Oxalate precipitation titration

Thereafter, analysis was done by faster techniques based on oxalate-permanganate titrations which involved a precipitation procedure followed by quick titration of pre-heated solutions containing oxalate ions. 

Complexometric titration with EDTA is the usual winery procedure for determining calcium (4, 6, 22, 113), but atomic absorption spectrophotometry (51,53,112) and flame photometry and a rapid micro method based on oxalate precipitation (114) have been used successfully. 

Rare earths Oxalate to precipitate metal ions Precipitate titrated with KMn04 KMn04 titration [123]... 

The calcium in a 5.00-mL serum sample is precipitated as CaC204 with ammonium oxalate. The filtered precipitate is dissolved in acid, the solution is heated, and the oxalate is titrated with 0.00100 M BQVln04, requiring 4.94 mL. Calculate the concentration of calcium in the serum in meq/L (equivalents based on charge). 

Applications of precipitation titrations are listed in Table 6-1. The Volhard method is an argentometric titration, but the Fajans method has wider application. Because the Volhard titration is carried out in 1 M acid, it avoids some interference that affects other titrations. Silver salts of anions such as C03 (carbonate), C204 (oxalate), and AsO (arsenate) are soluble in acidic solution, so these anions do not interfere with the analysis. 

The former furnished an obvious check on accuracy, since no accuracy greater than that achieved iyy titrating the titrant could be expected. The determinations listed in Table 11 are representative. The results agree favorably with those obtained by Toribara and Koral [7] (using visual detection of the fluorescence end point and oxalate precipitation of calcium), and by Ashby and Roberts [8] (who used Lab-trol without oxalate precipitation of calcium and visual detection of fluorescence end point). 

Calcium may be determined by atomic absorption spectrophotometry. This technique has been used for the estimation of calcium in biological fluids and agricultural materials. It is a speedy method, superior to the more tedious chemical determination by oxalate precipitation and more specific than the EDTA titration method. The accuracy is of the order of 2 per cent and sensitivity limits have been reported at 0 08 to 1 p.p.m. of calcium in solution. The interference problems are very similar to those experienced with the emission method although not quite so formidable. 

M EDTA 0-01712 g can be assayed for their calcium content either by precipitation as oxalate and titration with permanganate or by EDTx titration, both described above. 

The combined filtrate and washings from the oxalate precipitation (calcium estimation) are diluted to a known volume (say, 250 ml). 50 ml of this solution is titrated with a standard solution of EDTA at a pH around 10, using Eriochrome Black-T as indicator (3.3.2). The results are reported as Magnesium oxide. 

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. 

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. 

Telluric acid may also be estimated alkalimetrieally 2 by the addition of a large excess of standard barium hydroxide, or sodium hydroxide containing barium chloride, when barium tellurate is quantitatively precipitated. The excess of the hydroxide is determined by titration with oxalic acid, phenolphthalein being used as indicator. 

Solutions of telluric acid give a quantitative precipitation of barium tellurate, BaTe04.3H20, on the addition of barium hydroxide solution, and the use of a standard barium hydroxide solution, followed by titration of the excess of alkali with a standard solution of oxalic acid, using phenolphthalein as indicator, forms a convenient process for... 

Precipitate the metal oxalate. Dissolve in acid and titrate the H2C204. 

A 50.00-mL sample containing La3+ was treated with sodium oxalate to precipitate La2(C204)3, which was washed, dissolved in acid, and titrated with 18.04 mL of 0.006 363 M KMn04. Calculate the molarity of La3+ in the unknown. 

Magnesium, formerly determined by precipitation as magnesium ammonium phosphate and determining P in the latter, can be analyzed readily by EDTA titrations. It can be obtained either as the difference between titrations for (Ca and Mg) and Ca alone or by titrating the supernatant after Ca is precipitated as oxalate (White and Davies 1962). 

Fractionation of milk and titration of the fractions have been of considerable value. Rice and Markley (1924) made an attempt to assign contributions of the various milk components to titratable acidity. One scheme utilizes oxalate to precipitate calcium and rennet to remove the calcium caseinate phosphate micelles (Horst 1947 Ling 1936 Pyne and Ryan 1950). As formulated by Ling, the scheme involves titrations of milk, oxalated milk, rennet whey, and oxalated rennet whey to the phenolphthalein endpoint. From such titrations, Ling calculated that the caseinate contributed about 0.8 mEq of the total titer of 2.2 mEq/100 ml (0.19% lactic acid) in certain milks that he analyzed. These data are consistent with calculations based on the concentrations of phosphate and proteins present (Walstra and Jenness 1984). The casein, serum proteins, colloidal inorganic phosphorus, and dissolved inorganic phosphorus were accounted for by van der Have et al (1979) in their equation relating the titratable acidity of individual cow s milks to the composition. The casein and phosphates account for the major part of the titratable acidity of fresh milk. 

Lime.—This could be determined by precipitation with ammonium oxalate and subsequent titration with permanganate, but in practice use is generally made of Pellet s method, which is carried out as follows ... 

In the analytical determination of the hyponitrites, W. Zorn treated soln. of the free acid, or of its salts in cold dil. nitric acid with an excess of silver nitrate, and just neutralized the free acid with ammonia or sodium carbonate. The washed and dried precipitate was weighed as silver hyponitrite, or weighed as metal or chloride. A. Thum treated the aq. soln. of the acid with an excess of a soln. of permanganate, and in 15 minutes added sulphuric acid after standing for another hour the soln. was warmed to 30°, and treated with a known quantity of 0-1A-oxalie acid sufficient to decolorize the permanganate. The excess oxalic acid was determined by permanganate titration. A. Hantzsch and A. Sauer, and... 

A 0.518-g sample of limestone is dissolved, and then the calcium is precipitated as calcium oxalate, CaC2C>4. The precipitate is filtered, washed, and dissolved in sulfuric acid solution. It then requires 40.0 mL of 0.0500 M KMn( >4 solution to titrate it. What is the percent CaO in the limestone The unbalanced equation for the titration is... 

The first of these, utilized by Yoder, McCalip and Seibert,34 and by Balch, Broeg and Ambler,37 provides for the extraction of the aconitic acid from the sample being investigated, usually with diethyl ether, and the subsequent isolation of the acid from the solvent. In dealing with solid samples, e.g. alkaline earth aconitates, evaporator scale, etc., the prescribed procedure is to dissolve the material in aqueous mineral acid and to extract the acid solution exhaustively with ether. The ether extract is then evaporated under reduced pressure, the dried residue titrated with standard alkali and the titratable acid calculated as aconitic acid. In dealing with such solid samples it is often necessary to make an additional determination for oxalic acid which otherwise would be assumed to be aconitic acid.37 The aconitic acid in liquid samples is usually precipitated as the insoluble lead salt which is separated and treated as any other solid sample. In some cases this procedure is unnecessary and the liquid samples are merely acidified with a mineral acid and then extracted with ether.37 This method for the determination of aconitic acid, however, requires a considerable amount of time and is further complicated by the interference of ether-soluble waxes and non-volatile acids. 

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