Oxalate titration

The so-called Ling oxalate titration indicates that CCP consists of 80% Ca3(P04)2 and 20% CaHP04, with an overall Ca P ratio of 1.4 1 (Pyne, 1962). However, the oxalate titration procedure has been criticized because many of the assumptions made are not reliable. Pyne and McGann (1960) developed a new technique to study the composition of CCP. Milk was acidified to about pH 4.9 at 2°C, followed by exhaustive dialysis of the acidified milk against a large excess of bulk milk this procedure restored the acidified milk to normality in all respects except that CCP was not reformed. Analysis of milk and CCP-free milk (assumed to differ from milk only in respect of CCP) showed that the ratio of Ca P in CCP was 1.7 1. The difference between this value and that obtained by the oxalate titration (i.e. 1.4 1) was attributed to the presence of citrate in the CCP complex, which is not measured by the oxalate method. Pyne and McGann (1960) suggested that CCP has an apatite structure with the formula ... 

Uranyl oxalate actinometer. This actinometer has a range of 208-435 nm with an average quantum yield of about 0.5. Since the UO + ion acts as a photosensitizer for the oxalate decomposition the light absorption remains constant, but rather long exposures. re required for final accurate oxalate titrations. It is now mainly of histo al interest. 

The hydrogen peroxide problem When oxygen of the air is present, the reactions are more complicated H2O2 is formed during the permanganate-oxalate titration according to the overall reaction... 

It is now out of use due to low quantum yield ( 0.5) and other drawbacks. The actinometer works in wavelength range from 208 to 435 nm. The ion acts as a photosensitiser for oxalate decomposition. The light absorption remains constant by required long exposure to final accurate oxalate titration. 

The calcium chelation value of a substance can be determined by an oxalate titration. In this method [12], a sample of the product to be tested is added to a sodium oxalate solution at pH 10 to 12. The mixture is then titrated with a solution containing calcium ions. When the chelate becomes depleted the endpoint can be noted by the precipitation of insoluble calcium oxalate. The calcium chelation value (CV) is reported as mg of calcium chelated per gram of chelate. The CV of Na LED3 A was determined at a range of concentrations. The results are presented in Figure 7. EDTA chelates 260 mg CaCOj/g irrespective of system concentration. 

Even though the mechanism of the reaction between oxalate and permanganate is extremely compHcated, titration under acidic conditions is extremely accurate. This is the recommended method for standardi2ation of permanganate solutions. 

Quantitative Analysis. OxaUc acid is precipitated as calcium oxalate from a solution containing oxaUc acid, and the calcium oxalate obtained is then weighed. If there are no organic substances other than oxaUc acid present, oxaUc acid can be titrated quantitatively with potassium permanganate. 

Oxalate Acid Number. A metal soap solution is treated with a measured excess of organic acid. Potassium oxalate solution is added to precipitate the metal and the total sample is back-titrated with alkaU to determine its acidity. Acidity is expressed ia acid number units, equivalent to mg KOH per g. A neutral soap gives a 2ero acid number, an acidic soap solution a positive acid number, and a basic soap solution a negative acid number. 

Dry air is blown through the solution to remove the excess of ammonia, and the solution is then dissolved in its own volume of absolute alcohol. A sample of this solution is titrated with standard oxalic acid, litmus being used as an outside indicator (Note 3). The amount of oxalic acid (Note 4) necessary to form the acid salt is placed in a large evaporating dish and dissolved in 4 1. of 95 per cent alcohol. The amine solution is then slowly run into the acid with constant stirring. During the addition of the last half of the amine solution, the container must be cooled in order to avoid the formation of the neutral oxalate,... 

The process is one of electrolytic reduction and the apparatus is similar to that shown in Fig. 77, p. 144. It consists of a small porous cell (8 cm. x 2 cm. diam.) surrounded by a narrow beaher (ii cm. X 6 cm. diam.). The oxalic acid, mixed w lth too c.f. 10 per cent sulphuric acid (titrated against standard baryl.a solution] forms the cathode liquid and is placed in Iht beakei. The porous cell is filled with the same strength of siilphuiic acid and foims the anode liquid. The electrodes ara made from 01 dinary clean sheet lead. The anode consists of i thiu strip projecting about two inches from the cell and tliu... 

An example of catalytic action is provided by the titration of oxalates with potassium permanganate solution referred to above. It is found that even though the oxalate solution is heated, the first few drops of permanganate solution are only slowly decolorised, but as more permanganate solution is added the decoloration becomes instantaneous. This is because the reaction between oxalate ions and permanganate ions is catalysed by the Mn2+ ions formed by the reduction of permanganate ions ... 

With a knowledge of the pH at the stoichiometric point and also of the course of the neutralisation curve, it should be an easy matter to select the appropriate indicator for the titration of any diprotic acid for which K1/K2 is at least 104. For many diprotic acids, however, the two dissociation constants are too close together and it is not possible to differentiate between the two stages. If K 2 is not less than about 10 7, all the replaceable hydrogen may be titrated, e.g. sulphuric acid (primary stage — a strong acid), oxalic acid, malonic, succinic, and tartaric acids. 

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. 

Notes. (1) For elementary students, it is sufficient to weigh out accurately about 1.7 g of sodium oxalate, transfer it to a 250 mL graduated flask, and make up to the mark. Shake well, Use 25 mL of this solution per titration and add 150mL of ca 1M sulphuric acid. Carry out the titration rapidly at the ordinary temperature until the first pink colour appears throughout the solution, and allow to stand until the solution is colourless. Warm the solution to 50-60 °C and continue the titration to a permanent faint pink colour. It must be remembered that oxalate solutions attack glass, so that the solution should not be stored more than a few days. 

An approximate value of the volume of permanganate solution required can be computed from the weight of sodium oxalate employed. In the first titration about 75 per cent of this volume is added, and the determination is completed at 55-60 °C. Thereafter, about 90-95 per cent of the volume of permanganate solution is added at the laboratory temperature. 

The excess of oxalic acid is titrated with standard potassium permanganate solution. 

Weigh out accurately 0.3-0.4 g potassium persulphate into a 500 mL conical flask, add 50 mL of 0.05 M-oxalic acid, followed by 0.2 g of silver sulphate dissolved in 20 mL of 10 per cent sulphuric acid. Heat the mixture in a water bath until no more carbon dioxide is evolved (15-20 minutes), dilute the solution to about 100 mL with water at about 40 °C, and titrate the excess of oxalic acid with standard 0.02 M potassium permanganate. 

Weigh out accurately about 0.2 g sodium oxalate into a 250 mL conical flask and add 25-30 mL 1M sulphuric add. Heat the solution to about 60 °C and then add about 30 mL of the cerium(IV) solution to be standardised dropwise, adding the solution as rapidly as possible consistent with drop formation. Re-heat the solution to 60 °C, and then add a further 10 mL of the cerium(IV) solution. Allow to stand for three minutes, then cool and back-titrate the excess cerium(IV) with the iron(II) solution using ferroin as indicator. 

Oscillometry 527 as analytical tool, 528 titrations (H.F.), 527 Osmium tetroxide catalyst 381 Ostwald s dilution law 31 Ovens electric, 97 microwave, 97 Overpotential 506 Overvoltage see Overpotential Oxalates, D. of as calcium carbonate via oxalate, (g) 484... 

K permanganate by titration into Na oxalate/ dil sulfuric acid soln... 

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