Chromate precipitation titration

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

Mohr titration analychem Titration with silver nitrate to determine the concentration of chlorides in a solution silver chromate precipitation is the end-point indicator. mor tT tra-shon ... 

The sample of salt is dissolved in water and titrated with standardised silver nitrate solution until all the silver salt has precipitated. Titrations of this type can be self-indicating, but usually an indicator is chosen that gives a coloured precipitate at the end point. In the assay of NaCl, potassium chromate is added to the solution once all the NaCl has reacted, the first drop of AgN03 in excess results in the precipitation of red silver chromate, which changes the colour of the sample to brown-red. 

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,... 

Chromates have been used in the precipitation titration of acridine and other organic bases. 

A common type of precipitation titration uses silver nitrate to determine the concentration of chloride ions. Silver nitrate solution is added to a chloride solution in the presence of potassium chromate(vi), which acts as an indicator . 

The determination of lead by chromate precipitation is usually adopted for official salts, but titration of the oxalate by the general method given on p. 369 can be used. For chromate precipitation ... 

Indicators for silver-halide precipitation titrations are of two types. The first react specifically when an excess of titrant becomes present immediately after the end point - for example, if a small amount of potassium chromate is added, it will react with excess silver ions to produce deep red silver chromate in neutral solutions (Mohi s method). In acid solutions, the silver is titrated with potassium thiocyanate (KCNS) solution (Volhard s method). Iron (III) ammonium sulfate solution is added and reacts with an excesss of thiocyanate to produce a deep red iron thiocyanate species. 

Addition of silver nitrate to a solution of a chloride in dilute nitric acid gives a white precipitate of silver chloride, AgCl, soluble in ammonia solution. This test may be used for gravimetric or volumetric estimation of chloride the silver chloride can be filtered off, dried and weighed, or the chloride titrated with standard silver nitrate using potassium chromate(VI) or fluorescein as indicator. 

Chloride. The chloride concentration is determined by titration with silver nitrate solution. This causes the chloride to be removed from the solution as AgCl, a white precipitate. The endpoint of the titration is detected using a potassium chromate indicator. The excess Ag present after all Cl" has been removed from solution reacts with the chromate to form Ag CrO, an orange-red precipitate. 

The theory of the process is as follows. This is a case of fractional precipitation (Section 2.8), the two sparingly soluble salts being silver chloride (Xsol 1.2 x 10 10) and silver chromate (Kso] 1.7 x 10 12). It is best studied by considering an actual example encountered in practice, viz. the titration of, say, 0.1M sodium chloride with 0.1M silver nitrate in the presence of a few millilitres of dilute potassium chromate solution. Silver chloride is the less soluble salt and the initial chloride concentration is high hence silver chloride will be precipitated. At the first point where red silver chromate is just precipitated both salts will be in equilibrium with the solution. Hence ... 

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. 

Note. Lead or barium can be determined by precipitating the sparingly soluble chromate, dissolving the washed precipitate in dilute sulphuric acid, adding a known excess of ammonium iron(II) sulphate solution, and titrating the excess of Fe2 + ion with 0.02M potassium dichromate in the usual way. 

In actual practice, however, such titrations are more or less restricted to those involving precipitation of Ag+ with anions, for instance halogens (Cl , Br, I-) and thiocyanate (SCN-). Generally, it is quite difficult and tedious to locate the exact point at which further addition of reagent affords no more precipitation. Therefore, the choice and wisdom of a chemical reaction is preferably sought so as to result in either a coloured solution or a coloured precipitate at the end point. A typical instance may be cited by application of potassium chromate (K2Cr04) solution in the above case whereby any extra drop of silver nitrate, after all the chloride has been precipitated, immediately causes precipitation of red chromate showing that the end point has been duly achieved. 

Procedure Weigh accurately about 0.25 g of potassium chloride in a conical flask and dissolve it in 50 ml of DW and titrate with 0.1 N silver nitrate solution, using 2-3 drops of potassium chromate solution as indicator till precipitation of red chromate is indicated. Each ml of 0.1 N silver nitrate solution is equivalent to 0.007455 g of KC1. 

Chlorpromazine was determined by precipitation with an excess of 0.2% ammonium reineckate solution, and titration of the unconsumed reagent bromatometrically after alkaline hydrolysis [67]. Excess hexathiocyanato-chromate was used as a precipitating reagent for chlorpromazine, whereupon the unconsumed reagent was determined with KBr03 in the filtrate [68]. 

