Zinc with EDTA

Transition ranges for magnesium and calcium are indicated on the titration curves in Figure 17-6. As can be seen, the indicator is ideal for the titration of magnesium but quite unsatisfactory for calcium. Note that the formation constant for Cain" is only about that for Mgin. As a consequence, significant conversion of Cain" to Hln- occurs well before equivalence. A similar calculation. shows that Eriochrome Black T is also well suited for the titration of zinc with EDTA (see Figure 17-10). 

Figure 7.2 Chemical speciation for a titration of a solution containing lead and zinc ions with EDTA. Speciation of EDTA in the first stage of the titration of 10.0 ml of 50.0 mmol dm lead + 10.0 ml of 50.0 mmol dm zinc with EDTA (25.0 mmol dm ) at pH = 5.25...

Alum interferes with the direct titration of zinc with EDTA. Triethanolamine has been used to mask aluminium in the determination of magnesium by complexometric titration" and a similar method is used in the B.P.C. for the determination of zinc oxide in this preparation. 

BackTitrations. In the performance of aback titration, a known, but excess quantity of EDTA or other chelon is added, the pH is now properly adjusted, and the excess of the chelon is titrated with a suitable standard metal salt solution. Back titration procedures are especially useful when the metal ion to be determined cannot be kept in solution under the titration conditions or where the reaction of the metal ion with the chelon occurs too slowly to permit a direct titration, as in the titration of chromium(III) with EDTA. Back titration procedures sometimes permit a metal ion to be determined by the use of a metal indicator that is blocked by that ion in a direct titration. Eor example, nickel, cobalt, or aluminum form such stable complexes with Eriochrome Black T that the direct titration would fail. However, if an excess of EDTA is added before the indicator, no blocking occurs in the back titration with a magnesium or zinc salt solution. These metal ion titrants are chosen because they form EDTA complexes of relatively low stability, thereby avoiding the possible titration of EDTA bound by the sample metal ion. 

The method may also be applied to the analysis of silver halides by dissolution in excess of cyanide solution and back-titration with standard silver nitrate. It can also be utilised indirectly for the determination of several metals, notably nickel, cobalt, and zinc, which form stable stoichiometric complexes with cyanide ion. Thus if a Ni(II) salt in ammoniacal solution is heated with excess of cyanide ion, the [Ni(CN)4]2 ion is formed quantitatively since it is more stable than the [Ag(CN)2] ion, the excess of cyanide may be determined by the Liebig-Deniges method. The metal ion determinations are, however, more conveniently made by titration with EDTA see the following sections. 

Discussion. Minute amounts of beryllium may be readily determined spectrophotometrically by reaction under alkaline conditions with 4-nitrobenzeneazo-orcinol. The reagent is yellow in a basic medium in the presence of beryllium the colour changes to reddish-brown. The zone of optimum alkalinity is rather critical and narrow buffering with boric acid increases the reproducibility. Aluminium, up to about 240 mg per 25 mL, has little influence provided an excess of 1 mole of sodium hydroxide is added for each mole of aluminium present. Other elements which might interfere are removed by preliminary treatment with sodium hydroxide solution, but the possible co-precipitation of beryllium must be considered. Zinc interferes very slightly but can be removed by precipitation as sulphide. Copper interferes seriously, even in such small amounts as are soluble in sodium hydroxide solution. The interference of small amounts of copper, nickel, iron and calcium can be prevented by complexing with EDTA and triethanolamine. 

Single-phase ZnS films of a fine grain size (no XRD shown) and a band gap of 3.7 eV were electrodeposited from aqueous alkaline (pH 8-10) solutions of zinc complexed with EDTA, and thiosulfate as a sulfur source [101]. The voltammet-ric data implied that deposition occurred either by S-induced UPD of Zn or by a pathway involving both Zn " and thiosulfate concurrently. 

M with respect to ammonia. Can zinc be titrated quantitatively with EDTA solution at this pH ... 

