Masking agent, EDTA

Probably the most extensively applied masking agent is cyanide ion. In alkaline solution, cyanide forms strong cyano complexes with the following ions and masks their action toward EDTA Ag, Cd, Co(ll), Cu(ll), Fe(ll), Hg(ll), Ni, Pd(ll), Pt(ll), Tl(lll), and Zn. The alkaline earths, Mn(ll), Pb, and the rare earths are virtually unaffected hence, these latter ions may be titrated with EDTA with the former ions masked by cyanide. Iron(lll) is also masked by cyanide. However, as the hexacy-anoferrate(lll) ion oxidizes many indicators, ascorbic acid is added to form hexacyanoferrate(ll) ion. Moreover, since the addition of cyanide to an acidic solution results in the formation of deadly... 

Show that CN is an appropriate masking agent for Nb+ in a method in which nickel s complexation with EDTA is an interference. 

Ni(CN)4 is greater than that for the Ni-EDTA complex. In fact, the equilibrium constant for the reaction in which EDTA displaces the masking agent... 

Show that F is an effective masking agent in preventing a reaction of AP+ with EDTA. Assume that the only significant forms of fluoride and EDTA are F and Y. ... 

Hydroxyquinoline, having both a phenolic hydroxyl group and a basic nitrogen atom, is amphoteric in aqueous solution it is completely extracted from aqueous solution by chloroform at pH < 5 and pH > 9 the distribution coefficient of the neutral compound between chloroform and water is 720 at 18 °C. The usefulness of this sensitive reagent has been extended by the use of masking agents (cyanide, EDTA, citrate, tartrate, etc.) and by control of pH. 

Many of the heavy metals give slightly soluble products (some white, some coloured) with the reagent, most of which are soluble in the organic solvents mentioned. The selectivity of the reagent may be improved by the use of masking agents, particularly EDTA. 

Discussion. Silver can be extracted from a nearly neutral aqueous solution into nitrobenzene as a blue ternary ion association complex formed between silver(I) ions, 1,10-phenanthroline and bromopyrogallol red. The method is highly selective in the presence of EDTA, bromide and mercury(II) ions as masking agents and only thiosulphate appears to interfere.8... 

By the use of masking agents, some of the cations in a mixture can often be masked so that they can no longer react with EDTA or with the indicator. An effective masking agent is the cyanide ion this forms stable cyanide complexes with the cations of Cd, Zn, Hg(II), Cu, Co, Ni, Ag, and the platinum metals, but not with the alkaline earths, manganese, and lead ... 

A masking agent is a reagent that protects some component of the analyte from reaction with EDTA. For example, Al3+ in a mixture of Mg2+ and Al3+ can be measured by first masking the Al3+ with F , thereby leaving only the Mg2+ to react with EDTA. 

Virtually all chelate complexes of the type M(Ox)m derived from the cation Mm+ are extractable into chloroform and, being coloured, form the basis of spectrophotometric determinations. The extractions can be made more selective by controlling the pH (cf. Section 10.2.2.1) and/or by using suitable masking agents. Thus extraction of the yellow aluminum trisoxinate can be made almost specific at pH 8.5—9.0 if EDTA, cyanide ions and sulfite ions (to reduce FeI I to Fe11) are present. Since the range for extraction of individual metal oxinates extends from pH 1.6 to 14, separation of individual species is facilitated.52 59... 

Another major step in many analyses is separation (Chapters 22 to 25). When, because of the method chosen or the nature of the sample, this unit operation is not required, much effort can be saved. For example, if a masking agent will complex an interfering metal ion in an EDTA titration, a separation step may be avoided. Where a separation is essential, a choice among several techniques is usually available. In general, separation involves the formation of two phases, physically separated, one containing the material of interest and the other the interference. Either phase may be a gas, liquid, or solid. Thus six major types of separation processes are possible. Once separation has been effected, the quantitative determination by physical means is often straightforward. 

Selectivity in spectrophotometric methods for determining metals with dithizone is attained by controlling the acidity of the medium and using masking agents such as cyanide, EDTA, thiosulphate, or iodide. 

Hydroxyquinoline is a group reagent often applied to the precipitation or extraction of a large number of metals. The selectivity of metal reactions with oxine may be enhanced by masking agents such as EDTA, tartrate, oxalate, or cyanide. 

Triphenylmethane and azo reagents are used in most methods for determining beryllium. Methods using Chrome Azurol S or Eriochrome Cyanine R and some cationic surfactants are very sensitive. The selectivity of methods for beryllium determination is improved by the use of EDTA as masking agent. 

