24 TESTING MISCELLANEOUS SUPPLIES. 



(e) STANDARDIZING THE FEBBOCYANID SOLUTION. 



Prepare a solution of potassium ferrocyanid containing 22 grams of the 

 crystallized salt to the liter. Standardize this solution as follows: Weigh 

 carefully about 0.2 gram of pure ziiic and dissolve in 10 cc of strong hydro- 

 chloric acid (sp. gr. 1.20), using a 400 cc covered beaker. Dilute somewhat, 

 place a bit of litmus paper in the solution as an indicator, and make faintly 

 alkaline with ammonium hydroxid. Again acidify faintly with hydrochloric 

 acid and then add 3 cc excess of the strong acid. Dilute now to about 250 cc 

 and heat nearly to boiling. 



Titrate the hot liquid with the ferrocyanid solution as follows: Pour off 

 about one-third of the zinc solution and set it aside in a beaker. Titrate the 

 remainder by running in a few cubic centimeters at a time until a drop, when 

 tested on a porcelain plate with a drop of a 5 per cent solution of uranium 

 nitrate, shows a brown tinge. Now add the greater part of the reserved por- 

 tion and continue the titration more cautiously until the end-point is again 

 passed. Finally, add the last of the reserved portion, and to save rinsing out 

 the beaker, pour a large part of the solution back into it again and then empty 

 it once more. 



Finish the titration very carefully, ordinarily by testing after each addition 

 of two drops. Instead of using a single drop of the zinc solution for the test, 

 the reaction is sharper if a quantity equivalent to several drops is taken. If 

 this is done near the end of the titration the amount of zinc lost thereby will 

 be insignificant. A convenient way of making the test is to use a medicine- 

 dropper and place a single drop of the uranium solution in each depression 

 of the test-plate at the outset. By using a glass tube instead of a rod for a 

 stirrer, any desired quantity of the solution can be quickly removed for a test. 

 When the final brown tinge is obtained, note the reading of the burette, and 

 then wait a minute or two and observe if one or more of the preceding tests 

 do not also develop a color. The end-point is always passed by a test or two 

 and the burette reading must be corrected accordingly. A further correction 

 must also be made for the amount of ferrocyanid required to produce a color 

 under the same conditions when no zinc is present. This is ordinarily 2 drops. 

 One cubic centimeter of the standard solution will equal about 0.005 gram 

 of zinc. 



3. Calcium Pigments. 



Whiting is calcium carbonate and is used extensively as an adulterant. 

 Plaster of Paris is dehydrated calcium sulphate and gypsum is the natural 

 hydrated calcium sulphate (CaSO4+2H 3 O). Besides these a precipitated cal- 

 cium sulphate IB used, especially as a basis upon which to precipitate anilin 

 colors. A microscopic examination is therefore of importance. 



Free lime is objectionable and is especially to be looked for in whiting. Boil 

 2 grams of the sample for five minutes with 100 cc of water and add phe- 

 nol phthalein. If red color develops, free lime may be assumed to be present, 

 though the alkaline reaction may possibly be due to hydrates of sodium or 

 potassium. 



For quantitative analysis, when such is necessary, boil 1 gram of the sam- 

 ple with 26 cc of hydrochloric acid '(1:1), add 150 cc of water, boil, filter, 

 wash, ignite, and weigh the insoluble. Precipitate iron and aluminum hydrates 

 from the filtrate with ammonium hydroxid, dissolve in hydrochloric acid and 

 n precipitate, wash, ignite, and weigh. Unite the filtrates from the iron and 



