CHLOROMETRY. (Fa., Chlorometrie ; GER., ChlorbestImmung.) Chlorometry signifies the methods by which the amount of free or available chlorine ga,s, con tained in bleaching powder, and other disinfecting and bleaching agents, is determined. Commercial bleaching powder consists of a mixture, in variable proportions, of hydrochlorite, chloride, and hydrate of lime ; it is valued and sold in this country by the percentage of available chlorine. This percent age is usually determined by a process known as Penot's method, first proposed by Gay Lussac. It depends upon the fact that arsenious acid in solution is oxidized, by free chlorine, into arsenic acid, the end of the reaction being determined by iodide of potassium and starch paste.
The arsenious acid is employed in the form of an alkaline arsenite, and is prepared in the follow ing manner ;—Dissolve 4.95 grm. of the purest sublimed arsenious anhydride, free from sulphide, in about 250 cc. of distilled water in a flask, with about 25 grm. of the purest crystallized carbonate of soda, free from sulphide, hyposulphite, ca. sulphite. The acid should be finely powdered, and the liquid needs boiling and shaking for some time, in order to complete the solution. When the arsenic is entirely dissolved, the solution is allowed to cool, and diluted carefully to 1 litre.
The solution used to determine the end of the reaction is prepared by boiling 1 part of clean starch with 150 or 200 parts of water, adding a small quantity of iodide of potassium, aud allowing the mixture to cool. Slips of ordinary filtering paper are moistened with this solution, as required, and used while still moist, when it is far more sensitive than the dried paper recommended by Penot.
The operation of testing is then performed as follows :— The sample of bleaching powder is well mixed ; 3.55 grm. are weighed out, put into a small mortar, a little water is added, and the mixture rubbed to a smooth cream. More water is then stirred in with the pestle, allowed to settle for a, few moments, then poured off into a 250 co. flask, the sediment is again rubbed with water, poured off, and so on, repeatedly, until the whole of the powder has been conveyed into the flask without tho loss of a particle, and the mortar and pestle have been washed clean. The flask is then filled to the containing mark with water, wdl shaken, and 25 cc. of the turbid liquid is taken out with a pipe, and transferred to a beaker with a little distilled water. The arsenious solution is then run in from a burette, until a drop of the mix ture, taken out upon a glass rod and brought in contact with the moist starch-paper, gives no blue coloration. Towards the end of the operation, the arsenious acid must be introduced drop by drop very cautiously, until the colour is just discharged.
The following table shows a comparison of the English and French chlorometric degrees. The French degrees indicate how many litres, at 0° and 760 mm., are yielded by 1 kilo. of bleaching powder ; while the English degrees, which are also used in Germany, Russia, and America, show the percentage of " active" chlorine:—