DETECTION OF HEAVY METALS. 1039 



METALLIC CONTAMINATIONS. 



In searching for the metallic combinations of the samples of canned 

 vegetables examined in this laboratory, the method generally used was 

 as follows : As large a quantity as possible (50 grams when size of sam- 

 ple permitted) of the dried sample was burned or charred, this depend, 

 ing on the nature of the sample, in a capacious porcelain crucible. The 

 charred mass was extracted with weak nitric acid in the cold, to avoid 

 solution of organic matter, filtered, and washed. The mass of char and 

 insoluble matter on the filter was dried, together with the filter, and 

 then burned to a white ash in a porcelain crucible. This ash was trans- 

 ferred to a platinum dish and covered with a mixture of hydrofluoric 

 acid and normal potassium fluorid. 



It was then heated over a low flame till all the water was driven off, 

 and then the heat pushed till the mass in the dish after fusing finally 

 became infusible. The heat was then raised to redness, and kept at 

 that point for a few minutes. After cooling, the fritted mass was treated 

 with dilute sulphuric acid and heated till white fumes came off. It was 

 next taken up with dilute hydrochloric acid and the solution added to 

 that first obtained. From the mixed solutions copper, tin, and lead 

 were thrown down, after adopting the usual precautions, by means of 

 hydrogen sulphid. In the filtrate, zinc when present was determined 

 by the Low ferrocyanid method (see page 11GG). The mixed sulphids of 

 lead, tin, and copper were treated with hot dilute solution of sodium 

 sulphid, the resulting solution filtered, and the residue once more 

 extracted in the same way. From the mixed filtrates tin was precipi- 

 tated by hydrochloric acid, regulating the addition by congo paper. 

 The tin sulphid was converted to oxid by careful ignition, and weighed 

 in that shape. The residual sulphids of lead and copper were oxidized 

 by nitric acid and the lead separated as sulphate or chromate. In the 

 filtrate copper was determined electrolytically. 



In many cases copper was determined color iinetrically, but lead was 

 invariably estimated gravimetrically. 



Tin and lead estimations in the case of canned goods are always sub- 

 ject to a grave element of doubt, from the fact that it is an extremely 

 difficult thing to remove all traces of solder. Of course lead and tin 

 in this shape are not particularly dangerous to health, or, at all events, 

 not nearly so much so as are these metals when occurring in foods in 

 the dissolved or combined state. Extreme care was taken in the effort 

 to avoid this source of error, but the attempt can not have been always 

 successful. 



LEAP, 



As just stated, the estimation of lead in canned goods is a matter of 

 extreme difficulty that is, the estimation of the lead existing as salts 

 jn contradistinction to that existing in the metallic form. Careless 



