The assay solution is prepared by separating the bismuth with sulphuretted hydrogen, boiling the precipitate with nitric acid, and evaporating with sulphuric acid. Take up with water, add 10 or 20 c.c. of solution of potassium iodide, boil off any iodine liberated, dilute, filter, and make up to 100 c.c. According to the depth of colour take 10, 20, or 50 c.c. and transfer to the Nessler tube. Add a few c.c. of the solution of sulphurous acid. Into the other Nessler tube put as much potassium iodide solution as is contained in the assay tube, with sulphurous acid and water to within a few c.c. of the bulk. Then add the standard bismuth solution till the tints are equal. The student must be careful not to confuse the colour of the bismuth iodide with that of free iodine. If the yellow colour is removed by boiling and returns on standing it is due altogether to iodine; if it is lessened by the addition of a few drops of the dilute sulphurous acid, it is in part due to it. Hence the necessity of having a little free sulphurous acid in each tube. A strong solution must not be used, since it liberates iodine from potassium iodide. The following experiments illustrate the effect of variation in the conditions of the assay:-- ~Effect of Varying Temperature.~--At a higher temperature the colour is somewhat lessened. 1.0 c.c. at 15° C. showed the colour of 0.8 c.c. at 70° C. 2.5 " " " " 2.0 " 5.0 " " " " 5.0 " ~Effect of Free Acid.~-- 2.5 c.c. with 5 c.c. of nitric acid equalled 2.5 c.c. 5.0 " " sulphuric acid " 5.0 " Hydrochloric acid almost completely removes the colour, which, however, is restored by the addition of a few crystals of potassium iodide. ~Effect of Alkalies.~--Ammonia, soda, or potash destroys the colour, but it is restored on acidifying with nitric or sulphuric acid. ~Effect of Ammonic Salts.~--The following table shows the results after addition of ammonic salts:-- -------------+----------------+-----------------+----------------- C.c. present.|With 10 grams |With 10 grams |With 10 grams |Ammonic Nitrate.|Ammonic Sulphate.|Ammonic Chloride. -------------+----------------+-----------------+----------------- 1.0 c.c. | 0.9 c.c. | 1.1 c.c. | -- 2.5 " | 2.5 " | 2.7 " | -- 5.0 " | 5.0 " | 5.5 " | -- -------------+----------------+-----------------+----------------- Ammonic chloride, like hydrochloric acid, removes the colour, which may be restored on the addition of more potassium iodide. Nitrates and sulphates do not thus interfere.