Determination of Nickel and Chromium

Although laborious, the classical chemistry methods for nickel and chromium in steel provide good accuracy and precision. Truth be told, if I had access to quality XRF I’d probably go with that, but I don’t, and there is a certain beauty in old-school wet chemistry that keeps me engaged.

I start with a one-gram sample of metal because overall accuracy hinges on that of the initial weighing. My analytical balance if good for 0.1mg, which, in this case, gives an accuracy of one part in 104. This factors through as +/- 0.01% in the final weight percentage. That’s good enough for what I need.

I digest the approx. 1g steel sample in aqua regia and then analyse a tiny aliquot of that for the metal of interest. In the case of nickel, I employ solvent extraction of the dimethylglyoximate complex, followed by photometric measurement. For chromium, I measure it as dichromate by redox titration against ferrous sulphate.

Digestion:

  1. weigh out approx. 1g of metal. Record weight to the nearest 0.1mg
  2. add 15ml conc. HCl
  3. add 5ml conc. HNO3
  4. cover and heat at 160ºC on hotplate
  5. when effervescence ceases, uncover and heat further until almost dry
  6. add 20ml conc. HCl and heat at 230ºC on hotplate to expel oxides of nitrogen. Continue heating until almost dry
  7. add 30ml DIW and warm for 5min
  8. filter through 103 (gravity) and wash x3 with DIW
  9. make the filtrate up to exactly 250ml

Nickel:

  1. take 200ul aliquot of digest with a precision micro-pipette
  2. add 5ml ammonium citrate buffer
  3. add 5ml chloroform (exact using pipette)
  4. add approx. 20mg of dimethylglyoxime
  5. shake sufficiently vigorously to ensure mixing of the phases for 10min
  6. separate the chloroform phase and dry over solid anhydrous sodium sulphate
  7. measure absorbance at 360nm against pure chloroform

A single standard of 1.0umole is sufficient for calibration given the response in this range is linear and the blank is close to zero. However, a standard series using 0.001M NiSO4 can be used if so desired.

Ammonium Citrate Buffer:

Dissolve 12.5g of citric acid in 250ml of DIW. Add 30% NH3 solution until a pH of 7.5 is achieved. (approx. 15ml).

Chromium:

  1. take 10.0ml aliquot of digest
  2. add 60ml of freshly prepared ammonium persulphate solution (10%)
  3. add 40mg of silver nitrate
  4. heat on hotplate at 330ºC (just bubbling) for 1hr
  5. stand to cool
  6. add 100ml H2SO4 (2.5%)
  7. add exactly 25ml of freshly prepared standard ferrous sulphate (0.01M)
  8. add 3ml ortho phosphoric acid (conc)
  9. add 4 drops of DPAS indicator
  10. titrate against standard potassium dichromate (0.01N)

umoles of Cr in aliquot = (umoles Fe2+ added – uequivs K2Cr2O7 titrated )/3

DPAS Indicator:

  • weigh out 200mg of sodium diphenylamine sulphonate
  • dissolve in 100ml DIW

Notes:

  • The addition of silver nitrate is necessary to ensure quantitative oxidation of chromium salts to the dichromate. See Vogel’s Quantitative Analysis, section III, 70.
  • Phosphoric acid is used to lower the potential of the ferric/ferrous couple by forming a complex, and thus sharpening the end point with DPAS. See Vogel’s Quantitative Analysis, section III, 63.
  • Note that the persulphate is present in very large excess and must be completely decomposed before attempting the titration. If the end point is slow/unclear, then step 4. needs to be carried out more vigorously.
  • 11May24: The oxidation step (4) is far more reproducible if the flask is fitted with a reflux condenser and boiling continued for 3 hours.