Ammonia gravimetric analysis

Reactive amines other than ammonia have also been employed to distinguish Bronsted acid sites in various zeolites using a combination of TPD and thermo-gravimetric analysis (TGA) techniques [135]. The method is based on the fact that surface Bronsted sites may induce thermal decomposition of aliphatic amines to alkenes and ammonia over a narrow temperature range. The number of amine molecules reacted is equated to the number of strong Bronsted acid sites. By choosing amines of appropriate sizes it is possible to discriminate between acid sites located in pores of different diameters. 

In gravimetric analysis, the solution-preparation step has its own special significance. The analyte must be separated from interfering species, or the interferents must be masked. For example, if iron(III) is to be estimated as its hydrated oxide in the presence of chromium(III), then the mixture is initially treated with perchloric acid so as to oxidize chromium(III) to chromate (chromium(VI), Cr04 ), followed by addition of ammonia to precipitate the hydrated iron oxide. Sometimes it is necessary to remove interferents, for example, when calcium is to be estimated as calcium sulfate in the presence of barium. The barium is removed as its chromate and the calcium is precipitated quantitatively as its sulfate. 

In the determination of sulfate, 2 to 5 g of the analysis sample is mixed with HC1 (2 volumes concentrated HC1 + 3 volumes of water), and the mixture is gently boiled for 30 minutes. After filtering and washing, the undissolved coal may be retained for the determination of pyrite sulfur, or it may be discarded and a fresh sample used for pyrite sulfur. Saturated bromine water is added to the filtrate to oxidize all sulfur forms to sulfate ions and ferrous ions to ferric ions. After boiling to remove excess bromine, the iron is precipitated with excess ammonia and filtered. This precipitate must be retained for the determination of nonpyrite iron if a fresh sample of coal was used for the determination of the pyrite iron. The sulfate is then precipitated with ISaCE, and the BaSC>4 is determined gravimetrically. 

A new gravimetric method is developed for iron(III) in which the iron is precipitated in crystalline form with an organoboron cage compound. The accuracy of the method is checked by analyzing the iron in an ore sample and comparing with the results using the standard precipitation with ammonia and weighing of Fe Oa. The results, reported as % Fe for each analysis, were as follows ... 

A crystalline precipitate of magnesium ammonium phosphate hexahydrate (struvite) is produced when other magnesium salts are added with ammonia to weakly basic or neutral phosphate solutions (5.39). The ignition of this salt to give the pyrophosphate forms the basis of a gravimetric method of analysis for P (Chapter 14.1). 

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