Quantitative chemical analysis and inorganic preparations / by R.M. Caven.

  • Caven, R. M. (Robert Martin), 1870-1934.
Date:
1923
Catalogue details

Licence: In copyright

Credit: Quantitative chemical analysis and inorganic preparations / by R.M. Caven. Source: Wellcome Collection.

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    however, on the use of the balance as well, for solutions of standard strength are required, i.e. solutions containing in unit volumes weighed quantities of pure chemical substances, whereby the unknown may be compared with the known through some chemical reaction carried to completion. There is often a choice between gravimetric and volumetric methods. For example, it may be necessary to determine the amount of iron present in combination in a certain volume of a given solution. This may be done gravimetrically or volumetrically. Gravimetric ally the iron in a convenient volume of the solution may be converted into and precipitated as an insoluble compound, which can be filtered off and weighed in the same or a modified form after suitable treatment. All that is neces¬ sary is for the iron to be obtained finally in the form of a single, definite, chemical compound. In this case the metal in the ferric condition is precipitated as hydroxide, and weighed after drying and ignition in the form of ferric oxide, Fe203. Volumetrically the amount of iron in another measured amount of the same solution may be estimated by the quantity of oxidizing agent required to convert it completely from the ferrous to the ferric state. For this purpose the iron is first obtained entirely in the ferrous state, by reduction if necessary, and is then oxidized by an observed volume of a standard solution of potassium permanganate or dichromate. In all cases of volumetric analysis it is necessary to know when the reactions taking place have been completed; this is shown, as a rule, by some visible change brought about by another reaction which occurs only when the main reaction is at an end. The reagent which undergoes this visible change is called an indicator. An indicator may be internal or external, i.e. it may be present in the solution during the titration* — as the operation of adding the required volume of one solution to a measured volume of the other is called—because it changes * French, titre, standard.