Dichromate Titration Methods

DICHROMATE METHODS

Potassium dischromate (K₂Cr₂O₇) is a strong oxidizing agent, quite comparable to KMnO₄ that nor-mally shows only one pertinent reduced oxidation state : Thus, chemically we have :

Ionically we have :

Potassium dichromate exhibits much greater stability in aqueous solution in comparison to potassium permanganate. Potassium dichromate possesses an inherent oranage colour that is not intense enough to serve its own end-point signal, specifically in the presence of the green Cr³⁺ ion, which is supposed to be present at the end-point. Hence, redox indicators are usually employed to locate the exact end-point e.g., barium diphenylamine sulphonate.

1. Preparation of 0.1 N Potassium Dichromate Solution

Materials Required : Potassium dichromate : 4.930 g.

Procedure : Weigh accurately 4.93 g of potassium dichromate previously powdered and dried at 20°C for 4 hours and dissolve in sufficient DW to produce 1 litre in a volumetric flask.

Note : Potassium dichromate can be obtained as a primary standard reagent and hence, standard solu-tions may be prepared determinately and stored for long periods of time.

Equations : Chemically we have :

Ionically we have :

From this equation it follows that the equivalent weight of potassium dichromate is 1/6th of the molecular weight i.e., 294.22/6 or 49.03 g.

2. Standardization of 0.1 N Potassium Dichromate Solution

It can be achieved by following these steps, namely :

(a) Preparation of Standard Solution of Mohr’s Salt FeSO₄(NH₄)₂.SO₄.6H₂O :

Materials Required : Mohr’s salt : 4.9 g ; dilute sulphuric acid (1 in 3, approx. 9 N) : 20 ml.

Procedure : Weigh accurately about 4.9 g of pure sample of Mohr’s salt and transfer it to a 250 ml volumetric flask. Add 20 ml of dilute sulphuric acid and make up the volume to the mark with DW and finally mix the contents of the flask thoroughly.

Calculations : The quantity of Mohr’s salt required for 250 ml of the solution having a normality of 0.05 N can be calculated as follows :

(b) Standardization of 0.1 N K₂Cr₂O₇ Solution :

Materials Required : Standard solution of Mohr’s salt (0.05 N) : 250 ml, sulphuric acid (2 N) : 20 ml ; potassium dichromate solution (0.1 N) : 1 litre.

Procedure : Transfer 20 ml of the primary standard solution (Mohr’s salt) to the titration flask and add 20 ml of 2 N sulphuric acid. Take the potassium dichromate solution in the burette. Put drops of freshly prepared potassium ferricyanide, K₃[Fe(CN)₆], solution in the grooves of a porcelain tile. Now, proceed with the titration of Mohr’s salt solution against K₂Cr₂O₇ solution. Transfer drops of the titrated solution by means of a glass rod and mix with drops of the indicator, already taken in the groove-tile. Alternatively, pre-soaked and dried filter paper with K₃[Fe(CN)₆] solution can also be used in place of the groove-tile method.

In order to arrive at the exact end-point the above titration may be carried out at three stages, namely :

Stage 1 : Spot tests are carried out at intervals of 1-2 ml until a blue colour is no longer produced with K₃[Fe(CN)₆], which provides an altogether rough estimate of the K₂Cr₂O₇ solution required for the titration,

Stage 2 : Spot tests are only performed near the approach of the end of titration at intervals of 0.1-0.2 ml, and

Stage 3 : Spot tests are finally done only at the end-point.

The above sequential steps give fairly accurate results because the error caused by the removal of part of the solution for the spot tests is made negligibly small. However, the titration is repeated to get a set of concordant readings.

By applying the relationship between N₁V₁ (K₂Cr₂O₇) and N₂V₂ (Mohr’s salt), the normality of the former may be calculated.

2.1. Iron Ore

Materials Required : Iron ore : 0. 1 g ; hydrochloric acid (conc.) : 15 ml ; diphenylamine (1% w/v in conc. H₂SO₄) ; zinc metal (granulated) : 4 g ; ammonium thiocyanate solution (0.1% in water) ; mixture of sulphuric acid and phosphoric acid [dissolve 15 ml of H₂SO₄ (sp. gr. 1.84) in 50 ml of DW, cool and add 15 ml of H₃PO₄ (sp. gr. 1.70) and make the volume to 100 ml with DW] : 25 ml.

Procedure :

(a) Preparation of Standard K₂Cr₂O₇ Solution : Instead of using solutions having definite normal-ity, routine industrial laboratories make use of ‘emperical solution’ which is normally expressed in terms of ‘titer for the substance determined’. For this assay, let us prepare an emperical K₂Cr₂O₇ solution (250 ml) of such a concentration that 1 ml of the same exactly correspond to 0.0025 g Fe.

Calculations :

By Law of Equivalence, we have :

1 gram-equivalent of K₂Cr₂O₇ (49.03 g) ≡ 1 gram-equivalent of Fe (55.85 g)

Therefore, weigh accurately 0.5488 g of pure K₂Cr₂O₇ and transfer it quantitatively into a 250 ml volumetric flask, dissolve in DW, make up the volume and mix thoroughly.

Hence, the ‘iron titer’ of this solution is :

(b) Preparation of Ore Solution : Weigh accurately 0.1 g of powdered and dried ore on a clean watch glass and transfer it quantitatively into a 100 ml-volumetric flask. Add 15 ml of concen-trated hydrochloric acid, warm the contents of the flask carefully over a sand-bath until most of the dark grains of ore get dissolved completely and only a whitish silica precipitate settles at the bottom of the flask.

(c) Reduction of Fe³⁺ to F²⁺ in the Ore Solution : Introduce carefully a few pieces of granulated pure zinc metal into the flask, place a funnel in the neck of the flask to avoid splashes and boil the solution gently until the yellow colour has disappeared completely, thereby ascertaining that com-plete reduction of Fe³⁺ to Fe²⁺ is affected.

Note : It may be further confirmed by doing a spot test with NH₄CNS solution which only shows a blood-red colour with Fe³⁺.

The contents of the flask is cooled, filtered through cotton wool, washings done with DW and the filtrate diluted to about 350 ml with DW. This dilution is a must so as to avoid any interference caused by its inherent green colour with the estimation of the equivalence point in the titration as per the following chemical reaction :

K₂Cr₂O₇  +  6FeCl₂  +  14HCl   →     2CrCl₃  +  2KCl  +  6FeCl₃  + 7H₂O

(d) Final Titration : The 350 ml solution obtained in (c) above is now quantitatively titrated against K₂Cr₂O₇ solution employing diphenylamine as an internal indicator. Add 25 ml of a mixture of sulphuric acid and phosphoric acid to the solution along with 2 drops of diphenylamine indicator and titrate the solution with K₂Cr₂O₇ solution carefully, by adding small lots at intervals with constant shaking, until a persistant blue-violet colour appears.

Note : (a) The acidity of the solution must be maintained fairly high which can be achieved by adding orthophosphoric acid, H₃PO₄,

(b) The quantity of diphenylamine must not exceed 2 drops by virtue of the fact that at higher concentration with lower acidity during very slow titration, the indicator undergoes an altogether different type of chemical change that ultimately gives a green colour instead of the desired blue-violet colour.

(e) Calculations : Multiply the number of millilitres of K₂Cr₂O₇ Solution consumed in the titration by the ‘iron titer’ and therefrom determine the amount of iron present in the sample. Finally, the percentage of iron present in the ore may be calculated.