The manner in which the processes Involved in volumetric analysis are carried out, will be readily seen on referring to any of the articles above mentioned, or in considering a generally applicable process by Bunsen, presently to be described. The only apparatus neces sary are some pipettes and a few accurately graduated measuring glasses of small and largo capacity. One of the latter should be capable of delivering fluid both in a free stream and slowly in small drops. A special measuring-tube, or burette, has been contrived for this purpose by Mohr, and is now generally used for the purpose. Fig. 1 is a drawing of the instrument.
It consists of a glass tube, a, from three- to five.eighths of an inch wide, contracted at the lower extremity, b, and graduated. To the contracted portion is fitted a email piece of vulcanised caontehoue tube, c, into the other extremity of which a small spout, d, made of narrow glass tube, is inserted. A wire clamp, e, effectually prevents any fluid from passing out of the burette, unless the knobs, E, é, are pressed by the finger and thumb of the operator, when the liquid either flows or drops as may be wished. The accurate reading off of the height of the solution in the burette is a matter of great Importance ; Erdman recommends, for this purpose, the use of a float, 2. It is a hollow glass bulb, and is used as indicated in 3. which represents the upper part of the burette. It must more freely' within the burette. and its specific gravity be so adjusted, by enclosing la it a small quantity of mercury, that the upper edge of the liquid in the burette may cut it uniformly on all sides ate line, o, marked round the central portion of it. In order also that the line on the float may be parallel with the graduation on the burette, and that no undue friction may exist bo tween the two glass surfaces, the axis of the float must coincide with that of the tube. In cases where the teat-liquid contains anything that may act upon the catontehoue, the end (b, jig. 1) of the tube may be more finely drawn out and fluid expelled by gradually compressing an india.rubber ball attached to the upper extremity of the burette. Burette. may be specially graduated for a particular class of operations, as seen under Atxamarrav, and Acimmtvur ; the degrees may indi cate measures of ten grains each, as described under Stier-Tear ; or. as now usually adopted iu chemical laboratories, the division may be into cubic centimetres.
By classifying the reactions by which reducing or oxidising volu metric determinations are effected, Bunsen has succeeded in preparing three teet.solutions, by which any of the following substances can bo quantitatively estimated : mixtures and compounds of iodine, chlorine, bromine, chlorites, hypochlorites, sulphide of hydrogen, sulphites chromates, chlorates, arsenites ; the peroxides of iron, manganese, lead, nickel, &c., protoxide of iron, &c. The fact on which the method is based is as follows :—that when iodine is brought into contact with an aqueous solution of sulphurous acid, containing not more than 0.04 or 0.05 per cent. by weight of anhydrous acid, hydriodic and sulphuric acids are respectively formed :— The standard solution of iodine is made by dissolving five grammes of perfectly pure iodine in a concentrated solution of pure iodide of potassium, and making up the liquid to exactly one litre with pure distilled water. This solution ( =1000 cub. cent.) will obviously con
tain 0•005 gram. of iodine in every cubic centimetre. The sulphurous acid liquid.is made by adding 35 or 40 cub. cent. of a saturated solu tion of sulphurous acid to 5000 cub. cent. of water, introducing a little starch paste into some of the solution, and then pouring In the standard solution of iodine until after brisk agitation a distinct blue coloration remains. The value of this sulphurous solution having been calculated according to the above equation, it must be diluted till it contains 0'03 gram. of sulphurous acid in 100 cub. cent. The third test-liquid necessary is one containing about one gramme of pure iodide of potassium in 10 cub. cent. of water. In using these standard solutions for estimating an unknown quantity of free iodine in any substance, a weighed quantity of the latter is taken, the iodide of potassium solution added to dissolve the iodine, (if not already in solution) and the sulphurous acid solution then added until the brown colour of the iodine has disappeared. To effect this, an excess of the sulphurous acid is necessary, but that excess is determined by mixing a small quantity of starch paste with the liquid and then adding the standard solution of iodine until the blue colour appears and remains permanent. The quantity of iodine equivalent to the total amount of sulphurous acid used, less the quantity of iodine afterwards added to neutralise excess of sulphurous acid, will be the amount of iodine present in the portion of substance that was submitted to examination. The great value of this volumetric method of Bunsen depends upon the circumstances that so many bodies may be made directly or indirectly to liberate iodine from iodide of potassium, and that the reactions which occur in the processes are constant, accurately defined, and well known. Any body also which is decomposed by iodine in a known manner can be thus estimated. For example, an unknown quantity of sulphuretted hydrogen in any fluid is at once determined on adding to a given amount of the fluid, first a little starch paste and then the standard iodine solution until n permanent blue (iodide of starch) is produced : as the equivalent of iodine is to that of sulphuretted hydrogen, so is the amount of iodine used to the amount of sulphu retted hydrogen sought for. Hypochlorites are valued according to this method, by mixing a solution of a known weight of the salt with the test solution of iodide of potassium; adding hydrochloric acid till an acid reaction is obtained, and then determining the amount of liberated iodine in the manner already described. Besides a large number of substances which directly liberate chloriue,—and indirectly therefore, iodine,—those substances which are readily and perfectly oxidised by chlorine also come within the range of this method: such bodies may be heated with a known weight of bichromate of potash and hydrochloric acid; every two equivalents of the chromic acid yield under these circumstances three equivalents of chlorine, excess of which must of course be used, and that excess determined, as above indicated, by the amount of iodine it will liberate. For other examples of the adaptation of Bunsen's method, see the English translation of Quantitative Analysis,' third edition.