Chlorides, bromides, and iodides can be quantitatively determined by treatment with silver nitrate, and, with suitable precautions, the precipitated halide is washed, dried, and weighed. Chlorides in neutral soln. can be determined by F. Mohr s volumetric process 27 by titration with a standard soln. of silver nitrate with a little potassium chromate or sodium phosphate as indicator. When all the chloride has reacted with the silver nitrate, any further addition of this salt gives a yellow coloration with the phosphate, and a red coloration with the chromate. In J. Volhard s volumetric process, the chloride is treated with an excess of an acidified soln. of silver nitrate of known concentration. The excess of silver nitrate is filtered from the precipitated chloride, and titrated with a standard soln. of ammonium thiocyanate, NH4CN8—a little ferric alum is used as indicator. When the silver nitrate is all converted into thiocyanate AgN03-fNH4CNS=AgCNS +NH4NOS, the blood-red coloration of ferric thiocyanate appears. 

The extremely low solubility of lead phosphate in water (about 6 x 10 15m) again suggests potentiometric analysis. Selig57,59 determined micro amounts of phosphate by precipitation with lead perchlorate in aqueous medium. The sample was buffered at pH 8.25-8.75 and a lead-selective electrode was used to establish the end-point. The detection limit is about 10 pg of phosphorus. Anions which form insoluble lead salts, such as molybdate, tungstate or chromate, interfere with the procedure. Similar direct potentiometric titrations of phosphate by precipitation as insoluble salts of lanthanum(III), copper(II) or cadmium(II) are suggested, the corresponding ion-selective electrodes being used to detect the end-point. 

Other reagents which may be applied to the volumetric determination of sulphuric acid and sulphates are barium chromate and benzidine. In the case of the former, the solution of sulphate is precipitated by a solution of barium chromate in hydrochloric acid on subsequent neutralisation of the filtrate, a quantity of chromic acid, equivalent to the barium sulphate which has been precipitated, remains in solution and may be estimated iodometrically.6 Benzidine, on the other hand, is an organic base which forms a very sparingly soluble sulphate the solution of mineral sulphate is treated with a solution of benzidine hydrochloride and the precipitated benzidine sulphate removed by filtration when subsequently suspended in pure water the benzidine sulphate undergoes hydrolysis to a sufficient extent to permit titration of the sulphuric acid with standard alkali.7 Lead nitrate may also be... 

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. 

Fractional precipitation The calculation as to which of two sparingly soluble salts will be precipitated under given experimental conditions may be also made with the aid of the solubility product principle. An example of great practical importance is the Mohr method for the estimation of halides. In this process a solution of chloride ions is titrated with a standard solution of silver nitrate, a small quantity of potassium chromate being added to serve as an indicator. Here two sparingly soluble salts may be formed, viz. silver chloride (a white precipitate) and silver chromate (which is red) ... 

This expression shows that under equilibrium conditions the concentration of chromate ions in the solution is always much greater than that of the chloride ions. If therefore to a mixture of chloride and chromate ions, silver ions are added, these will combine with chloride ions, forming silver chloride precipitate until the concentration of chloride ions in the solution decreases to such an extent, that the ratio expressed in equation (iii) is achieved. From then onwards the two precipitates will be formed simultaneously. If a 01m solution of sodium chloride is titrated with silver nitrate in the presence of 0 002m potassium chromate, the concentration of chloride ions at which silver chromate starts to precipitate can be expressed from equation (iii) ... 

Mohr titration, which involves the formation of a coloured precipitate by reaction with the indicator. For example, in the determination of chloride concentration with silver nitrate a small amount of potassium chromate solution is added as an indicator. This results in the formation of a red silver chromate (Ag2Cr04) precipitate at the end-point ... 

In this case, the precipitate may form slightly after the end-point, but this error can usually be neglected. Also, the titration should be done in neutral or slighly alkaline solution (pH 6.5-9) otherwise silver chromate might not be formed. 

The formation of a second, highly colored precipitate is the basis of the Mohr method of endpoint detection. Chloride and bromide ions are titrated with standard silver nitrate using chromate ion as indicator, the endpoint being indicated by the appearance of brick-red silver chromate. ... 

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... 

The Mohr titration must be carried out at a pH of 7 to 10 because chromate ion is the conjugate base of the weak chromic acid. Consequently, in more acidic solutions, the chromate ion concentration is too low to produce the precipitate near the equivalence point. Normally, a suitable pH is achieved by saturating the analyte solution with sodium hydrogen carbonate. 

Indicators used in precipitation reactions. In the titration of sodium chloride with silver nitrate, a small quantity of K2Cr04 solution is added to serve as indicator. At the end point the chromate ion combines with Ag+ ions to form sparingly soluble AggCrO (brick red colour). Both AgCl and AggCrO are insoluble, but they are... 

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