158 PROVISIONAL METHODS FOR ANALYSIS OF FOODS. 



Class II. Mixtures of the above (I) with sugar. To this class belong many of the 

 effervescing compounds of the Pharmacopoeia and most of the similar proprietary 

 preparations. 



Class III. Mixtures of members of Class I with modifying agents and traces of 

 optically active substances. To this class belong mixtures containing alum, those 

 containing traces of iron and aluminum, and those of which starch is a constituent, 

 the latter containing almost invariably traces of active substances soluble in cold 

 water. Consequently all baking powders and mixtures of cream of tartar with 

 cream of tartar substitutes fall into this division. 



METHODS OF ANALYSIS. 



Class I. The method employed in the analysis of materials of this group is based 

 on the fact that in the presence of excess of ammonia the rotation of the solution is 

 proportional to the concentration of the tartaric acid, and is independent of the other 

 bases and acids present. 



(a) The substance is completely soluble in dilute ammonia. A weighed quantity of 

 the material containing not more than 1 gram tartaric acid is placed in a 25 cc measur- 

 ing flask, moistened with 3 or 4 cc of water, and cone, ammonia (sp. gr. 0.880) added 

 in quantity sufficient to neutralize all acids that may be present and leave about 1 cc 

 in excess. The actual amount of the excess is not of importance, but a greater quan- 

 tity than 1 cc of free ammonia should be avoided. The solution is then made up to 

 25 cc with water, filtered, if necessary, through a dry filter, and measured in a 20 cm 

 tube in the polari meter. 



The amount of tartaric acid (C 4 H 6 O 6 ) in grams (y) in the material taken is given 

 by the formula: 



?/=0.00519a; 



where x is the rotation in minutes. 



(b) The substance is not completely soluble in dilute ammonia. In this case calcium 

 tartrate is probably present and may be determined as follows: Treat 1 gram of the 

 substance (or an amount containing not more than 1 gram tartaric acid) in a small 

 beaker with 15 cc of water and 10 drops of cone, hydrochloric acid. Heat gently till 

 both the potassium and calcium tartrates have passed into solution, and then, while 

 still hot, add 2 cc of cone, ammonia (or enough to produce an ammoniacal smelling 

 liquid) and about 0.1 gram of sodium phosphate dissolved in a little water. Trans- 

 fer to a 25 cc measuring flask, cool, make up to the mark with water, filter through 

 a dry filter, and polarize the filtrate in a 20 cm tube. The tartaric acid is calculated 

 from the formula given under (a). 



The precipitation of the calcium by means of sodium phosphate is not absolutely 

 necessary, but when this is not done, in cases where the proportion of calcium in the 

 sample is high, there is a great tendency for the calcium tartrate to crystallize out 

 from the ammoniacal solution before the reading is made. 



The tartaric acid present as bitartrate of potash may be determined by proceeding 

 as in (a), the calcium tartrate being practically insoluble in cold ammonia solution. 



The tartaric acid present as calcium tartrate is given, with sufficient accuracy for 

 most purposes, by the difference between the results of (a) and (6). If more accurate 

 results are required, the residue insoluble in ammonia in (a) may be dissolved in a 

 little hydrochloric acid and treated as above with sodium phosphate and ammonia. 



It may be noted that the method given ^below, under Class III, is applicable to 

 this class also, but in most cases the above procedure will be found simpler. 



Class II. The determination of tartaric acid in substances of this class is an exten- 

 sion of the method given under I. In ammoniacal mixtures containing both tartaric 

 acid and sugar the rotation of each is unaffected by the presence of the other 

 substance, and consequently the rotation of the tartaric acid may be obtained by 

 subtracting from the total rotation that part due to the sugar. The cane sugar may 



