GASTRIC TESTS. 85 



c. Add Congo red solution. The red changes to blue. 

 Lactic acid may produce a purple color. 



Test-paper can be made from this reagent by dipping 

 a porous paper in the solution and drying. It can be used 

 in testing for HC1 like the solution. 



d. Add to a few drops of liquid an equal volume of an 

 alcoholic solution containing 2 per cent, phloroglucin and 

 1 per cent, vanillin. Evaporate to dryness in a porcelain 

 dish on a water-bath or by carefully warming over a flame. 

 A rose-red color remains. (Gunzburg's test.) 



e. To a few drops of the HC1 solution in a porcelain 

 dish add a little of an alcoholic solution of resorcin and 

 sugar. Evaporate to dryness and a red color appears. 

 (Boas's test.) (See page 221.) 



199. TOEPFER'S METHOD FOR DETERMINING THE 

 FREE AND COMBINED HC1. Into each of three beakers 

 (A., B, and G) measure with a pipette 5 cubic centimeters 

 of gastric juice. Titrate each of them with decinormal 

 ISTaOH (4 grammes to the liter), using, as indicators, in 

 A phenol-phthalein, and adding the alkali until the liquid 

 is a faint red; in B 3 or 4 drops of a 1-per-cent. solution 

 of alizarin sodium sulphonate in water. The NaOH must 

 be added until the liquid is a violet color, not stopping 

 with the reddish shade. The exact shade is very nearly 

 that given by the indicator to a 1-per-cent. solution of the 

 common sodium phosphate. With C the indicator is 3 or 

 4 drops of a 0.5-per-cent. alcoholic solution of dimethyl- 

 amido-azobenzene. HC1 gives a red color with this, and 

 the NaOH is added until this disappears and the color 

 changes to yellow. 



The phenol-phthalein is turned red by all the com- 

 pounds which have an alkaline reaction. A, therefore, rep- 

 resents total acidity. The alizarin sodium sulphonate re- 