Xue, H.-B. and Sigg, L. (1994). Zinc speciation in lake waters and its determination by ligand exchange with EDTA and differential pulse anodic stripping voltammetry, Anal. Chim. Acta, 284, 505-515. 

However, even with a molar phytic acid to zinc ratio of 22.3, average balances were positive (Table II). In contrast, at a much lower level of phytic acid intake, Cossack and Prasad (41) observed negative zinc balances with a soy protein diet. In the latter study, the soy products were "washed" with EDTA the possibility exists that some residual EDTA remained which could have affected mineral utilization. 

Titration with EDTA is used in the pharmacopoeial assays of bismuthsubcarbonate, calcium acetate, calcium chloride, calcium gluconate, magnesium carbonate, magnesium hydroxide, magnesium trisilicate, bacitracin zinc, zinc chloride and zinc undecanoate. 

Using vesicles where the amphiphilic chromophore is embedded only in the outer wall, electron transfer across the bilayer is not observed in the absence of mediators whether the EDTA is in the inner pools with MV2+ in the bulk phase or vice versa.338 339 With EDTA in the inner pools, electron transfer can, however, be effected on addition of, for example, ubiquinone, ZnTPP or H2TPP to the bilayer. In the last two cases, electron transfer is much more efficient than when the amphiphilic zinc porphyrin is omitted.339... 

Nerve growth factor is commonly found in snake venoms. This is a protease with specificity for lysyl and arginyl bonds, which contains one Zn2+ per molecule. The function of the zinc is to prevent the activation of NGF, presumably through control of the structure. Removal of Zn11 from NGF with EDTA leads to activation of the zymogen. 

Hirose, K., Dokiya,Y. and Sugimura, Y., 1982. Determination of conditional stability constants of organic copper and zinc complexes dissolved in seawater using ligand exchange method with EDTA. Mar. Chem., 11 343-354. 

In the back-titration method, a measured amount of an excess standard EDTA solution is added to the sample. The analyte ion combines with EDTA. After the reaction is complete, the excess EDTA is back-titrated against a standard solution of magnesium or zinc ion. Eriochrome Black T or Calmagite is commonly used as an indicator. After all the remaining EDTA chelates with Mg2+ or Zn2+, ary extra drop of the titrant solution imparts color to the indicator signifying the end point. The cations that form stable complexes or react slowly with EDTA can also be measured by the back-titration method. 

Polymerization. Monomers. The cyclopropane type monomers are prepared either by addition of the dichlorocarbene or by the Simmons-Smith reaction on the corresponding olefins. Most of these compounds have been described. Spiropentane is prepared by the Applequist method (I, 2), by the reaction of zinc with C(CH2Br)4 in alcohol in the presence of ethylenediaminetetraacetic acid (EDTA). This hydrocarbon is purified until a single NMR signal is obtained at t = 9.28. 

An on-stream atomic absorption method for the determination of zinc and manganese in flotation liquors containing calcium sulphate has been reported [27]. The nebuliser to an air/acetylene flame was continuously fed, by gravity, with a portion of the process stream buffered with EDTA to overcome calcium sulphate interference. The atomic absorption monitor operated continuously for 3h before salting-up of the burner occurred. 

Calculate the conditional formation constant for the Zn-EDTA complex at pH 9.0. in a solution 0.100 M with respect to ammonia. Can zinc be titrated quantitatively with EDTA solution at this pH ... 

Assume that log P4, for chloride, bromide, and iodide with Zn is — 1, — 0.74, and — 1.25 and with Cd" " is 0.9, 2.53, and 6.1. (a) Calculate the masking index for Zn" " and Cd" " in 1.0 Af chloride, 1.0 Af bromide, and 0.1 Af and 1.0 Af iodide. Assume activity coefficients of unity and that the metal ions are present predominately as their highest complexes, (b) Assuming that the conditional formation constants of Zn" " and Cd with EDTA are the same (Figure 11-3), which condition is most favorable for the titration of zinc in the presence of cadmium ... 

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