Titrations with Ceric Ions

Titrations with ceric salts, symbolized here by Ce, are sometimes grouped under the generic term cerimetry. Most of the time, ceric sulfate is used. 

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IVe need to think in terms of both kinetics and thermodynamics when considering restrictions on the choice of redox systems. For simplicity, we will continue to employ the case where ferrous ion is titrated with ceric ion. 

The titration of excess standard ceric ammonium nitrate solution after addition of azide either as a slurry, solution, or as distilled hydrazoic acid gives precise and accurate results. The ferroin endpoint is sharp and virtually instantaneous. Loss of hydrazoic acid in the distillation is apparently nil, and the reaction of hydrazoic acid with ceric ion is extremely rapid. 

This is an indirect titration. Peroxodisulfate ions cannot be directly oxidized with ceric ions [ °(S20g /S04 ) = 2.01 V]. From another standpoint, they cannot be directly titrated with ferrous ions since the following reaction is too slow ... 

Although gravimetric methods have been used traditionally for the determination of large amounts of tellurium, more accurate and convenient volumetric methods are favored. The oxidation of teUurium(IV) by ceric sulfate in hot sulfuric acid solution in the presence of chromic ion as catalyst affords a convenient volumetric method for the determination of tellurium (32). Selenium(IV) does not interfere if the sulfuric acid is less than 2 N in concentration. Excess ceric sulfate is added, the excess being titrated with ferrous ammonium sulfate using o-phenanthroline ferrous—sulfate as indicator. The ceric sulfate method is best appHed in tellurium-rich materials such as refined tellurium or tellurium compounds. 

There is no redox couple in solution at the start of the ferrous-ceric titration because the solution contains only Fe ". The oxidation of ferrous to ferric occurs as soon as an aliquot of ceric ions enter the solution to effect the redox reaction shown in equation (4.1). The bulk of the initially present ferrous ions remain, with the ferric products of the redox reaction residing in the same solution, i.e. a Fe " ", Fe + redox couple is formed. This couple has the electrode potential Epf + pg2+. 

An acidic solution of tellurium (IV) or tellurium (VI) is treated with sulfur dioxide and hydrazine hydrochloride. Tellurium precipitated from solution can be estimated by gravimetry. Selenium interferes with this test. A volumetric test involves converting tellurium to tellurous acid and oxidizing the acid with excess ceric sulfate in hot sulfuric acid in the presence of Cr3+ ion as catalyst. The excess ceric sulfate is measured by titration with a standard solution of ferrous ammonium sulfate. 

Some of the volumetric methods described above may also be adapted to the electrometric determination of arsenic. Such methods have been described for titration of arsenites with ceric sulphate,9 iodine in the presence of sodium bicarbonate,10 chloramine (p-toluene-sulphone chloramide),11 alkaline potassium ferricyanide solution,12 potassium bromate13 or potassium iodate14 in the presence of hydrochloric acid, silver nitrate15 (by applying a secondary titration with 01N alkali to maintain the desired low H+-ion concentration), and with... 

Determination of Adsorbed Ceric Ion. In determining the amount of ceric ion adsorbed on the graft copolymers the volumetric titration was conducted. Here, a given excess amount of ferrous sulfate solution (0.01 N) was added into the aqueous solution of hydrolysis and then the solution was back-titrated with 0.01 IT ceric ammonium sulfate in a 1 IT sulfuric acid solution using o-phenanthroline as an indicator. In so doing the amount of ceric ion adsorbed on the grafted bamboo sample could thus be determined. 

The peracetic acid content of the reagent may be determined by adding an aliquot to an equal volume of ice and water and titrating first with ceric sulfate solution until the orange color of the ceric ion remains (to eliminate hydrogen peroxide) and then adding potassiiun iodide and titrating with standard sodium thiosulfate solution. 

