David S. Price, Esq., on a New Test for Iodides. 299 



to describe the application of nitrites for the detection of 

 iodides, and at the same time give one or two instances of 

 the practical application of the test in cases where the quan- 

 tity of iodine is exceedingly small. 



The method of employing the test is the following : — The 

 liquid suspected to contain an iodide is mixed with starch 

 paste, and acidified with hydrochloric acid, a solution of 

 nitrite of potassa is then added, when, if much iodine be 

 present, a dark blue colour will be instantly produced ; if a 

 very small quantity only, as, for instance, the two or three 

 millionth part, then a few seconds elapse before the blue 

 colour makes its appearance. In this manner I have detected 

 ^liG T,zyir^,?jir(jth part of iodine dissolved in water as iodide of 

 potassium. It will be seen that the test admits of a degree 

 of delicacy not attainable by any of the other methods for 

 detecting iodides, as well as being at the same time free 

 from the disadvantages to which they are more or less subject; 

 as, for instance, in the employment of chlorine, which, unless 

 added very carefully to a liquid containing a trace of an 

 iodide only, is almost sure to afford a negative result, from 

 the chlorine combining with the iodine, and so preventing its 

 acting on the starch. The same error may also arise by the 

 use of nitric acid, should the suspected liquid contain a large 

 amount of chlorides. 



I will now detail the two cases in which I have applied this 

 test, in the one for the purpose of detecting iodine in cod liver 

 oil, the object being to see how small a quantity of the oil 

 would suffice ; in the other, for the purpose of detecting iodine 

 in marine vegetation. One ounce of ordinary brown cod liver 

 oil was saponified by a concentrated solution of caustic 

 potash, and then carbonized in an iron spoon over an open 

 fire ; the residue was removed into a covered porcelain cru- 

 cible and strongly heated, so as efi^ectually to destroy all 

 organic matter, and when cold, was digested with a small 

 quantity of water and thrown upon a filter ; the filtrate being 

 acidified with hydrochloric acid, was then mixed with starch- 

 paste, and tested with nitrite of potassa, which almost imme- 

 diately produced a pale plum colour. 



Sea-water contains so small an amount of iodine, that it is 



