Cyanide precipitation titration

The only difficulty in obtaining a sharp end point lies in the fact that silver cyanide, precipitated by local excess concentration of silver ion somewhat prior to the equivalence point, is very slow to re-dissolve and the titration is time-consuming. In the Deniges modification, iodide ion (usually as KI, ca 0.01 M) is used as the indicator and aqueous ammonia (ca 0.2M) is introduced to dissolve the silver cyanide. 

The potentiometric detection of the endpoint of precipitation titrations is very often used because not many visual indicators are available, in particular when mixtures of analytes are titrated. Halides, cyanide, sulfide, chromate, mercaptans, and thiols can be titrated with silver nitrate, using the silver sulfide-based ISE. Also complex mixtures, such as sulfide, thiocyanide, and chloride ions, or chloride, bromide, and iodide ions, can be titrated potentio-metrically with silver(I) ions. When the solubility of a compound formed during titration is too high, nonaqueous or mixed solvents are used, for example,... 

The concentration of cyanide, CN, in a copper electroplating bath can be determined by a complexometric titration with Ag+, forming the soluble Ag(CN)2 complex. In a typical analysis a 5.00-mL sample from an electroplating bath is transferred to a 250-mL Erlenmeyer flask, and treated with 100 mL of H2O, 5 mL of 20% w/v NaOH, and 5 mL of 10% w/v Kl. The sample is titrated with 0.1012 M AgN03, requiring 27.36 mL to reach the end point as signaled by the formation of a yellow precipitate of Agl. Report the concentration of cyanide as parts per million of NaCN. 

A simple example of the application of a complexation reaction to a titration procedure is the titration of cyanides with silver nitrate solution. When a solution of silver nitrate is added to a solution containing cyanide ions (e.g. an alkali cyanide) a white precipitate is formed when the two liquids first come into contact with one another, but on stirring it re-dissolves owing to the formation of a stable complex cyanide, the alkali salt of which is soluble ... 

Determination of calcium. Pipette two 25.0 mL portions of the mixed calcium and magnesium ion solution (not more than 0.01M with respect to either ion) into two separate 250 mL conical flasks and dilute each with about 25 mL of de-ionised water. To the first flask add 4 mL 8 M potassium hydroxide solution (a precipitate of magnesium hydroxide may be noted here), and allow to stand for 3-5 minutes with occasional swirling. Add about 30 mg each of potassium cyanide (Caution poison) and hydroxylammonium chloride and swirl the contents of the flask until the solids dissolve. Add about 50 mg of the HHSNNA indicator mixture and titrate with 0.01 M EDTA until the colour changes from red to blue. Run into the second flask from a burette a volume of EDTA solution equal to that required to reach the end point less 1 mL. Now add 4 mL of the potassium hydroxide solution, mix well and complete the titration as with the first sample record the exact volume of EDTA solution used. Perform a blank titration, replacing the sample with de-ionised water. 

Other dyestuffs have been recommended as adsorption indicators for the titration of halides and other ions. Thus cyanide ion may be titrated with standard silver nitrate solution using diphenylcarbazide as adsorption indicator (see Section 10.44) the precipitate is pale violet at the end point. A selection of adsorption indicators, their properties and uses, is given in Table 10.8. 

Discussion. The theory of the titration of cyanides with silver nitrate solution has been given in Section 10.44. All silver salts except the sulphide are readily soluble in excess of a solution of an alkali cyanide, hence chloride, bromide, and iodide do not interfere. The only difficulty in obtaining a sharp end point lies in the fact that silver cyanide is often precipitated in a curdy form which does not readily re-dissolve, and, moreover, the end point is not easy to detect with accuracy. 

At higher levels, cadmium may be estimated gravimetrically following precipitation with sulfide (20), P-naphthoquinoline (21), or after plating from a cyanide-containing solution onto a stationary platinum cathode. Volumetric procedures rely on preliminary precipitation of the sulfide that is purified and then dissolved in acid whereupon the liberated H2S may be titrated with iodine. An alternative, should zinc be a likely contaminant, is to precipitate with diethyldithiocarbamate and then to redissolve in acid and titrate with sodium ethylenetriaminetetraacetate (HDTA) using Eriochrome Black T as indicator (22). 

A rapid method of dealing with the precipitated selenium is to wash it and dissolve it in a cold saturated solution of sodium sulphide. The red colloidal solution obtained can be titrated with a standard potassium cyanide solution,7 the end-point being marked by a change of colour from red to yellow ... 

To measure hardness, the sample is treated with ascorbic acid (or hydroxylamine) to reduce Fe3+ to Fe2+ and with cyanide to mask Fe2+, Cu+, and several other minor metal ions. Titration with EDTA at pH 10 in NH3 buffer then gives the total concentrations of Ca2+ and Mg2+. Ca2+ can be determined separately if the titration is carried out at pH 13 without ammonia. At this pH, Mg(OH)2 precipitates and is inaccessible to EDTA. Interference by many metal ions can be reduced by the right choice of indicators.21... 

Other typical reagents generated for coulometric titrations are hydrogen and hydroxyl ions, redox reagents such as ceric, cuprous, ferrous, chromate, ferric, manganic, stannous, and titanous ions, precipitation reagents such as silver, mercurous, mercuric, and sulfate ions, and complex-formation reagents such as cyanide ion and EDTA [8-10]. 

