MIXED COMPOUNDS 77 



in which the acid is found in grapes and is the common domestic 

 household article cream of tartar. It is a constituent of most 

 baking powders. Its use in cooking is due to its acid character, 

 one of the acid hydrogens in tartaric acid still remaining. As 

 an acid it reacts in solution with carbonates, such as sodium 

 bicarbonate or baking soda, liberating carbon dioxide ; and this 

 evolution of carbon dioxide gas expands or raises the dough of 

 bread or cake and makes a light porous product. 



Rochelle Salt. — This compound is the mixed potassium 

 and sodium salt of tartaric acid, i.e. being dibasic the acid 

 has one acid hydrogen replaced by potassium and the other by 

 sodium. 



CH(OH) - COOK 



CH(OH) - COONa 



Potassium-sodium tartrate (Rochelle salt) 



The salt forms large crystals containing four molecules of 

 water of crystallization. It is a very important reagent as 

 a constituent of Fehling^s Solution. This will be described 

 later in discussing methods of analyzing sugars. It is also 

 used in medicine as a constituent of Seidlitz powders, free tar- 

 taric acid and sodium bicarbonate being the other constituents. 



Tartar Emetic. — This is a compound of tartaric acid used 

 in medicine as an emetic. It is a mixed potassium and anti- 

 monyl salt of tartaric acid, one acid hydrogen being replaced 

 by potassium and the other by the antimonyl group (SbO). 



CH(OH) - COOK 

 CH(OH)-COO(SbO) 



Potassium-antimonyl tartrate (Tartar emetic) 



Free tartaric acid, or its salts, have several properties which 

 enable them to be easily detected. When the acid, or a salt 

 plus H2SO4, is heated it decomposes, chars, and gives forth 

 an odor resembling burnt sugar. When a neutral solution of 

 a tartrate is added to an ammoniacal solution of silver nitrate, 



