Determination of Iron and Aluminium

I came across a method whereby someone had worked out a tricky sequence that allowed you to analyse a bunch of cations in a single EDTA titration. It seemed more clever than practical, but drew my attention to the fact that calcium and magnesium are very weakly bound to EDTA at pH 5, while aluminium, and both ferrous and ferric iron, are strongly bound.

I found that lead nitrate, in conjunction with xylenol orange as an indicator, proved to be an excellent back-titrant for a solution of EDTA partially complexed with aluminium and iron

The initial end-point measures both aluminium and iron (both oxidation states). At that point an excess of fluoride can be added and the aluminium is quantitatively decomplexed allowing further lead nitrate to be titrated and thus quantify the aluminium.

The process is as follows –

  1. 10ml aliquot of sample
  2. add 50ml DIW
  3. add exactly 5.0ml of standardised 0.01M EDTA
  4. add 2.5ml of pH 5.0 acetate buffer
  5. boil for 5mins
  6. stand to cool
  7. add 4 drops of xylenol orange
  8. titrate against standard 0.01M lead nitrate

gmoles of Al and Fe in aliquot = gmoles of EDTA added – gmoles of Pb titrated

  1. add approx. 0.5g of AR sodium fluoride
  2. boil 5mins
  3. stand to cool
  4. titrate against standard 0.01M lead nitrate

gmoles of Al in aliquot = gmoles of Pb titrated

EDTA Solution 0.01M:

  • weigh out close to 2.92g of AR EDTA
  • add 1.3g of AR sodium hydroxide
  • add 500ml of DIW and wait until it has dissolved
  • make up to 1L with DIW
  • standardise against 0.01M magnesium nitrate

Lead Nitrate Solution 0.01M:

  • weigh out close to 1.655g of AR Pb(NO3)2
  • make up to 500ml with DIW
  • calculate standard concentration applying purity of 99%

Xylenol Orange Indicator:

  • make up stock in DIW at 100mg/ml
  • dilute for use at 1ml of stock to 20ml DIW

Acetate Buffer pH 5.0:

  • measure out 29ml AR glacial acetic acid
  • add 71.5g of AR anhydrous sodium acetate
  • make up to 250ml with DIW

Notes:

  • No significant difference could be found between determination of ferric and ferrous iron
  • No interference was measurable when calcium or magnesium were present in the sample to the amounts of several hundred umoles