(Take of purified iodine, in powder, 111.125 grains; iodide of potassium, one hundred and fifty grains; distilled water, a sufficiency. Mix the iodide of potassium and iodine in a bottle with eighteen ounces of the water, agitate until both are dissolved, and when the solution is complete, add as much more distilled water as will make the total bulk exactly one pint.) The volumetric solution of iodine is of such a strength, that one hundred measures include 12.7 grains (or 1/10 of an equivalent) of iodine. It is dropped into the solution to be tested until free iodine begins to appear, and the number of measures added before this colour appears is carefully noted. It is principally used for the quantitative estimation of sulphurous and arsenious acid, all the sulphurous acid being converted into sulphuric, and the arsenious into arsenic acid, before any free iodine appears. The following equation represents the decompositions which occur, So2 + I + HO=So3 + HI; so that one equivalent of iodine is required to each equivalent of sulphurous acid, before any free iodine appears, and each equivalent of iodine will correspond to one equivalent of the sulphurous acid, or one hundred measures containing 12.7 grains (or 1/10 of an equivalent) of iodine will represent 3.2 grains (or 1/10 of an equivalent) of sulphurous acid. For the conversion of arsenious acid into arsenic, two equivalents of iodine will be required (As O3 + 2 HO + 2 I = As O5 + 2 HI). So that one hundred measures of the volumetric solution will represent 1/10 of half an equivalent of arsenious acid, or 4.95 grains.

It is also used for the estimation of sulphuretted hydrogen, one hundred measures of the solution representing 1.7 grains (or 1/10 of an equivalent in grains) of sulphuretted hydrogen.