1 BBased on formation of tetracyanonickelate A number

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B-Based on formation of tetracyanonickelate: A number of titrations are based on the fact that the tetracyanonickelate does not react rapidly with EDTA. Example 1: Determination of cyanide: Cyanide may be determined by adding the cyanide solution to known excess of standard Ni 2+ solution, in presence of ammonia buffer, and titrating rapidly with EDTA, using murexide indicator. Although the tetracyanonickelate complex is less stable than the EDTA, it decomposes slowly enough to allow the determination of the free Ni 2+ when the titration is completed rapidly Determination of silver salt: Silver salt is treated with [Ni(CN)4] 2 -, where the more stable [Ag(CN)2] is formed on expense of [Ni(CN)4] 2 - liberating equivalent amount of Ni 2+ which is titrated against EDTA using murexide indicator. 2 Ag + + [Ni(CN)4] 2 - Ni 2+ + H 3 Ind 2 - 2 [Ag (CN)2] - + Ni 2+ i. e. 2 Ag + = Ni 2+ Ni-H 2 Ind - + H + Ni-H 2 Ind - + H 2 Y 2 - + OH - Ni. Y 2 -2 + H 3 Ind 2 - + H 2 O

Determination of halides (Cl-, Br- , I-): First halides are precipitated as Ag. X, filtered. Then, Ag. X is dissolved in [Ni(CN)4]2 - complex, where equivalent amount of Ni 2+ is liberated. Ni 2+ which is equivalent to Ag+ and X- can be titrated against standard EDTA. Ag. NO 3 + Na. X Ag. X + Na. NO 3 2 Ag. X + [Ni(CN)4] 2 - 2[Ag(CN)2]- + 2 X- + Ni 2+ 3

Complexometric titration of mixtures To use EDTA in titration of mixture , its Selectivity can be improved by : [1] Suitable adjustment of p. H. [2] Use of masking and demasking agents. [3] Removal of interfering ion. Improvement of selectivity of EDTA [1] Suitable adjustment of p. H Metal-complexes can be classified roughly according to their stability at different p. H into the following groups. [a] First group: Form stable complexes at p. H = 1 -3 (using conc. HNO 3) such as, Th 4+ , Ti 4+ , Zr 4+(tetravalent), Fe 3+ , Bi 3+ (trivalent) and Hg 2+ (divalent).

[b] Second group Form stable complexes at p. H = 4 -6 (using acetate buffer or hexamine) such as Cu 2+, Pb 2+, Cd 2+ , Zn 2+ , Co 2+ , Ni 2+ , …) [c] Third group: Form stable complexes at p. H ≥ 10 (using ammonia buffer or 8% Na. OH) such as alkaline earth metal ions (Ba 2+ , Ca 2+) and Mg 2+. 5

Masking and demasking agents Masking agents are reagents which prevent interfering ion during determination. Demasking is the process in which the masking substance regains its ability to enter into a particular reaction. It is a wide range of cations can be chelated by EDTA, so that selectivity of the method is poor. When it is required to determine selectivity one or more ions in a mixture of cations and to eliminate the effects of possible impurities, masking agents are used. These acts either precipitation or by formation of complexes more stable than the ion-EDTA chelate. [1. a] Masking by precipitation : 1. Many heavy metals as (pb 2+ Hg 2+, Cu 2+ , Cd 2+ , Zn 2+, Co 2+ , Ni 2+ ) form of insoluble sulphides using sodium sulphide or thioacetamide. These are filtered, washed and decomposed prior to titration with EDTA. 2. Sulphate for ( pb 2+, Ba 2+). 3. Oxalate for ( pb 2+, Ca 2+). 4. Thioglycerol masks copper by precipitation in the analysis of lotions containing (Cu 2+ , Zn 2+). 6

