186 On a Method of Estimating Minute Quantities of Iodine. 



well known, produces in solutions of iodine and the soluble 

 iodides a brown or brownish-black precipitate of iodide of palla- 

 dium. When, however, the quantity of iodine is small, the 

 iodide instead of being immediately precipitated, remains sus- 

 pended in the solution, to which it communicates a brownish 

 tinge, more or less deep according to the proportion of iodine 

 present. Consequently, then, by ascertaining the depth of tint 

 of such a solution, by comparing it with that of standard solu- 

 tions properly prepared with known quantities of iodine, the 

 proportion of the latter substance that is contained in the mat- 

 ters tested may be ascertained with the greatest exactness. 



In an investigation of this kind, therefore, the first thing to 

 be done is to prepare certain standard solutions. 1-309 grain 

 of perfectly pure iodide of potassium — which is equivalent to 

 one grain of iodine — is accordingly dissolved in 10,000 grains 

 of water. This constitutes solution No. 1, and contains a little 

 less than *01 gr. per cent, of iodine. By diluting this with 

 water, other solutions — Nos. 2, 3, 4, 5, 6, &c. — are formed. The 

 iodine in the substance to be tested having been converted into 

 hydriodic acid or a soluble iodide, the latter is introduced into a 

 colorimeter and diluted with water to the necessary degree, — that 

 is to say, until it occupies exactly 100, 500, 1000, 10,000 or 

 more water grain measures ; the precaution being taken to first 

 add the palladium solution drop by drop until no deepening of 

 colour ensues. The tint is then compared with that of the 

 standard solutions before mentioned contained in tubes or phials 

 of similar diameter, in which certain known quantities of iodine 

 are contained in the same bulk of water. Though it might 

 rarely be possible to identify it with either one of two solutions 

 in the scale, there can be no difficulty in deciding between which 

 two it should fall, or nearest to which one of two it should be 

 placed. 



These standard solutions, it should be observed, may be sealed 

 up in glass tubes, and thus rendered available in future investi- 

 gations; the only precaution necessary to be taken in such 

 cases being to well agitate the contents of the tubes, so as to 

 again get the precipitated iodide of palladium in suspension in 

 the fluid. Operating in this way, it is possible to estimate the 

 2o~5oo tn °f a gr™ °f iodine with the greatest readiness. It is 

 sometimes preferable to employ but one standard solution. The 

 roportion of iodine in the liquid analysed is then determined 

 y measuring the volume of water that is required to lighten the 

 tint, so as to render it identical with that of the normal solution 

 or vice versd. 



Mansion House, Old Park, Bristol, 

 Julv 27, 1853. 



I 



