3 



Our work indicated that the following was the most satisfactory 

 method of making a sulphuric acid digestion: Weigh 25 grams of 

 fish or meat or 50 grams of vegetables^ into a beaker, wash into an 

 800 or 1,000 c. c. Kjeldahl flask, add from 100 to 150 c. c. of water, 25 

 grams of potassium sulphate, 50 c. c. of sulphuric acid (sp. gr. 1.84), 

 and a few glass beads. Place over a small flame, rotate a few times till 

 it boils,, then increase heat and boil vigorously until the water is 

 boiled off and fumes of sulphur trioxid appear in the neck of the 

 flask and settle back, remove it from the flame before it foams or 

 caking will ensue and the flask break in subsequent boiling. Add 50 

 c. c. of sulphuric acid (sp. gr. 1.84) turn the flame down so that it just 

 touches the flask, and heat gently until the mass boils quietly. Again 

 increase the heat and boil vigorously until decolorized. If the flask is 

 placed in an asbestos ring, cracking can often be avoided. This 

 digestion requires at least six hours. 



To determine the accuracy of duplicate determinations by the 

 sulphuric add method, samples of canned fish were very carefully 

 prepared by passing them through a sausage grinder and then thor- 

 oughly mixing them by hand. From these mixtures duplicate 

 weighings of 25 grams were made on an analytical balance to 0.05 

 gram and the following results were obtained : 



TABLE 3. Duplicate determinations of tin by the sulphuric add or wet combustion method. 



To determine tlie effect of the presence of sodium chlorid on 

 results obtained by the wet combustion method, 10 grams of salt 

 were added to the material to be analyzed. The figures given in the 

 following table prove that this had no effect on the results: 



TABLE 4. Determination of tin by the sulphuric-acid method in the presence . of salt. 

 [Grams of tin in 50 grams of sample.) 



[Cir. 67] 



