ANALYSIS OF TIN PLATE. 1037 



lead and tin found together and ascertaining the per cent of the sum 

 due to lead. This method suffers from the radical defect that alloys of 

 tin and iron, such as are found under the outermost layer and in imme- 

 diate contact with the iron, can not be parted by the usual nitric acid 

 separation. 1 Tin goes into solution and iron remains with the tin oxid, 

 from which it can not be extracted by nitric acid. In the tin oxid ob- 

 tained by following Pinette's method the amount of ferric oxid ranged 

 from 4 to 12 per cent of the total, running usually between 4 and 10. 

 In filtering the tin oxid the nitric acid solution was difficult to filter 

 clear and when wash water was used the filtrate became turbid. Of 

 course all tin going into the filtrate would count as lead when using the 

 customary sulphuric acid precipitation, and of course the error due to 

 ferric oxid retained by the stannic oxid tends to counterbalance that 

 due to dissolved tin, but for the increase in lead there is no such com- 

 pensation. It may therefore be safely concluded that the method is 

 not accurate. 



In the analyses given the method used was to dissolve the plating 

 with weak aqua regia, neutralize the solution with ammonia, add excess 

 of ammonium sulphid, digest on the water bath, filter, once more digest 

 the solid residue with sulphid, and refilter. In the united filtrates tin 

 sulphid was precipitated by hydrochloric acid, filtered and weighed as 

 stannic oxid. The solid residue from the sulphid separation was 

 dissolved in nitric acid and lead determined as sulphate. This method, 

 although accurate, was too cumbrous and time-consuming to permit the 

 examination of many samples. 



The tin-iron alloy is more difficult of solution than the surface alloy 

 of tin and lead, so that in stripping the tin plate by acids, as in the 

 methods just described, there is a tendency to leave this interior layer 

 to a greater or less extent. For this reason, unless care in this respect 

 be exercised, the subsequent analysis will show the ratio of lead to tin 

 greater than it was in the original alloy used for tinning. 



There is a qualitative method often proposed, 2 which consists in put- 

 a drop of weak nitric acid on the tin, evaporating to dryuess, and 

 moistening the spot with solution of potassium iodid. A yellow col- 

 oration resulting is supposed to indicate lead and by its intensity to 

 give an approximate idea of the amount present. An alloy of 99.5 

 parts tin and 0.5 parts lead thus treated gave a yellow color which did 

 not materially differ from that given by an alloy of 88 parts tin and 12 

 parts lead. 



USE OF SOLDER, IN CANNING. 



The analysis of the solder presented no difficulty. Weighed portions 

 were treated with nitric acid and the tin oxid filtered and weighed. In 

 the filtrate the lead was estimated as sulphate. Traces of copper, comin g 



1 Fremy, Traite cle chiin., 3, (pt. 1), 795. 



2 Perron, Chem. CentrbL, 1890, 1, 731. 



