Water sulphide determination

Samples should be taken from the ordinary hydrocast samplers immediately after the oxygen sample and in the the same manner as these samples (see Chapter 4). The hydrocast samplers should preferably be all-plastic, since sulphide reacts with many metals. When no oxygen is present in the water, samples for hydrogen sulphide determination should be taken first (see also Chapter 2). We use 50 mL oxygen bottles. Hydrogen sulphide is probably present in the water when the manganese hydroxide precipitate in the oxygen sample is completely white instead of brownish. 

The method may be applied to those anions (e.g. chloride, bromide, and iodide) which are completely precipitated by silver and are sparingly soluble in dilute nitric acid. Excess of standard silver nitrate solution is added to the solution containing free nitric acid, and the residual silver nitrate solution is titrated with standard thiocyanate solution. This is sometimes termed the residual process. Anions whose silver salts are slightly soluble in water, but which are soluble in nitric acid, such as phosphate, arsenate, chromate, sulphide, and oxalate, may be precipitated in neutral solution with an excess of standard silver nitrate solution. The precipitate is filtered off, thoroughly washed, dissolved in dilute nitric acid, and the silver titrated with thiocyanate solution. Alternatively, the residual silver nitrate in the filtrate from the precipitation may be determined with thiocyanate solution after acidification with dilute nitric acid. 

The procedure is illustrated by determination of the concentration of hydrogen sulphide water. 

A determination of dimethyl sulphoxide by Dizdar and Idjakovic" is based on the fact that it can cause changes in the visible absorption spectra of some metal compounds, especially transition metals, in aqueous solution. In these solutions water and sulphoxide evidently compete for places in the coordination sphere of the metal ions. The authors found the effect to be largest with ammonium ferric sulphate, (NH4)2S04 Fe2(S04)3T2H20, in dilute acid and related the observed increase in absorption at 410 nm with the concentration of dimethyl sulphoxide. Neither sulphide nor sulphone interfered. Toma and coworkers described a method, which may bear a relation to this group displacement in a sphere of coordination. They reacted sulphoxides (also cyanides and carbon monoxide) with excess sodium aquapentacyanoferrate" (the corresponding amminopentacyanoferrate complex was used) with which a 1 1 complex is formed. In the sulphoxide determination they then titrated spectrophotometrically with methylpyrazinium iodide, the cation of which reacts with the unused ferrate" complex to give a deep blue ion combination product (absorption maximum at 658 nm). 

As esters the alkyl halides are hydrolysed by alkalis to alcohols and salts of halogen acids. They are converted by nascent hydrogen into hydrocarbons, by ammonia into amines, by alkoxides into ethers, by alkali hydrogen sulphides into mercaptans, by potassium cyanide into nitriles, and by sodium acetate into acetic esters. (Formulate these reactions.) The alkyl halides are practically insoluble in water but are, on the other hand, miscible with organic solvents. As a consequence of the great affinity of iodine for silver, the alkyl iodides are almost instantaneously decomposed by aqueous-alcoholic silver nitrate solution, and so yield silver iodide and alcohol. The important method of Ziesel for the quantitative determination of alkyl groups combined in the form of ethers, depends on this property (cf. p. 80). 

Samples of natural water should either be analysed immediately or be stored (not for a very long time) at a decreased temperature to suppress microbial processes. For the determination of nitrate and nitrite it is useful to conserve the samples by addition of 1 ml chloroform or 0.1% phenylmercuric acetate per Utre. To prevent oxidation of sulphide and some other substances in water samples, reductants are added [5, 147]. If the distribution of a species between the f ree ionic form and various complexes is to be studied, as is of ten the case, care must be taken not to shift the equiUbrium by adding substances that would enter into side reactions with the studied species. 

Several classical ion-selective electrodes (some of which are commercially available) have been incorporated into continuous systems via suitable flow-cells. In fact, Lima et al. [112] used a tubular homogeneous crystal-membrane (AgjS or AgCl) sensor for the determination of sulphide and chloride in natural and waste waters. However, the search for new active materials providing higher selectivity and/or lower detection limits continues. Thus, Smyth et al [113] tested the suitability of a potentiometric sensor based on calix[4]arene compounds for use in flow injection systems. They found two neutral carriers, viz. methyl-j3-rerr-butylcalix[4]aryl acetate and... 

The solubility of nitroguanidine in organic solvents is limited. Desvergnes [31] determined its solubility in various solvents water, acetone, methyl and ethyl alcohols, ethyl acetate, ether, benzene, toluene, pyridine, chloroform, carbon tetrachloride and carbon sulphide. In all these liquids the solubility of nitroguanidine is negligible, the highest value—for pyridine—being 1.75 g/100 ml at 19°C. 

---