In this procedure [3, 12, 13] a known excess of standard ceric ammonium nitrate solution is added to an azide solution or slurry. The excess ceric ammonium nitrate is titrated with standard ferrous ammonium sulfate or sodium oxalate, using ferroin as indicator. The method is extremely simple and flee from hazard once the reagents have been mixed. A serious drawback is that dextrin and polyvinyl alcohol are oxidized by ceric ion. Blay [1] reports gelatin and carboxymethyl cellulose are not oxidized. The method is as follows. 

The latter observation was reaffirmed by determining the average valence of the molybdena in a catalyst (8.7 % M0O3) that had been reduced in a flow of hydrogen for 60 hrs. at 480° and 1 atm. A wet-chemical method of analysis was used in which the reduced molybdenum was oxidized by ceric sulfate, excess ceric ion being back titrated with ferrous sulfate. It was found that the average valence of the molybdenum corresponded to M0O2.36 -... 

Suppose it is desired to titrate ceric ion with ferrous ion according to the reaction... 

The number-average molecular weights of starches have been determined by a procedure that involves oxidation of the polysaccharide with potassium hexa-cyanoferrate solution and potentiometric titration of ferrous ions with ceric sulphate. ... 

Poly(acrylic acid) (PAA) was grafted onto the ultra-fine cellulose fiber surfaces via ceric ion initiation (Scheme 5). The carboxylic acid concentration could be controlled by varying monomer AA and/or ceric ion concentrations, and was determined by NaOH-HCl titration using phenophthlein as the indicator. Unless otherwise indicated, fibers with 3.6 mmol of COOH per g cellulose (lAA]/[Ce(IV)]-30) were used for enzyme adsorption and assay. The PAA activated cellulose fibers were immersed in 1.0 mg/ml lipase solution (pH 7.0) at room temperature for 24 hrs. The lipase adsorbed cellulose fibers were rinsed by pH 7 buffer and deionized water to remove loose lipase, then dried under vacuum at room temperature for 12 hrs. 

Reduction of Sn to Sn, followed by titration of the Sn with a standard I2 solution, is a standard volumetric procedure for tin which may be found in any quantitative inorganic textbook as well as in a number of literature references. 55,24, 125,34,176, 136 volumetric procedures for tin involve the use of ceric ion, " chromous ion,hy-... 

At first, we must again mention some usual species that quantitatively oxidize iodide ions to give iodine. They permit the extemporaneous standardization of thiosulfate solutions (see Sect. 18.8). It is the case of dichromate, ceric, hexacyanoferrate(III), permanganate, and peroxodisulfate ions. Quite evidently, they can be titrated by indirect iodometry. This is also the case with iodate ions, which is the matter of Chap. 19. 

Oxidizations with Permanganate, Dichromate, and Ceric Ions Some Titration Methods Involving a Reduction Reaction... 

We begin this chapter with the study of three important titration methods involving quantitative oxidization reactions. They are based on oxidizations with permanganate, dichromate, and ceric ions. These methods are sometimes (notably in the french literature) called manganimetry, chromimetry, and cerimetry, respectively. Next, we very briefly mention three titration methods involving reduction reactions. 

When two indicator electrodes are used, the potential difference between them is plotted as a function of titrant concentration. The shapes of the individual titration curves vary with the degree of irreversibility of the couples as shown in Fig. 6.9. The titration of ferrous ion by ceric ion gives a curve of the type shown in Fig. 6.9a and the reactions occurring at various points on the curve will be briefly considered. 

N ceric sulphate solution. Add 5 g of the ceric sulphate to 35 ml of ION sulphuric acid and warm for ten minutes to effect solution. Dilute to 100 ml with redistilled water and filter through a Whatman No. 42 paper. Standardise by adding 0 5 ml of standard iodide solution B (=0-5 mg iodide) to 10 ml of the ceric sulphate solution and titrating at 40° with 0 15N arsenious acid reagent (below) to a colourless endpoint. Adjust the solution to O IN w4th 3-5N sulphuric acid. 

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