Chromate EDTA titration

Chemicals used were of standard analytical grade. Fe(lll) was analysed by complexomentric (EDTA) titration [5] with NH4SCN indicator at pH 3. Fe(Il) was estimated by deducting Fe(III) from total Fe found after oxidation of a mix of Fe(IlI) and Fe(II) solution. Chromium was estimated spectrophotometrically at mO 373 as chromate with molar extinction co-efficient of 4815 in IM NaOH solution. Sulphate was analysed by a Nephelometer against a calibrated curve of standard BaS04 suspension. 

Masking by oxidation or reduction of a metal ion to a state which does not react with EDTA is occasionally of value. For example, Fe(III) (log K- y 24.23) in acidic media may be reduced to Fe(II) (log K-yyy = 14.33) by ascorbic acid in this state iron does not interfere in the titration of some trivalent and tetravalent ions in strong acidic medium (pH 0 to 2). Similarly, Hg(II) can be reduced to the metal. In favorable conditions, Cr(III) may be oxidized by alkaline peroxide to chromate which does not complex with EDTA. 

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]. 

Lead as a major constituent may be determined gravimetrically as the sulphide or the chromate. More frequently it is determined titrimetrically, either by precipitation of lead oxalate which is then titrated with potassium permanganate or, better, by titration with EDTA. The oxalate method for lead acetate is as follows ... 

Volumetric methods of lead estimation involving titration with molybdate, chromate, phosphoric acid, ferrocyanide, fluoride and iodide are all available, in addition to the complexometric titrations with reagents such as EDTA. 

Lead(II) ions in aqueous solution can be titrated with EDTA solution using xylenol orange indicator and hexamine to control the pH. They can be precipitated as chromate which may be weighed or it may be dissolved in dil. HCI, adding excess KI and titrating... 

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