the manner in which the reagent was added. 
What the literature taught me on the determinations of SO, by 
weight-analysis was far from encouraging. It is evidently among the 
worst that exists. When we read TREADWELIs book on quantitative 
analysis 4th Ed. (1907) p. 353, then it appears what great obstacles 
the usual gravimetric method encounters; even when BaCl, is added 
to H,SO, the weight of the precipitate is not always the same. 
BaCl, is closed in, and that in amounts which depend for instance 
on the manner in which the BaCl, is added, by drops or at once. 
If we have to deal with a sulphate containing other salts, the matter 
becomes more complicated still. Besides the BaCl, being closed in, 
other salts are adsorbed: especially iron and calcium-salts are weighed 
with the precipitate, even after the BaSO, has been carefully washed. 
The quantity of HCl added is by no means immaterial, nor the 
dilution with water, which makes itself the more felt in proportion 
as the erystals are smaller. 
A few years ago the matter was taken up again by M. J. van‘? 
Kruys 5, who submitted it to a detailed systematic investigation; he 
1) M. J. vaN ’r Kruys. Zeitschrift für anal. Chemie, 49 (1910), 395. 
