CAUSTIC SODA AND POTASH 641 



bottom of which is covered about one-half inch deep with recent- 

 ly ignited, perfectly dry sand, and in which is a small glass rod. 

 Place in the beaker about five grams of the sample; add 25 

 cubic centimeters of alcohol or more if necessary, and dissolve 

 the soap in the alcohol by constant stirring on the water bath. 

 Evaporate the alcohol and finally dry in an oven at 110 until 

 the weight is constant. A few precautions should be taken which 

 are not mentioned in the above method, namely : If the soap is 

 hard the five grams should be cut off in very thin strips so that 

 it will dissolve more readily in the alcohol ; also most samples of 

 soap never come to a constant weight on drying, but gain or 

 lose nearly indefinitely. It is, therefore, best to heat the soap 

 at 110 until it is nearly dry and weigh, then return the soap to 

 the oven and dry another half hour. Continue this alternate 

 drying and weighing until the weight changes only a few milli- 

 grams during the course of a half hour's drying. 



Total Alkali. 18 Dissolve a weighed quantity of the soap in 

 water; decompose with hydrochloric acid, filter off the water 

 from the fat, and wash with cold water. Determine both po- 

 tassium and sodium in the filtrate first as mixed chlorids in the 

 ordinary manner and then determine the potassium by means of 

 platinum chlorid. 



A rapid but only approximate determination of the alkali in 

 soap is made in the following manner: Weigh a small quan- 

 tity of the soap, treat with concentrated sulfuric acid, burn, re- 

 peat treatment with sulfuric acid, and burn again. Add a small 

 amount of ammonium carbonate to the dish, cover; and heat. 

 Repeat this a number of times till all bisulfates have changed 

 to sulfates. Test the residue qualitatively to determine whether 

 it is sodium or potassium sulfate, and calculate the residue to 

 soda or potash, as the case may be. 



537. Caustic Soda and Potash. These two substances are some- 

 times used in water solutions as winter washes. They are more 

 often used by the entomologist, however, in preparing resin and 

 fish oil soaps, lye-sulfur mixtures, etc. To judge of their value 

 for any of the above purposes and to calculate how much of 

 18 Bureau of Chemistry, Bulletin 107, 1907 : 31. 

 21 



