Oxalate titration 5-Oxidation

Sodium oxalate has been used as a primary standard substance for Ce(IV) in sulfuric acid. In the absence of a catalyst a temperature of 70 to 75°C is necessary. Smith and Getz found that in 1 to 2 M perchloric acid solution, sodium oxalate can be titrated at room temperature with Ce(IV) perchlorate or nitrate but not with sulfate. Rao, Rao, and Rao carried out the titration at room temperature in the presence of barium chloride to remove sulfate, which retards the reaction between oxalate and Ce(IV) and between oxalate and oxidized ferroin. Alternatively, some Fe(III) was added, and the trace of Fe(II) produced photochemically then reacted with the indicator. Rao, Rao, and Murty carried out the titration in 0.5 M HNOj with ammonium hexanitratocerate(IV) instead of the sulfate. With a small amount of KI and KIO3, a satisfactory end point was obtained at room temperature with ferroin as indicator. 

Titrations based on oxidation-reduction reactions enjoy wide use. Permanganate, dichromate, and iodine and iron(II), tin(II), thiosulfate, and oxalate are commonly used oxidizing and reducing titrants, have been employed to determine components in both inorganic and organic analysis. As we saw in Chapter 7, solvent water does not play as central a role as in acid-base titrations. Oxidants or reductants strong enough to decompose water are not practical as titrants. 

The strength (concentration) of a titrant can also be expressed as its titer. The titer of a titrant is defined as the weight (in milligrams) of substance titrated that is consumed by 1 mL of the titrant. Thus it is specific to a particular substance titrated, meaning that it is expressed with respect to a specific substance titrated. For example, if the analyst is using an oxalic acid solution to titrate a solution of calcium oxide, the titer of the oxalic acid solution would be expressed as its CaO titer, or the weight of CaO that is consumed by 1 mL of the oxalic acid solution. Titer is typically used for repetitive routine work in which the same titrant is used repetitively to titrate a given analyte. 

What is the titer (expressed in milligrams per milliliter) of a solution of oxalic acid dihydrate with respect to calcium oxide if 21.49 mL of it was needed to titrate 0.2203 g of CaO ... 

Elemental composition Mn 63.19%, 0 36.81%. The pure oxide may be characterized by x-ray crystallography. The Mn02 content in pyrolusite may be measured by titration. An excess of a standard solution of oxalic acid is added to a solution of Mn02 in sulfuric acid. After all solid Mn02 dissolves, the excess oxalic acid is measured by titrating against a standard solution of potassium permanganate (see Reactions). 

Oxidation processes involving the subsequent titration of an excess of ferrous sulphate,2 oxalic acid (in the presence of silver sulphate as catalyst), titanous chloride,4 or of the quantity of iodine liberated from potassium iodide,5 are also available but are less satisfactory. In the last-named method a large excess of potassium iodide is necessary to obtain complete reaction in a short time. The reaction may be accelerated by the addition of potassium chloride6 or ammonium chloride with 20 per cent, by weight of the latter salt present a large excess of the iodide is not necessary and the liberated iodine may be titrated after fifteen minutes.7... 

Selenious acid readily decomposes potassium permanganate, but analytical results are untrustworthy in the presence of more than a limited quantity of sulphuric acid.1 The oxidation should be carried out at 50° C., a known quantity of 0-lN potassium permanganate being used and the excess determined either by means of standard oxalic acid solution or by electrometric titration with ferrous sulphate. In the presence of tellurium, the latter is also oxidised and should be determined in a separate sample by oxidation with potassium di-chromate, which does not oxidise the selenium, and the necessary deduction can then be made.2... 

Tellurous acid cannot be determined by oxidation with potassium permanganate in acidified solution, but in alkaline solution accurate results may be obtained by cooling to 8°-10° C. after the oxidation and slowly acidifying with dilute sulphuric acid, with continual stirring. Excess of standard oxalic acid is then added and after warming to 50° C. the remaining excess is titrated with permanganate.6... 

The violet complexes Mn(H2P 07)73 and Mn(C204)73 (both presumably chelates) are also known. The latter, when heated or illuminated strongly, undergoes an internal oxidation-reduction reaction, yielding C02 and an oxalate complex (or complexes) of Mn(II). The stability of the Mn(C204)73 complex at room temperature makes it necessary to carry out the familiar titration of oxalate with permanganate at elevated temperature (>60° 0). 

Although sodium oxalate is commonly used as a primary standard, its use is recommended only when oxalate is to be determined. For best absolute accuracy As(III) oxide is recommended. The direct titration of As(III) in acid solution does not proceed readily without a catalyst, probably because of the stabilization of Mn(III) by complex formation with As(V). With potassium iodate (1 drop of0.0025 M solution) as catalyst, the potentiometric end point was found to coincide with the visual end point, using ferroin, to within 0.01%, and the accuracy, tested against pure potassium iodide, was within 0.02%. 

Reaction (18-6) is considered to be the rate-limiting step. This is analogous to the oxidation of oxalate by permanganate (Section 17-2), where a critical intermediate is a complex between oxalate and Mn(III). The rate-limiting step is reversible in that the 204" radical can reoxidize the Ce(III) to Ce(IV). The concentration of sulfate controls the amount of the Ce(IV)-oxalate reactive intermediate formed, but not its specific rate of decomposition. Thus the error in the Fe(II)-Ce(IV) titration in the presence of oxalate can be decreased by the addition of sulfate. Phosphate also inhibits the Ce(IV)-oxalate reaction. 

Ammonium hexanitratocerate(IV) as a standard oxidant for reactions at room temperature has been developed for several substances. " Oxalate can be titrated with Ce(IV) in 0.5 M HNO3 with an iodide catalyst mandelic acid at a lower acid concentration, 0.1 MHNO3 Mn(II) with excess Ce(TV) in 0.5 to 2M HNO3 with silver nitrate catalyst hydrazine or isonicotinic acid in HCl-KBr solution and As(III) in HCl, HNO3, or H2SO4 solution with a trace of iodine as catalyst. 

Verma and Bhuchar determined copper by reducing its tartrate complex with glucose to form insoluble CujO, which was treated with an excess of standard iodine and back-titrated with standard As(III). Oxalate was added as a complexing agent to aid in the oxidation of the CU2O, and precautions were taken to avoid air oxidation. The method has the advantage of avoiding interference from V(V). For the determination of copper in alloys, Rooney and Pratt separated copper by precipitation as its diethyldithiocarbamate from EDTA solution. 

The oxidations usually are carried out at room temperature with an excess of periodic acid or its salts. The analyses can be completed by several means. The lUPAC procedure for glycerol involves an alkalimetric titration of the formic acid generated, using a pH of 8.0 at the end point. More commonly, the excess periodate is determined by adding a slight excess of standard As(III) to reduce periodate to iodate then the excess As(III) is titrated with standard iodine. Both iodate and periodate can be determined by first masking the periodate with molybdate while iodate is titrated and then demasking the periodate with oxalate, followed by titration of the periodate. ... 

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