334 CONSIDERATION OF CARBON COMPOUNDS. 



There are three acids which are isomeric with common tartaric 

 acid, differing from it in physical, but not in chemical properties. 

 These acids are known as inactive tartaric acid, levotartaric acid, and 

 racemic acid, whilst the common tartaric acid is termed dextrotartaric 

 acid. Crude tartar sometimes contains racemic acid. 



Analytical reactions. 

 (Potassium sodium tartrate, KNaC 4 H 4 O 6 , may be used.) 



1. Neutral solutions of tartaric acid give with calcium chloride a 

 white precipitate of calcium tartrate, which, after being quickly col- 

 lected on a filter and washed, is soluble in potassium hydroxide ; from 

 this solution calcium tartrate is precipitated on boiling. (Calcium 

 citrate is insoluble in potassium hydroxide.) 



2. A strong solution of a tartrate, acidulated with acetic acid, gives 

 a white precipitate of potassium acid tartrate on the addition of potas- 

 sium acetate. (Precipitate forms slowly.) 



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

 precipitate of silver tartrate, Ag 2 C 4 H 4 O 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 be 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 



The above reaction, 3, can be used to advantage for silvering glass by operat- 

 ing as follows: Dissolve 1 gramme of silver nitrate in 20 c.c. of water, add 

 water of ammonia until the precipitate which forms is nearly redissolved, and 

 dilute with water to 100 c.c. Make a second solution by dissolving 0.2 gramme 

 of silver nitrate in 100 c.c. of boiling water, add 0.166 gramme of potassium 

 sodium tartrate, boil until the precipitate becomes gray, and filter. Mix the 

 two solutions cold and set aside for one hour, when a mirror of metallic silver 

 will be found. 



Potassium acid tartrate, Potassii bitartras, KHC 4 H 4 O 6 = 188 

 (Potassium bitartrate, 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 color- 

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

 acidulous taste; it is sparingly soluble in cold, easily soluble in hot 

 water. 



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



