Chromium (III) salts are oxidised to dichromate by boiling with excess of a persulphate solution in the presence of a little silver sulphate (catalyst). The excess of persulphate remaining after the oxidation is complete is destroyed by boiling the solution for a short time. The dichromate content of the resultant solution is determined by the addition of excess of a standard iron(II) solution and titration of the excess of the latter with standard 0.01 N potassium dichromate.
2Cr3+ + 3S2082-
+ 7H20 = Cr2O72- + 6HSO4-
+ 8H +
2S2082- +2H20
= 02 + 4HS04-
Potassium dichromate is not such a
powerful oxidising agent as potassium permanganate, but it can be obtained
pure, it is stable up to its fusion point, and it is therefore an excellent
primary standard. Weighing out the pure dry salt and dissolving it in the
proper volume of water can prepare standard solutions of exactly known
concentration. Furthermore, the aqueous solutions are stable indefinitely if
adequately protected from evaporation. Potassium dichromate is used only in
acid solution, and is reduced rapidly at the ordinary temperature to a green
chromium (III) salt.
Cr2O72- +
14H+ 6e- = 2Cr3+ + 7H2O
It is not reduced by cold hydrochloric acid, provided the acid concentration does not exceed 1 or 2M. Dichromate solutions are less easily reduced by organic matter than are those of permanganate and are also stable towards light. Potassium dichromate is therefore of particular value in the determination of iron in iron ores: the ore is usually dissolved in hydrochloric acid, the iron(III) reduced to iron(II), and the solution then titrated with standard dichromate solution:
Cr2O72- +
6Fe2+ + 14H+ = 2Cr3+ + 6Fe3+ + 7H2O
The green colour due to the Cr3 + ions formed by the reduction of potassium dichromate makes it impossible to ascertain the end-point of a dichromate titration by simple visual inspection of the solution and so a redox indicator must be employed which gives a strong and unmistakable colour change; this procedure has rendered obsolete the external indicator method which was formerly widely used. Suitable indicators for use with dichromate titrations include N-phenylanthranilic acid (0.1 per cent solution in 0.005M NaOH) and sodium diphenylamine sulphonate (0.2 per cent aqueous solution); the latter must be used in presence of phosphoric acid.
Analytical grade potassium dichromate
is satisfactory for most purposes. Powder finely about 6 g of the analytical
grade material in a glass or agate mortar, and heat for 30-60 minutes in an air
oven at 140-150 °C. Allow to cool in a closed vessel in a desiccator. Weigh out
accurately about 5.88 g of the dry potassium dichromate into a weighing bottle
and transfer the salt quantitatively to a 1 L graduated flask, using a small
funnel to avoid loss. Dissolve the salt in the flask in water, make up to the
mark and shake well. The molarity of the solution can be calculated directly
from the weight of salt taken.
Weigh out accurately an amount of the salt contain about 0.25 g of chromium, and dissolve it in 50 mL distilled water. Add about 1 g silver sulphate, followed by 50 mL of a 10 per cent solution of ammonium or potassium persulphate. Boil the liquid gently for 20 minutes. Cool, and dilute to 250 mL in a graduated flask. Remove 50 mL of the solution with a pipette, add 50 mL of a 0.1 M ammonium iron (II) sulphate solution, 200 mL of 1M sulphuric acid, and 0.5 mL of N-phenylanthranilic acid indicator. Titrate the excess of the iron(II) salt with standard 0.02 M potassium dichromate until the colour changes from green to violet-red.
Standardise the ammonium iron(II)
sulphate solution against the 0.1N potassium dichromate, using
N-phenylanthranilic acid as indicator. Calculate the volume of the iron(II)
solution which was oxidised by the dichromate originating from the chromium
salt, and from this the percentage of chromium in the sample.