
TRANSACTIONS OF THE SECTIONS. BO 
On the Estimation of Iodine. By Professor FrepErick Penny, PhD. 
Andersonian University, Glasgow. 
Among the many applications that may be made of bichromate of potash to the 
purposes of centigrade analysis, there is none more convenient or useful than its 
employment for the estimation of the amount of iodine in samples of commercial iodine 
and of iodide of potassium. In Glasgow especially, which is the principal focus of the 
manufacture of potash-salts and iodine from kelp, and where the problems referred 
to are frequently presented for solution, an expeditious and exact method for the deter- 
mination of iodine becomes truly valuable. 
According to the statements of a party well qualified to judge, it appears, that, in 
the kelp season of 1851, the total quantity of kelp brought to Glasgow was about 
6000 tons (222 ewt. to the ton), which may be considered a fair average of ordinary 
seasons. On account of the greatly increased demand for potash-salts, the arrivals in 
1850 amounted to nearly 10,000 tons, which is considerably higher than for several 
previous years*. The manufacture however is not confined to Glasgow; there are 
iodine works at Borrowstowness, Greenock, and Falkirk, in Scotland, at Ramelton 
in Ireland, and at Cherbourg and Brest in France. 
The centigrade process, here to be described, for the estimation of iodine, is based 
upon the fact, that chromic acid in presence of hydrochloric acid causes the complete 
decomposition of soluble metallic iodides, the chloride of chromium and the chloride 
of the other metal being produced, while the iodine is thrown down in the solid state. 
Bichromate of potash is taken as the most convenient and stable form of chromic acid. 
The reaction with iodide of potassium is exhibited in the following equation :-— 
3KI+ KO, 2Cr03+ 7HCI=I3 + 4KC1+ Cr? C+ 7HO. 
The action is immediate, and provided that the solutions are kept cool, no secondary 
result is formed. The precipitated iodine speedily subsides, leaving the supernatant 
liquid quite clear, though slightly coloured, witha few floating particles of iodine on the 
surface. When the solution of the bichromate is overdosed with the iodide, it becomes 
dark red, from a portion of the iodine being dissolved by the excess of the iodide. 
This change of colour is useful as indicating when the addition of the iodide has been 
carried too far. 
Iodide of Potassium.—The process for iodide of potassium is conducted as follows :— 
10 grs. of bichromate of potash are dissolved in half an ounce of cold water, and half 
an ounce by measure of hydrochloric acid is afterwards poured in. An alkalimeter 
of 100 measures is then made up in the usual manner with 50 grs. of the iodide of 
potassium dissolved in water, and the solution is added to that of the bichromate of 
potash until the chromic acid is completely decomposed. To hit the exact point at 
which the action is complete, a white plate is spotted with a solution containing a 
mixture of pure protosulphate of iron and sulphocyanide of potassium, slightly acidi- 
fied with hydrochloric acid, and by means of a glass rod a small quantity of the bi- 
chromate liquor is brought into contact with the mixture on the plate. So long as a 
red colour, or even dark tinge, is communicated to the spots by the application of this 
test, the presence of chromic acid is indicated; but when no dark shade is produced, 
the action is complete, and the operation is finished. The number of measures used 
is accurately read off, and the per-centage quantity of iodine in the sample subjected 
to trial is found by dividing 5080 by this number, 10 grs. of bichromate being equal 
(as will presently be shown) to 25:4 of iodine, and to 33:3 of iodide of potassium. 
Throughout the process the liquid should be kept quite cool, and towards the con- 
clusion the iodide solution in the alkalimeter should be added very slowly, in order to 
give ample time for the mutual action of the iodide and chromic acid, which should 
likewise be assisted by repeated and brisk stirring. 
* Kelp Imports in Glasgow. July to July. 
1841—42.,..ccseeccseeeeesee 2565 tons. 

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1843-44... ccssecceeseeeees .. 1965 ,, 
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1845—46...... eeeee saeererere 6086 ? 
1846-47 csssesecoserserees S627 45 
Mr. Glassford’s Kelp Manufacture. 