Chrome Azurol S (CAS, formula 4.18) forms a coloured chelate complex with Be, and this has been used for the determination of Be [18-21], In acetate (or hexamine) buffer and in the presence of EDTA as masking agent, the Chrome Azurol S method is highly selective for beryllium. The absorbance of the complex depends on the pH, and on the concentrations of CAS, EDTA, and the acetate buffer. The absorbance increases with increasing CAS concentration, and decreases with increasing EDTA and acetate concentrations. A pH of 5 is the most suitable. Below this pH, the absorbance of CAS increases considerably, and above it the absorbance of the beryllium complex is decreased more by EDTA. 

Eriochrome Cyanine R (ECR) (formula 4.17) reacts with beryllium ions [4,9,10,16,30] similarly to Chrome Azurol S (see Section 9.2.1). At pH 9.7, A-max of ECR is 435 nm and that of its water-soluble beryllium complex is 525 nm. The molar absorptivity of the complex is 1.5 10 . EDTA, tartrate and cyanide are used as the main masking agents for interfering metals. In the presence of cationic surfactants, the sensitivity is increased several times, and significant bathochromic shifts are observed. In the case of CTA, e = 8.7-1 O at 590 nm (pH 7) [31,32]. Beryllium was also sorbed on anion exchange resins impregnated with ECR [33]. 

The main interfering metals in the copper determination are Fe, Bi, Mn, Ni, Co, Cr, Mo and U, which form coloured complexes. The selectivity of the method is considerably enhanced by the use of EDTA as a masking agent. In a tartrate or citrate medium at pH 8-9, Fe, Mn, Ni and Co are masked by EDTA, as are Cd, Pb, Zn, and In, which form colourless complexes with DDTC. Of the metals forming coloured compounds with DDTC, only Bi, Tl(III), and Cu are not masked. Thallium, when reduced to T1(I), does not interfere. Bismuth can be stripped from the organic extract, containing copper and bismuth diethyidithiocarbamates, with 5 M HCl. Copper diethyidithiocarbamate is decomposed by cyanide, whereas the bismuth complex remains unaffected. 

Phenanthroline and 2,2 -bipyridyl form complexes, although not intensely coloured, with Ru, Os, and Cu(I). Many metals e.g., Zn and Cd) can form colourless complexes with phenanthroline and bipyridyl, which are more stable than the corresponding Fe(II) complexes. When determining Fe in the presence of Zn or Cd, EDTA should be used as a masking agent [31]. Copper can be masked with triethylenetetramine [32]. 

Metal ions e.g., Fe, Co", Cu ) which form coloured, water-soluble complexes with HiDm interfere. However, the complexes of these metals are decomposed by EDTA, and a preliminary extraction as Ni(HDni)2 allows nickel to be isolated from Cu, Co, Fe, Cr, Al, and Mn. The presence of hydroxylamine ensures the quantitative extraction of nickel and prevents interference from Cu and Mn. In the presence of large quantities of Co and Fe the use of triethanolamine as masking agent is recommended [13]. 

If the sample solution contains Nb separated from other metals, no masking agents, including EDTA and tartrate, are necessary. In this case the sensitivity of the Nb reaction with BPR is higher (e = 6.0-10 ) [42],... 

The molar absorptivity of the toluene solution of piazselenol at 420 nm is 1.02-10" (sp. abs. 0.13). This method is specific for selenium. Tellurium does not react with DAB, but V(V) and Fe(III) oxidize DAB to give coloured oxidation products. Iron(III) can be masked with fluoride or phosphate. EDTA is used as masking agent to prevent the precipitation of metals in the neutral medium. Substances capable of reducing selenium to the element interfere in the determination of selenium by the 3,3 -diaminobenzidine method. 

Other methods for separating silver involve extraction of the diethyldithiocarbamate into chloroform [pH 4-11 (EDTA as masking agent)] [1]. The thiocyanate complex of silver can be extracted from 0.1-1 M solutions of H2SO4, HCl, or HCIO4 into TBP [2]. 

The colour reaction can be carried out also in a weakly acid medium, at pH 2, which is then raised to 6.5 before extraction of the complex into chloroform. At this pH, extraction of free Bismuthiol II is negligible. EDTA is again used as masking agent. A 20% increase in sensitivity is achieved by adding ammonium sulphate to the aqueous solution [30]. 

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