76 ILLINOIS ACADEMY OF SCIENCE 



tion was explained to the managers of each plant. They were 

 asked to co-operate with us by furnishing a sample of the alum 

 used, together with the name of the manufacturer or dealer 

 supplying the same. Twenty-one of the plants very promptly 

 complied with our request, and in nearly every case, expressed 

 decided interest in the subject with a wish to know the results 

 of our investigation. The specimens of alum were carefully 

 sampled, ground and analyzed in duplicate by the following 

 methods : 



Methods of Analysis: The method used in obtaining most 

 of the data given is a modified Gutzeit Method, developed by 

 Claude R. Smith 1 in his work on coal tar dyes and other food 

 constituents. The results obtained by this method were in 

 several cases checked by the Marsh-Bezelius Method 2 and 

 were found to agree. The Gutzeit Method has been fnvesti- 

 gated by others for quantitative work, and, when proper care 

 is taken in the manipulation, has been found to give satis- 

 factory results. The chief modification proposed by Smith is 

 the use of paper sensitized with mercuric bromide in the place 

 of mercuric chloride, which had previously been generally used. 

 The bromide gives more permanent stains and the standards 

 can be kept longer. The method depends upon the formation 

 of a dark orange stain when the generated arsine is brought in 

 contact with the sensitized paper. Under uniform conditions, 

 the length of the stain varies with the amount of arsenic pres- 

 ent. A series of standard stains prepared from known amounts 

 of arsenic are used for comparison. A convenient series is 

 made from 2, 5, 7.5, 10, and 15 micro milligrams. The 

 amount of arsenic in the weight of alum taken is determined 

 by matching the stain it produces with the standards ; it is then 

 a matter of simple calculation to determine the percentage 

 arsenic content or the parts per million of arsenic. A one gram 

 sample will contain as many parts per million of arsenic as 

 there are micro milligrams of stain obtained. For example, if 

 one gram of alum produces a stain which matches the 5 micro 

 milligram standard stain, then that alum contains 5 parts per 

 million. One part per million is equivalent to .0001 of one 

 per cent. A stain representing between five and twenty-five 

 milligrams gives the most satisfactory results. A stain between 

 these limits can be obtained by varying the weight of alum 

 used. 



Experiments: Preliminary qualitative tests were run on 

 several samples of alum that were in the laboratory. All were 



1. U. S. Dept. Agr. Bur. of Chem., Circular No. 102. 



2. U. S. Dept. Agr., Bur. of Chem., Circular No. 99. 



