Silver halide titration

Silver-halide titrations, (a) Chloride alone (solid line) (b) iodide plus chloride (dashed... 

The method may also be applied to the analysis of silver halides by dissolution in excess of cyanide solution and back-titration with standard silver nitrate. It can also be utilised indirectly for the determination of several metals, notably nickel, cobalt, and zinc, which form stable stoichiometric complexes with cyanide ion. Thus if a Ni(II) salt in ammoniacal solution is heated with excess of cyanide ion, the [Ni(CN)4]2 ion is formed quantitatively since it is more stable than the [Ag(CN)2] ion, the excess of cyanide may be determined by the Liebig-Deniges method. The metal ion determinations are, however, more conveniently made by titration with EDTA see the following sections. 

Silver halides can be dissolved in a solution of potassium tetracyanonickelate(II) in the presence of an ammonia-ammonium chloride buffer, and the nickel ion set free may be titrated with standard EDTA using murexide as indicator. 

A disadvantage of adsorption indicators is that silver halides are sensitised to the action of light by a layer of adsorbed dyestuff. For this reason, titrations should be carried out with a minimum exposure to sunlight. When using adsorption indicators, only 2 x 10-4 to 3 x 10 3 mol of dye per mol of silver halide is added this small concentration is used so that an appreciable fraction of the added indicator is actually adsorbed on the precipitate. 

Iodides can also be determined by this method, and in this case too there is no need to filter off the silver halide, since silver iodide is very much less soluble than silver thiocyanate. In this determination the iodide solution must be very dilute in order to reduce adsorption effects. The dilute iodide solution (ca 300 mL), acidified with dilute nitric acid, is treated very slowly and with vigorous stirring or shaking with standard 0.1 M silver nitrate until the yellow precipitate coagulates and the supernatant liquid appears colourless. Silver nitrate is then present in excess. One millilitre of iron(III) indicator solution is added, and the residual silver nitrate is titrated with standard 0.1M ammonium or potassium thiocyanate. 

For the determination of chlorine ions add to the solution 5-10 drops of a 0101 per cent alcoholic solution of dichlorofluorescein and titrate with neutral 0-02-0-025 N silver nitrate solution, using a micro-burette with 0-02 c.c. scale divisions. At the beginning of the titration the solution is only slightly opalescent deep turbidity develops as the end-point is approached. At this stage continue to titrate cautiously with vigorous shaking until the silver halide sol suddenly coagulates to reddish-pink flocks. 

Titrate rather rapidly in diffused daylight. Avoid direct sunlight, because the sensitivity to light of the silver halide is greatly increased by the sensitising action of the dye. 

It remains to be determined to what extent the dye adsorption technique is applicable to other substrates. No evidence was obtained for Pseudocyanine adsorption to Mn02, Fe2Os or to pure silver surfaces, although this dye can be bound to mica, lead halides, and mercury salts with formation of a /-band (61). Not only cyanines but other dye classes can yield surface spectra which may be similarly analyzed. This is specifically the case with the phthalein and azine dyes which were recommended by Fajans and by Kolthoff as adsorption indicators in potentio-metric titrations (15, 30). The techniques described are also convenient for determining rates and heats of adsorption and surface concentrations of dyes they have already found application in studies of luminescence (18) and electrophoresis (68) of silver halides as a function of dye coverage. 

A 100.0 mL solution containing aqueous HC1 and HBr was titrated with 0.1235 M NaOH. The volume of base required to neutralize the acid was 47.14 mL. Aqueous AgN03 was then added to precipitate the Cl" and Br ions as AgCl and AgBr. The mass of the silver halides obtained was 0.9974 g. What are the molarities of the HC1 and HBr in the original solution ... 

S) Bishop, E., and R. G. Dhaneshwar Differential Electrolytic Potentiometry. VIII. The Behaviour and Energetics of Current-Carrying Silver and Silver Halide Electrodes in the Semi-Micro Scale Titration of Nanogram Amounts of Halides at Extreme Dilution. Analyst 87, 845 (1962). 

Even with the "well-behaved" silver halides and oxides, indifferent electrolyte. For Insoluble oxides In pure water, containing only H and OH Ions, these Ions constitute not only the surface charge but also the counter charge. (The situation Is a bit academic because carrying out a titration requires the Introduction of other ions anjnvay. Moreover, few oxides are completely insoluble and some Ions may be Introduced by the wall of the vessel (silicates from the glass).) We shall therefore only consider the realistic cases that c ., etc. An additional... 

The so-called primary titration technique is attempted only with electrodes of silver metal, silver-silver halide, or mercury amalgams, which are the source of the electrogenerated species. The substance to be determined reacts directly at the electrode or with a reactant electrogenerated from the working electrode. This class of titrations is limited generally to non-diffusible reactants such as mercury amalgams, silver ions generated by anodization of silver metal, and halides liberated by reduction of the appropriate silver-silver halide electrode. 

The most common method of determining the halide ion concentration of aqueous solutions is titration v /ith a standard solution of silver nitrate. The reaction product is solid silver halide. A titration curve for this method usually consists of a plot of pAg versus the volume of silver nitrate added. To construct titration curves, three type of calculations are required, each of which corresponds to a distinct stage in the reaction (1) preequivalence, (2) equivalence, and (3) postequivalence. Example 13-10 demonstrates how pAg is determined for each of these stages. 

In organic solvents, the possibility of complex formation appears to be enhanced over that in aqueous solutions. Such equilibria are conveniently studied by means of potentiometric titrations. Although most of the published data for organic systems involve silver halides, the method is quite general and is applicable to any system providing that the concentration of at least one ion can be independently monitored, say by e.m.f. measurements. (Since a detailed analysis of this method has been given by Butler only an outline will be given here.)... 

There are several other electrochemical measxirements in protic solvents which are of interest and have not been discussed above. In pure methanol, the data of Buckley and Hartley were noted to contain some errors and the values of for the silver halides must therefore be questioned. The solubility products of the silver halides have been redetermined more recently by potentiometric titration and these results are compared in Table 2.7.6. 

Gravimetric method such as AgCl or fire assay is used. Gay-Lussac method involves the titration of Ag with NaCl and/or KBr in acid solution. The endpoint is taken as the point at which no further precipitation of silver halide is observed upon the addition of more precipitant. Fajans method is based on the precipitation of Ag with chloride or bromide ions, using an adsorption indicator to detect the endpoint. Rhodamine 6G or dichlorofluorescein is used as the adsorption indicators. Volhard method is based on the titration of Ag with potassium or sodium thiocyanate to form insoluble silver thiocyanate. This titration is... 

Jordan and coworkers, beginning in 1959 (56) performed several thermochemical titrations in nitrate melts including precipitation of silver halides and silver chromate, measuring the temperature change in an adiabatic system. Zambonin and Jordan (57) used voltammetry to study oxygen and peroxide species in 1967. 

As discussed, the chlorinity is determined by titration with silver nitrate solution precipitating the halides dissolved in seawater as insoluble silver halides with silver ions ... 

The product Cv is the mole number of Ag+ ions added at saturation. By solving these relations, we find v 10 - ml. The volume of a drop is higher than ml. Hence, the titration reaction is indeed, from the beginning until the end, that of the silver halide precipitation. 

Due to the very weak values of silver halide s solubility products, titration reactions are quasi-totally displaced toward the right. This condition, which is required to carry out a good titration, is obeyed. 

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