INORGANIC cHEMISTRY

Experiment 10

Aim of the Experiment

To estimate iron as ferric oxide in the given solution of ferrous ammonium sulphate.

Principle

This estimation is based on the precipitation of iron as ferric hydroxide by adding NH,OH to ferric salt and subsequently igniting ferric hydroxide to ferric oxide which is weighed.

If the supplied solution is in the ferrous form, it is desirable to oxidise this solution before precipitation by boiling it with conc. nitric acid or hydrogen peroxide solution or bromine water.

Solution :

Solution for such an exercise may be prepared by dissolving 58.8 g or 78.4 g of or 98.0 g of Mohr's salt per litre 25 ml of this solution on oxidation with nitric acid and precipitation with ammonium hydroxide yields about 0.3 g of 0.4 g, 0.5 g of Fe2O3 residue on ignition.

Reagents

(1) Precipitating reagent: It is 50% ammonia and is prepared by adding 50 ml of liquid ammonia to 50 ml of water.

(ii) Washing solution : It is 1% ammonium nitrate in distilled water.

Procedure

(1) Precipitation : Measure out 25 ml of the provided solution by a standard

burette into a 400 ml beaker. To this add 50 ml of distilled water and 1 ml of conc. hydrochloric acid. Then, add 3-4 ml of conc. nitric acid. Boil the acidified solution to oxidise ferrous to ferric until the colour is clear yellow. Test a small drop of this solution with potassium ferricyanide to ensure complete oxidation. If a blue coloration appears, add some nitric acid and boil. Again test the solution with potassium ferricyanide and make sure that the oxidation is complete before porceeding further.

Now add 150 ml of distilled water and heat the solution to boiling. To the hot solution add slowly dil. ammonia solution (1 : 1) with constant stirring until the solution smells strongly of it. Now boil the solution to remove excess of ammonia. Discontinue heating and allow the ppt. to settle down. Now add ammoniacal paper pulp (see the estimation of aluminium) to the contents of the beaker and stir with a

glass rod. Again, heat the contents of the beaker for sometime and allow the ppt. to settle down.

Filtration and washing : Fit a quantitative filter paper (Whatman No.41) in to a funnel properly, so that the stem of the funnel could always remain filled with liquid during filtration. If this precaution is not taken, the filtration will be unduly low. Decant the supernatant liquid through the filter paper without disturbing the precipitate. Add about 100 ml of boiling 1% ammonium nitrate solution to the ppt in the beaker and stir the mixture well; then allow to settle. Pure off the supernatant liquid through the filter paper as before. Repeat washing by decantation in this manner, four times, and then transfer the ppt. to the filter paper.

Wash the ppt on the filter paper with a hot 1% solution of ammonium nitrate, until the filtrate is entirely free from sulphate ions (test with dil, nitric acid and barium chloride). Always allow each portion of the wash liquid to flow through before adding the next instalment.

Now place the funnel with the filter paper on a hot air cone for partial dryness. When almost dry, fold the filter paper with the ppt. And put it in a weighed crucible. Heat the crucible gently over a small flame till the paper gets charred. Next increase the flame slowly and continue heating till all carbonaceous matter has burnt away. Next heat the crucible by a Mecker burner for about į hour and finally heat it in a muffle furnace at 1000°C. Cool and weigh. Repeat the process of heating, cooling and weighing till the crucible with its constents attains a constant weight

Calculation:

Let weight of Fe2O3 residue be x g

Fe2O3 = 2Fe

159.68 = 111.68

159.68 g of Fe2O3 contains = 111.68 g of iron

X g of Fe2O3 contains = 111.68/159.68 X x g of iron

25 ml of the given solution contains = 111.68/159.68 X x g of iron

1000 ml of the given solution contains = 111.68 X x/159.68 X 1000/25

=_________ g of iron.

Result:

The provided solution contains _________ g of ion

Precautions

(1) If iron is to be estimated in a ferrous salt, the latter must be oxidised to the ferric state completely, before affecting the precipitation. This is done by boiling the solution containing the ferrous salt with nitric acid. The completion of oxidation must be proved by means of potassium ferricyanide used as an external indicator.

(ii) The precipitate must be obtained in a co-agulated form, or otherwise it would be difficult to filter. This is done by affecting the precipitation at, or near, the boiling point.

(ii) Ammonium nitrate should be added to wash solution so as to prevent

peptisation and formation of slimy material.

(iv) Since the carbon of the filter paper and the flame gases have reducing

action on Fe,O3, it is advisable to carry out charring of the filter paper at a low temperature under good oxidising conditions, i.e., in an open

crucible.

Reference Material

1. Vogel’s textbook of quantitative Chemical Analysis.

link:

Questions

1. Explain what are buffers.