Nitrate gravimetric analysis

A successful gravimetric analysis consists of a number of important operations designed to obtain a pure and filterable precipitate suitable for weighing. You may wish to precipitate silver chloride from a solution of chloride by adding silver nitrate. There is more to the procedure than simply pouring in silver nitrate solution and then filtering. 

As an example of a gravimetric analysis, consider the problem of determining the amount of lead in a sample of drinking water. Lead, if it occurs in the water, probably exists as the lead(II) ion, Pb. Lead(II) sulfate is a very insoluble compound of lead(II) ion. When sodium sulfate, Na2S04, is added to a solution containing Pb ", lead(II) sulfate precipitates (that is, PbS04 comes out of the solution as a fine, crystalline solid). If you assume that the lead is present in solution as lead(ll) nitrate, you can write the following equation for the reaction ... 

Fluoride ion, and weak acids and bases do not interfere, but nitrate, nitrite, perchlorate, thiocyanate, chromate, chlorate, iodide, and bromide do. Since analysis of almost all boron-containing compounds requires a preliminary treatment which ultimately results in an aqueous boric acid sample, this procedure may be regarded as a gravimetric determination of boron. 

No satisfactory direct gravimetric procedure is available but nitrite can be oxidised to nitrate by permanganate or cerium(IV) and then determined in that form. The determination of total nitrate + nitrite is an important analysis, e.g. for soil samples. Nitrite may be destroyed using urea, sulphamic acid or hydrazine sulphate the reaction with the former is ... 

To prepare the solution we used zirconium nitrate containing 1.5 to 2.1% of hafnium in a ratio to the sum of zirconium and hafnium oxides (2 Zr(Hf)02). Initial solutions of zirconium nitrate with different concentrations of salts and nitric acid have been prepared by dissolving a concentrated water solution of zirconium nitrate salts. TBF, dissolved by o-xylol has been used as extraction media. Trials have been done in batches. The extraction media, previously saturated by nitric acid, is mixed with water solution (20 ml at a time) for 15 minutes on a vibration apparatus. Re-extraction has been done by two-step vibration of the organic phase with equal volumes of distilled water. The water phase was, after extraction and reextraction, analyzed gravimetrically to find out the contents of the sum of zirconium and hafnium oxides. Hafnium contents were determined by spectral analysis. Nitric acid concentration has been determined by titration. 

Analysis of the compound for platinum is accomplished gravimetrically by reduction of a known weight of the anhydrous salt to metallic platinum with formic acid.3 For the determination of chloride, 0.3 g. of the anhydrous salt is dissolved in 40 ml. of distilled water containing 250 mg. of hydrazine sulfate the solution is boiled gently until platinum metal is formed and coagulated. Then, without removal of the platinum, the solution is made 1 f in nitric acid and is titrated potentiometrically with standard 0.2 F silver nitrate solution. Anal. Calcd. for Na2PtCl6 Pt, 42.99 Cl, 46.88. Found Pt, 42.78 Cl, 46.73. 

The analysis of simple fulminate-chlorate mixtures may be effected by extraction either with water or with pyridine. In the former method the mixture ts digested with c

mercury fulminate is dried at 50 for three hours and weighed. Some fulminate dissolves in the water. This is estimated by precipitation as sulphide. The chlorate may also be determined in the filtrate. The solution t boiled with nitric acid and tlwn treated with fbrntaldehyde and silver nitrate. Silver chloride is precipitated and the estimation may be made gravimetrically or by Vol hard s method. 

For the analysis of rhenium, the sample is fused with sodium peroxide and rhenium precipitated from hydrochloric acid solution as Re2S7 by hydrogen sulfide. The precipitate is dissolved in aqueous sodium peroxide and from dilute sulfuric acid solution, rhenium is determined by electrodeposition.8 Chlorine is determined gravimetrically by fusion of the sample with a sodium carbonate/ sodium nitrate mixture (95 5) in a platinum crucible and then precipitating as silver chloride. 

For the analysis of molybdenum, the sample is decomposed by fuming with a few drops of nitric acid and sulfuric acid in a platinum crucible and the molybdenum is determined gravimetrically7 as the 8-quinolinol complex. From the filtrate, potassium is determined gravimetrically as K2S04. Fluoride is determined by titration with a standard solution of thorium nitrate using sodium alizarinsulfonate as indicator, after steam distillation of fluorosilicic acid.8 The determination of the oxidation state of molybdenum is carried out by oxidizing a known amount of the compound with a known amount of potassium dichromate in hot 2 N sulfuric acid and titrating the excess dichromate with standard Fe2+ solution. 

Using the proper choice of separation column, ion chromatography appears to be applicable for the sequential multicomponent analysis of other anions such as sulfate, chloride, fluoride and nitrate. The detection limits will be substantially lower than the classical gravimetric and potentiometric methods currently used. Ion selective electrodes are available for chloride and fluoride. 

The most fundamental approach for quantitative analysis involves the use of a conventional analytical calibration curve. This curve establishes a functional relationship between the ion current of the analyte and known concentrations of carefully prepared calibration standards. For dilute solution analysis, these standards usually consist of gravimetrically prepared aqueous dilutions of high-purity metal salts. These compounds should be either nitrate or oxide salts. Chloride salts should be avoided whenever possible to prevent the possibility of molecular ion interferences (see chapter on interferences). An alternate approach is to use commercially available calibration standard solutions that have been obtained fix>m a trusted source. After dilution, the standards are preserved with high-purity nitric acid to prevent precipitation or adsorption of the trace analyte species onto the interior walls of the sample holding container. 

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