514 CONSIDERATION OF CARBON COMPOUNDS. 



Analytical reactions. 

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



1. A neutral solution of a tartrate gives 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.) 



Calcium tartrate is soluble in a solution of an alkali tartrate; 

 hence, unless a sufficient amount of calcium chloride is added, a pre- 

 cipitate will not be obtained. 



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. The precipitate, which forms slowly, is soluble in alka- 

 lies and in mineral acids. 



In the case of potassium sodium tartrate, or potassium tartrate, 

 addition of acetic acid alone precipitates potassium acid tartrate. 



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. 



Silver tartrate is soluble in a solution of alkali tartrate ; hence the 

 silver nitrate solution must be added in sufficient quantity to obtain 

 a permanent precipitate. 



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 

 ammonia water 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^O,, = 186.78 

 (Potassium bitartrate, Cream of Tartar). The formation of this salt in 



