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10. The gravimetric determination of nickel in alloys
Gravimetric analyses belong to the most precise, because contemporary analytical balances make possible determination of the mass of a sample with great accuracy. In these analyses one should obtain high purity compound of the analyzed element or a compound directly obtained from the analyzed substance). This reaction has to be exactly stoichiometric. It is also important that the weighed compound was non-hydroscopic and stable in air, it also better if it has relatively high molecular mass, because in this case the weighing is more precise.
Examples of gravimetric analyses of high precision and still often used in practice are, among others:
Analysis of iron precipitated as Fe(OH)3 and heated in electric oven at ca. 800⁰C to oxide Fe2O3;
Analysis of barium precipitated as BaSO4 (or analysis of sulphates precipitated by Ba2+ salts), the precipitate is heated to ca. 500⁰C;
Analysis of nickel precipitated in form of its complex with dimethylglyoxime (CH3C(NOH)C(NOH)CH3, known also as “Chugaev salt”1, the precipitate has to be dried at 110⁰C only.
In this exercise we will analyze an alloy containing Ni to determine its percent content in it.
Nickel(II) forms a precipitate with the organic compound dimethylglyoxime, C4H6(NOH)2. The formation of the red chelate occurs quantitatively in a solution in which the pH is buffered in the range of 5 to 9. The chelation reaction that occurs is illustrated below.
Although the loss of one proton occurs from one oxime group (NOH) on each of the two molecules of dimethylglyoxime, the chelation reaction occurs due to donation of the electron pairs on the four nitrogen atoms, not by electrons on the oxygen atoms. The reaction is performed in a solution buffered by either an ammonia or citrate buffer to prevent the pH of the solution from falling below 5. If the pH does become too low the equilibrium of the above reaction favors the formation of the nickel(II) ion,
Gravimetric analyses belong to the most precise, because contemporary analytical balances make possible determination of the mass of a sample with great accuracy. In these analyses one should obtain high purity compound of the analyzed element or a compound directly obtained from the analyzed substance). This reaction has to be exactly stoichiometric. It is also important that the weighed compound was non-hydroscopic and stable in air, it also better if it has relatively high molecular mass, because in this case the weighing is more precise.
Examples of gravimetric analyses of high precision and still often used in practice are, among others:
Analysis of iron precipitated as Fe(OH)3 and heated in electric oven at ca. 800⁰C to oxide Fe2O3;
Analysis of barium precipitated as BaSO4 (or analysis of sulphates precipitated by Ba2+ salts), the precipitate is heated to ca. 500⁰C;
Analysis of nickel precipitated in form of its complex with dimethylglyoxime (CH3C(NOH)C(NOH)CH3, known also as “Chugaev salt”1, the precipitate has to be dried at 110⁰C only.
In this exercise we will analyze an alloy containing Ni to determine its percent content in it.
Nickel(II) forms a precipitate with the organic compound dimethylglyoxime, C4H6(NOH)2. The formation of the red chelate occurs quantitatively in a solution in which the pH is buffered in the range of 5 to 9. The chelation reaction that occurs is illustrated below.
Although the loss of one proton occurs from one oxime group (NOH) on each of the two molecules of dimethylglyoxime, the chelation reaction occurs due to donation of the electron pairs on the four nitrogen atoms, not by electrons on the oxygen atoms. The reaction is performed in a solution buffered by either an ammonia or citrate buffer to prevent the pH of the solution from falling below 5. If the pH does become too low the equilibrium of the above reaction favors the formation of the nickel(II) ion,