Silver masking

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... 

Determination. To an aliquot of the silver(I) solution containing between 10 and 50 pg of silver, add sufficient EDTA to complex all those cations present which form an EDTA complex. If gold is present (>250 xg) it is masked by adding sufficient bromide ion to form the AuBr4 complex. Cyanide, thiocyanate or iodide ions are masked by adding sufficient mercury(II) ions to complex these anions followed by sufficient EDTA to complex any excess mercury(II). Add 1 mL of 20 per cent ammonium acetate solution, etc., and proceed as described under Calibration. 

Take a small piece of masking tape and use it to label each piece of wire. Write Ag on the wire to be used with the silver nitrate, and K on the wire to be used with the potassium nitrate. 

Both ways have been used in previously reported attempts to remove chloride interference. Three methods have been used in attempts to prevent chloride oxidation masking with mercury (II) [191,192] precipitation of chloride using silver (I) [188] or altering oxidation conditions [193]. 

Considerable inter-element interference effects have been reported on the actual hydride-forming step. Elements easily reduced by sodium borohydride (e.g. silver, gold, copper, nickel) give rise to the greatest suppressions. These interfering ions may be removed by the addition of masking agents that complex with them. 

AgBr films could be made in the same manner using various bromo-alco-hols. AgBr is colorless but is usually slightly brownish due to photolytic formation of small particles of metallic silver (this occurs much more strongly if deposition is carried out in room light, but formation of some brown coloration is stiU noticeable even if deposition is carried out in the dark). This coloration masked the optical absorption, which occurs mainly in the UV for AgBr. For bromides, triethanolamine was not needed. 

The extraction constant for primary silver(I) dithizonate, from aqueous solution to a range of organic solvents, has been found to he in the range log K = 5.8-8.94.442,444 It was concluded that changing the solvents did not achieve any worthwhile improvement in separability of silver from other metal ions compared to what could be achieved by addition of suitable masking agents. Carbon tetrachloride was the preferred extractive solvent (Amax = 463 nm).444... 

A familiar example is the reaction of dilute ammonium hydroxide with silver ions to give Ag(NH3)2+, which reduces the concentration of free Ag+ to a value below that needed to exceed the solubility product of AgCl (but not that of AgBr or Agl). If ammonia is replaced by cyanide ions, which form more stable complexes, only the more insoluble Agl can be precipitated. Hence by choice of the masking agent used a cation can be selectively masked towards some reagents but not others. 

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