344 METHODS OF ANALYSIS [Chap. 



utes. Filter on a Gooch crucible with a thin layer of paper pulp and wash with 95% 

 alcohol until 2 cc. of the filtrate do not change the color of litmus tincture diluted 

 with water. Place the precipitate in a small casserole, dissolve in 50 cc. of hot water 

 and add N/5 potassium hydroxid, leaving it still strongly acid. Boil for a minute. 

 Finish the titration, using phenolphthalein as an indicator and correct the reading 

 by adding 0.2 cc. One cc. of N/5 potassium hydroxid, under the above conditions, 

 is equivalent to 0.02641 gram of tartaric anhydrid, 0.03001 gram of tartaric acid, 

 or 0.03763 gram of potassium bitartrate. Standardize the N/5 potassium hydroxid 

 by means of pure potassium bitartrate. 



The accuracy of this method is indicated by the agreement of the percentages of 

 potassium bitartrate in cream of tartar powders containing no free tartaric acid, 

 obtained by calculation from the tartaric acid, with those obtained by calculation 

 from the potassium oxid''. 



FREE TARTARIC ACID. 



14 Qualitative Test. — Tentative. 



Extract 5 grams of the sample with absolute alcohol and evaporate the alcohol 

 from the extract. Dissolve the residue in dilute ammonium hydroxid, transfer to 

 a test tube, add a good sized crystal of silver nitrate and heat gently. Tartaric 

 acid is indicated by the formation of a silver mirror. If desired, the absolute alco- 

 hol extract may be tested as directed under 1 2. 



15 Quantitative Method. — Tentative. 



Calculate the percentage of tartaric anhydrid combined with the potash as bi- 

 tartrate, if any, and subtract this from the percentage of total tartaric anhydrid. 

 The difference is the tartaric anhydrid originally added as the free acid, although, 

 if the sample has been kept for a long time or has been improperly stored, a portion 

 or all of this acid may exist at the time of analysis as the sodimn salt resulting from 

 the reaction in the can with the sodium bicarbonate. Multiply by 1.137 to obtain 

 the percentage of tartaric acid. 



16 POTASSIUM BITARTRATE.— TENTATIVE. 



If, as is usually the case, potassium bitartrate is the only potassium salt present, 

 multiply the percentage of total potash, determined as directed under 24, by 3.994. 



STARCH. 



17 Direct Inversion Method. — Tentative. 



(For all baking powder ingredients free from lime.) 



Weigh 5 grams of the powder into a 500 cc. graduated flask and proceed as directed 

 under VIII, 60. 



18 Indirect Method^. — Tentative. 



(For phosphate, alum phosphate and all other baking powders containing lime.) 



Mix 5 grams of the powder with 200 cc. of 3% hydrochloric acid in a 500 cc. grad- 

 uated flask and allow the mixture to stand for an hour, with frequent shaking. 

 Filter on an 11 cm. hardened filter, taking care that a clear filtrate is obtained. 

 Rinse the flask once without attempting to remove all the starch, and wash the 

 paper twice with cold water. Carefully wash the starch from the paper back into 

 the flask with 200 cc. of water. Add 20 cc. of 25% hydrochloric acid and proceed as 

 directed under VIII, 60. 



