Lessons in qualitative chemical analysis / by F. Beilstein ; translated from the fifth edition, with copious additions ... by Charles O. Curtman.

  • Beilstein, Friedrich Konrad, 1838-1906.
Date:
1883
Catalogue details

Licence: Public Domain Mark

Credit: Lessons in qualitative chemical analysis / by F. Beilstein ; translated from the fifth edition, with copious additions ... by Charles O. Curtman. Source: Wellcome Collection.

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    VARIOUS METHODS OF DETERMINATION. Volumetric analysis may either be effected by direct or by in- direct determination or by the method of residues (rest-analy- sis.) a. Direct method. By this we determine the quantity of an acid directly by neutralization with normal alkali solution; or the quantity of iodine by adding a solution of sodium hyposul- phite, until the color of iodide of starch disappears; or in gene- ral terms by determining the quantity of the unknown body by direct measurement of tliec|iiantity of the reagents added to it until either the saturation or other effect is completed. This is a most accurate method, but only applicable to soluble sub- stances. b. Indirect method. Many insoluble substances cannot be directly determined. Some, as certain peroxides and oxygen- aeids (MllO,.. PbOs. BaOa, Cr08, HCIOs, etc., furnish, when boiled with strong HCl a definite amount of free chlorine. Now as iodine may be determined more accurately than chlor- ine, it is best to let (i be absorbed by KI, liberating an equival- ent amount of 1 to be measured bv standard solution of sodium hyposulphite. Even in such indirect measurements the quan- tity of the reagent used is in direct proportion to the quantity of the substance tested; hence some authors unite both methods under the title of direct. In the following method the process is reversed. c. Method of residues. In this process the substance is incapable of being determined directly, but other substances capable of volumetric determination are made to react upon it and an accurate measure of them is added in excess of the required quantity. A part is then so modified or destroyed, that it will no longer react with the standard solution, so that we can only measure the residue, the rest in excess of the require- ment, and from this calculate the quantity used up and the equivalent proportion of the original substance. To illustrate by an example: Calcium carbonate cannot well be directly determined. An accurately measured quantity of nitric acid, or hydrochloric acid, of known strength, more than sufficient to decompose the calcium carbonate is used and after the reaction is complete, standard solution of KOH is used to measure the rest or residue of free acid.