POLYBASIC AND HYDROXY-ACIDS. 515 



the crude state (argol) has been explained above. It is purified by 

 dissolving in hot water and crystallizing, when it is obtained in color- 

 less crystals, or as a white, somewhat gritty powder of a pleasant, 

 acidulous taste ; it is soluble in about 200 parts of cold, easily sol- 

 uble in hot water, but insoluble in alcohol. 



The name cream of tartar was given to the salt for the reason that 

 small crystals, which float on the liquid, separate on rapidly cooling 

 a hot solution of potassium bitartrate. 



Potassium tartrate, 2(K 2 C 4 H 4 6 ).H,0. Obtained by saturating a solution 

 of potassium acid tartrate with potassium carbonate : 



2KHC 4 H 4 O e + K 2 C0 3 = 2K 2 C 4 H 4 O 6 + H 2 O + CO,. 

 Potassium acid Potassium Potassium 

 tartrate. carbonate. tartrate. 



Small transparent or white crystals, or a white neutral powder, soluble in 

 less than its own weight of water. 



Potassium sodium tartrate, Potassii et sodii tartras, 

 KNaC 4 H 4 O 6 .4H 2 O = 280.18 (Rochelle salt). If in the above-described 

 process for making neutral potassium tartrate, sodium carbonate is 

 substituted for potassium carbonate, the double tartrate of potassium 

 and sodium is formed. It is a white powder, or occurs in colorless, 

 transparent crystals which are easily soluble in water. 



Experiment 59. Add gradually 24 grammes of potassium acid tartrate to a 

 hot solution of 20 grammes of crystallized sodium carbonate in 100 c.c. of 

 water. Heat until complete solution has taken place, filter, evaporate to about 

 one-half the volume, and set aside for the potassium sodium tartrate to crys- 

 tallize. How much crystallized sodium carbonate is required for the conversion 

 of 25 grammes of potassium acid tartrate into Eochelle salt? 



Seidlitz powders (Compound effervescing powders) consist of a mixture 

 of 7.75 grammes (120 grains) of Rochelle salt with 2.58 grammes 

 (40 grains) of sodium bicarbonate (wrapped in blue paper), and 2.25 

 grammes (35 grains) of tartaric acid (wrapped in white paper). 

 When dissolved in water the tartaric acid acts upon the sodium 

 bicarbonate, causing the formation of sodium tartrate, while the 

 escaping carbon dioxide causes effervescence. 



Antimony and potassium tartrate, Antimonii et potassii 

 tartras, 2(KSbO.C 4 H 4 O 6 ).H,O=659.8 (Potassium antimonyl tartrate, 

 Tartar emetic). This salt is made by dissolving freshly prepared 

 antimonous oxide (while yet moist) in a solution of potassium acid 