[1. b] Masking by Complex formation : 1. Cyanide for (Ag+ , Cu 2+, Hg 2+ , Fe 3+ , Zn 2+, Co 2+ , Cd 2+ , Ni 2+ ) in alkaline solution. Formaldehyde-acetic acid mixture, is used as a selective demasking agent for cyanocomplexes of cadmium and zinc. (not suitable for the highly stable Cu-Cyanide complex). OH [Zn(CN)4] 2 - + 4 HCHO + 4 H + → 4 CH 2 2. Triethanolamine for (Fe 3+ , Mn 2+). + Zn 2+ CN 3. Iodide specific for (Hg 2+) give Hg. I 4 2 -. Mixture of Mg 2+, Zn 2+ and Cu 2+: Total: a-add known excess standard EDTA, back titrate with standard Zn++ using EBT p. H 10 Va = Mg 2+, Zn 2+& Cu 2+ b-another portion mask Cu and Zn with CN -, titrate with EDTA, to get Mg++ alone Vb = Mg 2+, Va -Vb = Zn 2+, Cu 2+ c-above soln is demasked with HCHO & acetic acid → Zn++ , titrate with EDTA Vc = Zn 2+ (Va-Vb) - Vc = Cu++

[1. c] Suitable adjustment of p. H e. g. Mixture of Bi 3+and Pb 2+. Bi 3+ can be titrated in acid solution (p. H= 2 -3) by HNO 3 using xylenol orange as indicator. After addition of hexamine the p. H is raised to 5, Pb 2+ can be detected using xylenol orange. (red to yellow) e. g. Mixture of Ca 2+and Mg 2+ Ca 2+ is determined in presence of Mg 2+ at p. H 12 using murexide as indicator which responds to Ca 2+ only, Mg 2+being ppted as Mg(OH)2↓ [1. d] Separation by selective extraction: e. g. separation of Zn 2+ from Cu 2+ or Pb 2+ For determination of Zn 2+ in a mixture with either Cu 2+ or Pb 2+ , mixture is treated with SCN - then Zn(SCN)2 is extracted by isobutyl methyl ketone. 8

Determination of Ca 2+/Mg 2+ mixture: mixture 1 st step: step Determination of Ca 2+ is determined in presence of Mg 2+ at p. H 12 using murexide as indicator which responds to Ca 2+ only, Mg 2+being ppted as Mg(OH) ↓ 2 Mg 2+ + 2 OH - Mg (OH)2 Ca 2+ + H 3 ind 2 - Ca-H 2 ind - + H + Ca-H 2 ind- + H 2 Y 2 - + OH - Ca. Y 2 - + H 3 ind 2 - + H 2 O 2 nd step: step Determination of total mixture: Mixture is titrated against EDTA using EBT at p. H=10 using ammonia buffer Mg 2+ = Total – Ca 2+ 9

Other complexometric titration: [1] Cyanometric titrations [2] Mercurimetric titrations. [1] Cyanometric titrations It is the use of standard cyanide for the determination of Mn+ (Ag+ , Hg 2+ , Cu 2+, Cd 2+ , Zn 2+ , Co 2+ , Ni 2+). Standard Ag. NO 3 M n+ Known xs CNM 2+ + CN - M(CN) complex Ag+ + 2 CN- [Ag(CN)2] Ag+ + [Ag(CN)2]- Ag [Ag (CN)2] white ppt.

[2] Mercurimetric titrations. It is the use of highly ionized Hg(NO 3)2 as titrant for the determination of halides, thiocyanate and other ions which form very slightly ionized mercuric salts. [1] Determination of chloride (Cl-) give Hg. Cl 42 - colorless complex The end point is detected by using nitroprusside (white turbidity) or diphenylcarbazone(orange) form (bluish violet mercuric complex). * Advantage than Mohr’s, may be in quite acidic, and with dilute solution [2] Determination of iodide (I-) (no indicator is used). Hg 2+ + 4 I - Hg. I 4 2 - colorless complex excess Hg 2+ + Hg. I 4 2 - 2 Hg. I 2 red precipitate. [3] Determination of cyanide (CN-) (back titration method) Standard SCN Using ferric alum as indicator. CN The presence of Hg(CN)2 not interfere Known xs Hg(NO 3)2 , because it is less ionized than Hg(SCN)2 [4] Determination of thiocyanate (SCN-) by direct titration with Hg(NO 3)2 using ferric alum as indicator. Nitroprusside can not be used , mercuric nitroprusside ppt is hid by the 11 slightly soluble mercuric thiocyanate.

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