Ag+ preferentially reacts with the analyte to form a soluble salt or complex. During this addition, Ag+ reacts with the analyte only, and not the indicator. But when all the analyte is completely consumed by Ag+ and no more of it is left in the solution, addition of an excess drop of silver nitrate titrant produces an instant change in color because of its reaction with the silver-sensitive indicator. Some of the indicators used in the argentometric titrations are potassium chromate or dichlorofluorescein in chloride analysis and p -dime thy la m i nobe nzalrho da n i nc in cyanide analysis. Silver nitrate reacts with potassium chromate to form red silver chromate at the end point. This is an example of precipitation indicator, where the first excess of silver ion combines with the indicator chromate ion to form a bright red solid. This is also known as Mohr method. 

Volumetric Methods.—Nickel may be conveniently estimated volu-metrically in the absence of cobalt, copper, silver, gold, and the platinum metals by means of potassium cyanide.4 The solution containing the nickel is, if acid, neutralised with ammonia and some ammonium sulphate is added to render the indicator more sensitive. A little ammonia is now added, and a few drops of potassium iodide and silver nitrate. The solution becomes turbid in consequence of the precipitation of silver iodide. The liquid is now titrated with potassium cyanide solution until the turbidity just disappears. The reaction consists in converting the nickel salt into the double cyanide, Ni(CN)a.2KCN, after which any excess of potassium cyanide attacks the silver iodide, yielding the soluble double cyanide, AgCN.KCN. The disappearance of the turbidity therefore indicates the complete conversion of the nickel salt. A slight correction is necessary for the silver introduced. 

The potential of a silver electrode during the course of the titration of silver nitrate with potassium c an-ide is shown in Fig. 78 the first marked change of potential occurs when one equivalent of cyanide has been added to one of silver, so that the whole of the silver cyanide is precipitated, and the second, when two equivalents of cyanide have been added, corresponds to the complete formation of the Ag(CN)2 ion. It will be seen that the changes of potential occur very sharply in each case this means that the silver cyanide is very slightly soluble and that, t+vo complex ion is very stable. 

Indirect determinations of several types can be carried out. Sulfate has been determined by adding an excess of standard barium(II) solution and back-titrating the excess. By titrating the cations in moderately soluble precipitates, other ions can be determined indirectly. Thus sodium has been determined by titration of zinc in sodium zinc uranyl acetate, and phosphate by determination of magnesium in magnesium ammonium phosphate. Quantitative formation of tetracyano nickel-ate(II) has been used for the indirect determination of cyanide. ... 

Puschel and Stefanac ° use alkaline hydrogen peroxide in the oxygen flask method to oxidize arsenic to arsenate. The arsenate is titrated directly with standard lead nitrate solution with 4-(2-pyridylazo) resorcinol or 8-hydroxy-7-(4-sulpho-l-naphthylazo) quino-line-5-sulphonic acid as indicator. Phosphorus interferes in this method. The precision at the 99% confidence limit is within 0.67% for a 3-mg sample. In another variation, Stefanac used sodium acetate as the absorbing liquid, and arsenite and arsenate are precipitated with silver nitrate. The precipitate is dissolved in potassium nickel cyanide (K2Ni(CN)4) solution and the displaced nickel is titrated with EDTA solution, with murexide as indicator. The average error is within + 0.19% for a 3-mg sample. Halogens and phosphate interfere in the procedure. 

Sodium chromate can serve as an indicator for the argentometric determination of chloride, bromide, and cyanide ions by reacting with silver ion to form a brick-red silver chromate (Ag2Cr04) precipitate in the equivalence-point region. The silver ion concentration at chemical equivalence in the titration of chloride with silver ions is given by... 

Fig. 5.7-1 The (colored) titration curve for the von Liebig titration of 0.1M cyanide with 0.1M silver, using the equilibrium constants given in the text. The black curve shows how the complexometric titration curve would have continued if somehow the precipitation of solid AgCN could have been prevented. Open circles show the equivalence points.

For the determination of higher quantities of cyanides (2-40 mg 1 ) titration against silver nitrate can be used. After transforming all cyanides into the complex [Ag(CN)2] the excess of silver ions is indicated by p-dimethylbenzilidene rhodanine which reacts with silver to form a red precipitate [14]. 

Given the vride variety of ion-selective electrodes already commercially available and the many more specialized ones that can be fabricated, titrations involving the precipitation or complexation of ions are widely used. Halides, cyanide, thiocyanate, sulfide, chromate, and thiols can be titrated with silver nitrate, using the appropriate... 

Cyanide is determined by a modified Kjeldahl method. A suitable sample is distilled with 125 ml. of 3 A sulfuric acid 100 ml. of the distillate is caught in a solution of 0.1 N sodium hydroxide, which is then titrated with standard silver nitrate solution. Potassium and chromium are determined on the same sample. The sample is decomposed with aqua regia, evaporated to diyness, and taken up with water, and chromium(III) hydroxide is precipitated by addition of ammonia. The filtrate is evaporated with sul-... 

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