The analytical investigation and control of chroming baths is usually simple. A suitable volume of liquor is titrated with N/10 thiosulphate after acidifying with hydrochloric acid and adding potassium iodide. The operation should be conducted in a stoppered bottle, and the liquor allowed to stand for 10-15 minutes after adding the iodide and before titrating. A little fresh starch infusion should be added towards the end of the reaction. Each c.c. N/10 thiosulphate corresponds to 0.0033 gram CrO{3} or 0.0049 gram K{2}Cr{2}O{7}. The same volume of liquor should also be titrated with N/10 caustic soda and phenolphthalein. Potassium chromate is neutral to this indicator, _i.e._ chromic acid acts as a dibasic acid. Any excess of hydrochloric acid is also titrated. More indicator should be added towards the end of the titration, as it is often oxidized. Each c.c. N/10 soda corresponds to 0.005 gram CrO{3}, 0.01 gram "half-bound" CrO{3} (_i.e._ present as dichromate), 0.0147 gram K{2}Cr{2}O{7}, or 0.00365 gram HCl. If _a_ c.c. N/10 thiosulphate and _b_ c.c. N/10 soda be needed the type of chroming bath may be seen at a glance-- ---------------------------+-------------+----------------------- If | The type is | The bath contains ---------------------------+-------------+----------------------- _b_ is greater than 1/3_a_ | Schultz | potassium dichromate but is less than 2/3_a_ | | and chromic acid _b_ is greater than 2/3_a_ | Acid | chromic acid and free | | hydrochloric acid _b_ equals 1/3_a_ | Neutral | chromic acid only ---------------------------+-------------+----------------------- If 10 c.c. chrome liquor require _a_ and _b_ c.c. of thiosulphate and soda respectively-- I. 10 c.c. of a Schultz bath contain (b - 1/3×a) × 0.01 gram CrO{3} and [(a×0.0033) - [(b - 1/3×a) × 0.01]] × 1.47 grams K{2}Cr{2}O{7} II. 10 c.c. of an acid bath contain (a×0.0033) grams CrO{3} and [(b - 2/3×a) × 0.00365] grams HCl III. 10 c.c. of a neutral bath (a×0.0033) grams \ > CrO{3} _or_ (b×0.005) grams /