PREPARATION OF SOLUTIONS

!Approximate Strength 0.1 N!

It is possible to purify commercial potassium bichromate by recrystallization from hot water.  It must then be dried and cautiously heated to fusion to expel the last traces of moisture, but not sufficiently high to expel any oxygen.  The pure salt thus prepared, may be weighed out directly, dissolved, and the solution diluted in a graduated flask to a definite volume.  In this case no standardization is made, as the normal value can be calculated directly.  It is, however, more generally customary to standardize a solution of the commercial salt by comparison with some substance of definite composition, as described below.

Procedure.—­Pulverize about 5 grams of potassium bichromate of good quality.  Dissolve the bichromate in distilled water, transfer the solution to a liter bottle, and dilute to approximately 1000 cc.  Shake thoroughly until the solution is uniform.

To prepare the solution of the reducing agent, pulverize about 28 grams of ferrous sulphate (FeSO_{4}.7H_{2}O) or about 40 grams of ferrous ammonium sulphate (FeSO_{4}.(NH_{4})_{2}so_{4}.6H_{2}O) and dissolve in distilled water containing 5 cc. of concentrated sulphuric acid.  Transfer the solution to a liter bottle, add 5 cc. concentrated sulphuric acid, make up to about 1000 cc. and shake vigorously to insure uniformity.

INDICATOR SOLUTION

No indicator is known which, like methyl orange, can be used within the solution, to show when the oxidation process is complete.  Instead, an outside indicator solution is employed to which drops of the titrated solution are transferred for testing.  The reagent used is potassium ferricyanide, which produces a blue precipitate (or color) with ferrous compounds as long as there are unoxidized ferrous ions in the titrated solution.  Drops of the indicator solution are placed upon a glazed porcelain tile, or upon white cardboard which has been coated with paraffin to render it waterproof, and drops of the titrated solution are transferred to the indicator on the end of a stirring rod.  When the oxidation is nearly completed only very small amounts of the ferrous compounds remain unoxidized and the reaction with the indicator is no longer instantaneous.  It is necessary to allow a brief time to elapse before determining that no blue color is formed.  Thirty seconds is a sufficient interval, and should be adopted throughout the analytical procedure.  If left too long, the combined effect of light and dust from the air will cause a reduction of the ferric compounds already formed and a resultant blue will appear which misleads the observer with respect to the true end-point.

The indicator solution must be highly diluted, otherwise its own color interferes with accurate observation.  Prepare a fresh solution, as needed each day, by dissolving a crystal of potassium ferricyanide about the size of a pin’s head in 25 cc. of distilled water.  The salt should be carefully tested with ferric chloride for the presence of ferrocyanides, which give a blue color with ferric salts.