Qualitative Analysis 137 



QUALITATIVE ANALYSIS— TIN GROUP. 



Ralph W. Hufferd, DePauw University. 



Numerous complaints from students that their results in the analysis 

 of the tin division were not clear-cut caused the writer to investigate 

 the matter. Two difficulties were found with the method generally 

 employed. The first was in separating the tin group from the copper 

 group, where care in avoiding solution of the latter led to incomplete 

 solution of the former. The second was in separating tin and antimony 

 from arsenic where appreciable amounts of arsenic sulphide dissolved 

 to cause trouble later on. 



Since it is generally necessary to remove traces of copper and 

 mercury from the tin group precipitate, it was decided to make the 

 treatment with polysulphide so severe that this procedure would become 

 the rule. This would insure complete separation of the two divisions. 



A somewhat similar decision was made in the case of the second 

 part of the problem. It was decided to heat the reprecipitated tin 

 group sulphides with 12N HCl at the temperature of boiling water 

 since it is a simple matter to remove the arsenic sulphide which dissolves. 



In order to avcid the difficulty which students have with the use of 

 test-lead in reducing tin, aluminium was substituted for lead with good 

 results. 



The method finally adopted is as follows: 



Method. Wet the sulphides on the filter with ammonia in an evapor- 

 ating dish. Add about 15cc. polysulphide and set over a beaker of 

 boiling water for five minutes. During the heating, the dish should be 

 covered with a watch-glass. The contents should be stirred occasionally. 

 Filter and treat the residue a second time with ammonia and poly- 

 sulphide. Filter into the original filtrate. The residue contains all 

 but a trace of the copper group. 



Make the filtrate just acidic with dilute HCl and filter. Transfer 

 the precipitate and paper to a dish and heat over boiling water with 

 about 15cc. 15N ammonia for three to five minutes, adding more ammonia 

 from time to time to replace that which boils oflf. This treatment 

 dissolves the tin group sulphides. Filter, and wash the residue with 

 warm ammonia. The residue may be added to the copper group pre- 

 cipitate. Acidify the filtrate with 6N HCl, being careful not to add a 

 great excess and filter. 



Transfer the paper and precipitate to a large test-tube and add 

 lOcc. 12N HCl. Set the tube in a beaker of boiling water for ten 

 minutes. Cool, filter, and wash the precipitate on the filter with 5cc. 

 cold 6N HCl. Confirm arsenic in the residue. 



Add 20cc. water to the filtrate. Unless this dilution causes the 

 precipitation of orange antimony sulphide, pass in hydrogen sulphide 

 until it starts to precipitate or to saturation if it does not appear. 

 If antim.ony has precipitated, warm until it redissolves and filter. If 

 it is absent, filter. This treatment removes all arsenic. 



Boil the filtrate down to about 20cc. and transfer it to a large test- 



"Pioc. 38th Meeting-, 1922 (1923)." 



