38 REPORT—1852. 
The mode of preparing the mixture of protosulphate of iron and sulphocyanide of 
potassium, as well as its extreme delicacy.in detecting minute quantities of bichromate 
of potash, I have fully explained in a paper published in the Quarterly Journal of the 
Chemical Society of London, vol. iv. p. 244. It should be made very weak, and in using 
it the precaution should be observed of not spotting it on the plate until the moment 
of its being required. The spots moreover should be large, and thinly spread on the 
surface of the plate, and care should be taken to bring the drop of the liquor to be 
tested into contact with the centre of the spot, which it is allowed merely to touch, 
without any stirring or agitation. The colour or tinge is produced instantaneously if 
any chromic acid be present. It is not always easy to procure protosulphate of iron 
eee i free from peroxide, and then the mixture prepared with such impure sulphate 
as a pinkish colour. This is of no moment when the tinge is light, as it becomes 
extremely pale and scarcely perceptible on the mixture being spread upon the plate. 
In the case of the mixture, however, being dark-coloured from the impurity of the 
sulphate of iron, it is better to dissolve a small piece of iron wire in dilute hydrochloric 
acid, and to add a portion of the clear solution at once to the sulphocyanide of potas- 
sium previously dissolved in water. 
When minute accuracy is required, it will be advisable to add 30 grs. of the iodide, 
dissolved in a small quantity of water, at once to the bichromate solution; then to 
make up the alkalimeter with 10 or 20 grs. of the iodide, and to proceed with the 
operation as before directed. By using in this way a very weak solution of the iodide, 
it is perfectly easy to bring the result within a tenth of a grain of the proper quantity. 
I have tried this process repeatedly upon carefully-purified iodide of potassium, 
both in large and small quantities. The results, which never differed more than a 
tenth of a grain from each other, show that 100 parts of bichromate of potash are 
equal to 833 of iodide of potassium, and to 254 of iodine; and accordingly 10 grs. of 
bichromate are equivalent to 25°4 grs. of iodine. 
The following are some of the results obtained by subjecting different specimens 
of commercial iodide of potassium to trial by this process :— 
Samples. Iodide of potassium, 
+ lea i re a meie(erers 99:8 per cent. 
De dea ntotan tal aietaipie nPaiaieiatelesstatals 99°5 7 
eUeL einleie’s e[e nibs, sinefeimatele’ sintaie 93°6 as 
es meteyaereiataeieasieraiieierns: sisters 88°8 Ge 
Time saxArsenane Sater Sse aeier END rf 
Ur eeraltenterarertrsisteieieis tele, Sissons 30°2 OH 
The last sample consisted chiefly of carbonate of potash. 
Iodine.— When iodine is the subject of examination, it must be first converted into 
a soluble iodide. The iodide of zine appears to be best adapted for the purpose, 
from its stability and the readiness with which it may be prepared. 50 grs. of the 
iodine to be tested are put into a small flask with some fragments of pure zinc and 
half an ounce of water. The mixture is agitated till the iodine becomes completely 
converted into iodide of zinc, which is indicated by the solution losing the dark red 
colour that it first acquires, and becoming nearly colourless. The solution is then 
decanted into an alkalimeter, which is made up to 0 with rinsings from the excess of 
zinc. 
10 grs. of bichromate of potash are next dissolved in a small basin with half an 
ounce of water, and half an ounce of hydrochloric acid is subsequently added. 
The remaining operations are precisely similar to those directed for iodide of potas- 
sium: ‘The iodide solution is dropped into that of the bichromate till no coloration is 
produced with the sulphocyanide mixture; then 5080, divided by the number of mea- 
sures consumed, gives the amount of iodine per cent. in the sample. 
This process is obviously incompatible with proto-compounds of iron, arsenious 
acid, and substances having similar chemical relations; but their presence would be 
immediately disclosed and their influence easily prevented. 
Kelp and Kelp-liquor.—The direct application of the present process to kelp and 
kelp-liquors is evidently inadmissible, in consequence of the sulphides, sulphites, hypo- 
sulphites and sulphocyanides, which they invariably contain. All these ingredients 
act readily upon bichromate of potash, in presence of hydrochloric acid. They may 
however be effectually removed by cautiously treating the solution from the kelp with 

