Cyanide EDTA titration

Cu, Ni, Co, Cr, Fe, or Al, even in traces, must be absent when conducting a direct titration of the other metals listed above if the metal ion to be titrated does not react with the cyanide ion or with triethanolamine, these substances can be used as masking reagents. It has been stated that the addition of 0.5-1 mL of 0.001 M o-phenanthroline prior to the EDTA titration eliminates the blocking effect of these metals with solochrome black and also with xylenol orange (see below). 

Cyanide recovered from the refining of gold ore can be determined indirectly by EDTA titration. A known excess of Ni is added to the cyanide to form tetracyanonickelate(II) ... 

The most sensitive - and perhaps for the radiochemist, the most useful volumetric procedure is complexlmetrlc titration utilizing the lead EDTA complex. A number of Indicators have been used for the direct EDTA titration (W5) The most popular of these are Erlochrome Black T (Cl) (W5)j Eriochrome Red B and X-ylenol orange (W5)- The direct titration with the sodium salt of EDTA is carried out in a pH 10 buffer solution (P5)(p6). Iron, the alkaline earths and the earths interfere but bismuth, aluminum and antimony do not. Cyanide can be used to mask cobalt, nickel, copper, zinc, cadmium, mercury and platinum (Cl),... 

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... 

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. 

Determination of calcium. Pipette two 25.0 mL portions of the mixed calcium and magnesium ion solution (not more than 0.01M with respect to either ion) into two separate 250 mL conical flasks and dilute each with about 25 mL of de-ionised water. To the first flask add 4 mL 8 M potassium hydroxide solution (a precipitate of magnesium hydroxide may be noted here), and allow to stand for 3-5 minutes with occasional swirling. Add about 30 mg each of potassium cyanide (Caution poison) and hydroxylammonium chloride and swirl the contents of the flask until the solids dissolve. Add about 50 mg of the HHSNNA indicator mixture and titrate with 0.01 M EDTA until the colour changes from red to blue. Run into the second flask from a burette a volume of EDTA solution equal to that required to reach the end point less 1 mL. Now add 4 mL of the potassium hydroxide solution, mix well and complete the titration as with the first sample record the exact volume of EDTA solution used. Perform a blank titration, replacing the sample with de-ionised water. 

Mixtures of manganese, magnesium, and zinc can be similarly analysed. The first EDTA end point gives the sum of the three ions. Fluoride ion is added and the EDTA liberated from the magnesium-EDTA complex is titrated with manganese ion as detailed above. Following the second end point cyanide ion is added to displace zinc from its EDTA chelate and to form the stable cyanozincate complex [Zn(CN)4]2- the liberated EDTA (equivalent to the zinc) is titrated with standard manganese-ion solution. 

The complexometric method for determination of Ca(II) and Mg(II) is based on two titrations with EDTA in alkaline solution, one where both ions are determined together and the second after one of them has been masked with a specific complexing agent. The effect of interfering heavy metals such as Cu, Fe, Mn or Zn can be avoided by adding cyanide. The AOAC Official Method 964.01 for determination of acid-soluble... 

To measure hardness, the sample is treated with ascorbic acid (or hydroxylamine) to reduce Fe3+ to Fe2+ and with cyanide to mask Fe2+, Cu+, and several other minor metal ions. Titration with EDTA at pH 10 in NH3 buffer then gives the total concentrations of Ca2+ and Mg2+. Ca2+ can be determined separately if the titration is carried out at pH 13 without ammonia. At this pH, Mg(OH)2 precipitates and is inaccessible to EDTA. Interference by many metal ions can be reduced by the right choice of indicators.21... 

Other typical reagents generated for coulometric titrations are hydrogen and hydroxyl ions, redox reagents such as ceric, cuprous, ferrous, chromate, ferric, manganic, stannous, and titanous ions, precipitation reagents such as silver, mercurous, mercuric, and sulfate ions, and complex-formation reagents such as cyanide ion and EDTA [8-10]. 

Titer Determination Pipet 50.0 mL of Magnesium Sulfate Solution into a 400-mL beaker, and add 200 mL of water, 2 mL of Buffer Solution Initial Preparation, 1.0 mL of a 1 20 potassium cyanide solution, and 5 drops of eriochrome black TS or another suitable indicator. While stirring with a magnetic stirrer, titrate with the Standard EDTA Solution to a true blue endpoint. Record the volume, / , in milliliters, of Standard EDTA Solution equivalent to 50.0 mL of Magnesium Sulfate Solution. 

Puschel and Stefanac ° use alkaline hydrogen peroxide in the oxygen flask method to oxidize arsenic to arsenate. The arsenate is titrated directly with standard lead nitrate solution with 4-(2-pyridylazo) resorcinol or 8-hydroxy-7-(4-sulpho-l-naphthylazo) quino-line-5-sulphonic acid as indicator. Phosphorus interferes in this method. The precision at the 99% confidence limit is within 0.67% for a 3-mg sample. In another variation, Stefanac used sodium acetate as the absorbing liquid, and arsenite and arsenate are precipitated with silver nitrate. The precipitate is dissolved in potassium nickel cyanide (K2Ni(CN)4) solution and the displaced nickel is titrated with EDTA solution, with murexide as indicator. The average error is within + 0.19% for a 3-mg sample. Halogens and phosphate interfere in the procedure. 

A 0.3284-g sample of brass (containing lead, zinc, copper, and tin) was dissolved in nitric acid. The sparingly soluble SnOi -4H20 was removed by filtration, and the combined fdtrate and washings were then diluted to 500.0 mL. A 10.00-mL aliquot was suitably buffered titration of the lead, zinc, and copper in this aliquot required 37.56 mL of 0.002500 M EDTA. The copper in a 25.00-mL aliquot was ma.sked with thiosulfate the lead and zinc were then titrated with 27.67 mL of the EDTA solution. Cyanide ion was used to mask the copper and zinc in a 100-niL aliquot 10.80 mL of the EDTA solution was needed to titrate the lead ion. Determine the composition of the brass sample evaluate the percentage of tin by difference. 

The solubility of the complex anion Ag(CN)2 in contrast to the high insolubility of neutral silver cyanide (AgCN) was exploited for the determination of silver (and/or cyanide) by Justus von Liebig as long ago as 1851 this procedure may well be cited as the best established complexometric titration preceding the great developments brought about by G. Schwarzenbach in 1940 when he pioneered the use of ethylenediaminetetraacetic acid, EDTA, and other aminopolycarboxylic acids as titrants" and opened up new vistas in titrimetry (see Section 10.6). 

An aqueous solution containing Pb and Cd can be titrated using EDTA. If an excess of cyanide is added to the solution, the cadmium is said to be masked by the formation of a cyanide complex. Suppose 20.00 mL of a solution containing only lead and cadmium is first titrated against 45.94 mL of a 0.02000 M solution of EDTA. After adding an excess of sodium cyanide, only 34.87 mL of the same EDTA solution produces an end point. Calculate the molarities of both the lead and the cadmium in the original solution. 

In the second stage, an excess of a cyanide solution is added to another sample of the initial solution. A known quantity in excess of EDTA is then added to the obtained solution. The EDTA in excess is finally back-titrated with a standard solution of Mg +. This stage gives the Mg + concentration. 

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