Precipitation EDTA titration

Zirconium is often deterniined gravimetrically. The most common procedure utilizes mandelic acid (81) which is fairly specific for zirconium plus hafnium. Other precipitants, including nine inorganic and 42 organic reagents, are Hsted in Reference 82. Volumetric procedures for zirconium, which also include hafnium as zirconium, are limited to either EDTA titrations (83) or indirect procedures (84). X-ray fluorescence spectroscopy gives quantitative results for zirconium, without including hafnium, for concentrations from 0.1 to 50% (85). Atomic absorption determines zirconium in aluminum in the presence of hafnium at concentrations of 0.1—3% (86). 

Potentiometric titrations - continued EDTA titrations, 586 neutralisation reactions, 578, 580 non-aqueous titrations, 589, (T) 590 oxidation-reduction reactions, 579, 581, 584 precipitation reactions, 579, 582 Potentiometry 548 direct, 548, 567 fluoride, D. of, 570 Potentiostats 510, 607 Precipitants organic, 437 Precipitate ageing of, 423 digestion of, 423... 

Definitions. Titrimetric Reactions. Acid-base Titrations. Applications of Acid-base Titrations. Redox Titrations. Applications of Redox Titrations. Complexometric Titrations. Ethylenediaminetetraacetic Acid (EDTA). Applications of EDTA Titrations. Titrations with Complexing Agents Other Than EDTA. Precipitation Titrations. ... 

EDTA titration solutions frequently need to be buffered to a high pH to ensure stoichiometric formation of the complex. Unfortunately many metals will precipitate as hydroxides or hydrated oxides under these conditions. Use can be made of secondary or auxiliary complexing agents to retain the metal ion in solution. Typical reagents for this purpose are... 

If the analyte metal ion forms a stable EDTA complex rapidly, and an end point can be readily detected, a direct titration procedure may be employed. More than thirty metal ions may be so determined. Where the analyte is partially precipitated under the reaction conditions thereby leading to a slow reaction, or where a suitable indicator cannot be found, back titration procedures are used. A measured excess of EDTA is added and the unreacted EDTA titrated with a standard magnesium or calcium solution. Provided the analyte complex is stronger than the Ca-EDTA or Mg-EDTA complex a satisfactory end point may be obtained with eriochrome black T as indicator. An alternative procedure, where end points are difficult to observe, is to use a displacement reaction. In this case, a measured excess of EDTA is added as its zinc or magnesium complex. Provided the analyte complex is the stronger, the analyte will displace the zinc or magnesium. 

EDTA titrations are routinely used to determine water hardness in a laboratory. Raw well water samples can have a significant quantity of dissolved minerals that contribute to a variety of problems associated with the use of such water. These minerals consist chiefly of calcium and magnesium carbonates, sulfates, etc. The problems that arise are mostly a result of heating or boiling the water over a period of time such that the water is evaporated, and the calcium and magnesium salts become concentrated and precipitate in the form of a scale on the walls of the container, hence the term hardness. This kind of problem is evident in boilers, domestic and commercial water heaters, humidifiers, tea kettles, and the like. 

Figure 12-14 Guide to EDTA titrations of common metals. Light color shows pH region in which reaction with EDTA is quantitative. Dark color shows pH region in which auxiliary complexing agent is required to prevent metal from precipitating. Calmagite is more stable than Eriochrome black T (EB) and can be substituted for EB. [Adapted from K. Ueno, Guide tor Selecting Conditions of EDTA Titrations." J. Chem. Ed. 1965,42,432.]...

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). 

The number of reversible metal-metal ion electrodes is limited so that the accurate direct potentiometric measurement of the activity of a metal ion with an electrode of the same metal usually is not feasible, except perhaps with the Ag/Ag,(OH2)4 system. However, a number of metal ion-metal half-reactions are sufficiently reversible to give a satisfactory potentiometric titration with a precipitation ion or complexing agent. These couples include Cuu(OH2>6+/Cu, Pbn(OH2>4+/Pb, Cdu(OH2)l+/Cd, and Znn(OH2)i+/Zn. However, all these metals can be determined by EDTA titration and the mercury electrode that is described in the preceding section. 

Often, auxiliary complexing agents must be used in EDTA titrations to prevent precipitation of the analyte as a hydrous oxide. Such reagents cause the end points to be less sharp. 

Edta titration of A1 in A1 alloys may be carried out using iV-phenylan-thranilic acid as precipitant. ... 

Three general methods for performing EDTA titrations are (1) direct titration, (2) back titration, and (3) displacement titration. Method (1) is simple, rapid, but requires one standard reagent. Method (2) is advantageous for those metals that react so slowly with EDTA as to make direct titration inconvenient. In addition, this procedure is useful for cations for which satisfactory indicators are not available. Finally, it is useful for analyzing samples that contain anions that form sparingly soluble precipitates with the analyte under analytical conditions. Method (3) is particularly useful in situations where no satisfactory indicators are available for direct titration. 

Precipitated bismuth, is determined by EDTA titration after... 

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. 

Dissolve 0-2 g in 50 ml water and 2 ml hydrochloric acid, add 25 ml of dilute solution of ammonium acetate (7-2 per cent) and a slight excess of ammonium oxalate. After coagulating the precipitate by heat as usual, filter, wash, suspend in water acidified with dilute sulphuric acid, heat to 60 and titrate with 0-1N permanganate. 1 ml = 0 00517 g Ca3(P04)2. For EDTA titration ... 

The following method may be used for determination of the calcium. The total calcium and magnesium is determined in an aliquot by EDTA titration, the calcium precipitated as oxalate from a further aliquot and the residual magnesium titrated to give the calcium by difference. 

For lead, the EDTA titration given under lead acetate is also satisfactory using about 2 g. 1 ml 0-05M — 0 01036 g Pb. The alkalinity must then be done on a separate portion of the sample by precipitation with oxalic acid as above. 

Solution of Mercuric Chloride, B.Vet.C. A 0 1 per cent solution in water. This is assayed officially by the Rupp method, using 200 ml of solution, but it is better estimated gravimetrically by precipitation with hydrogen sulphide, HgS X 1 167 = HgClg, or by EDTA titration. 

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