128

DYEING AND CALICO PRINTING.

These impurities generally consist of sulphide of sodium, hyposulphite andsulphite of soda. They can be detected by the addition of ammonia andnitrate of silver, by which a black precipitate is thrown down. If, afterheating the solution and adding more ammonia there still remains a blackprecipitate, the solution contains hydrosulphuric acid and sulphite of soda.

If there be sulphide of sodium present, it will be recognised upon the additionof the normal sulphuric acid by the formation of sulphate of soda and sul-phuretted hydrogen, according to the following equation:—

NaS+H0,S0 3 = Na0,S0 3 + HS.

If sulphite of soda be present, it will be detedted by the peculiar odour of thesulphurous acid formed on the addition of the normal test acid, according tothe equation—•

NaO,S0 2 + H0,S0 3 = Na0,S0 3 + H0 + S0 2 .

In the case of hyposulphite of soda the addition of the normal acid causesdecomposition into sulphate of soda, water, sulphurous acid, and sulphur.

Na0,S a 0 2 + HO,S0 3 = NaO,S0 3 + HO+ S0 2 +S.

These impurities must be removed before an accurate test can be made,because their adtion upon the litmus solution would lead to error in the esti-mation of the amount of acid employed, and consequently the amount of purecarbonate contained in the sample of soda-ash. MM. Fordos and Gelisrecommend the changing of the sulphurets, sulphites, and hyposulphites byadding a little neutral chromate of potash to the alkaline solution, to obtainsulphate of chromium, water, and a separation of sulphur. These readlionsdo not affect the accuracy of the alkalimetrical process.

Estimation of Caustic Alkalies.

Caustic potash, KO,HO, is composed in loo parts of 83-97 of P ure potash,or anhydrous oxide of potassium, and 16-03 of water. It is prepared from acrude carbonate of potash obtained from chloride of potassium, carnallite,vinasse, or other sources, by lixiviating the crude carbonate, and rendering theliquid caustic with quick-lime; or by igniting a mixture of sulphate of potash,limestone, and small coal, and lixiviating with water at 50° to yield at oncea solution of caustic potash. It may also be obtained by decomposing sulphateof potash with caustic baryta; by converting chloride of potassium intosilico-fluoride of potassium, and decomposing this salt with caustic lime; orby igniting nitrate of potash with copper-scale. The following table showsthe relation of sp. gr. to strength in caustic potash solutions :—

ge of Potash.

Degrees Beaum6.

Specific Gravity.

47

9

I-o6

9'5

IS

I-II

13-0

19

1-15

16-2

24

1-19

19-5

28

1-23

23'4

32

1-28

32-4

41

i '39

42-9

50

1-52

46 7

53

i-6o

51-2

57

i-6o