DIBASIC AND TRIBASIC ORGANIC ACIDS. 291 



3. A neutral solution of a tartrate gives with silver nitrate 

 a white precipitate of silver tartrate, Ag 2 C 4 H 4 6 , which blackens 

 on boiling, in consequence of the decomposition of the salt with 

 separation of silver. If, before boiling, a drop of ammonia 

 water is added, a mirror of metallic silver will form upon the 

 glass. 



4. Sulphuric acid heated with tartrates chars them readily. 



5. Tartrates, when heated, are decomposed (blacken), and 

 evolve a somewhat characteristic odor resembling that of burnt 

 sugar. 



Potassium acid tartrate, Potassii bitartras, KHC 4 H 4 6 = 188 (Bi- 

 tartrate of potassium. Cream of tartar). The formation of this 

 salt in the crude state (argol) has been explained above. It is 

 purified by dissolving in hot water and crystallizing, when it is 

 obtained in colorless crystals, or as a white, somewhat gritty 

 powder of a pleasant, acidulous taste; it is sparingly soluble 

 in cold, easily soluble in hot water. 



Potassium tartrate, Potassii tartras, 2(K 2 C 4 H 4 6 )H 2 = 470 



(Tartrate of potassium). Obtained by saturating a solution of 

 potassium acid tartrate with potassium carbonate: 



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



Potassium Potassium Potassium 



acid 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 6 . 

 4H..O = 282 (Tartrate of potassium and sodium, 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. 



Seidlitz powders consist of a mixture of 120 grains of Rochelle 

 salt with 40 grains of sodium bicarbonate (wrapped in blue 

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

 When dissolved in water, the tartaric acid acts upon the sodium 



