CH. X.] 



CHLORIDES. 



259 



2. Coagulation of the proteins. Place the paper in a clean, dry, 

 rather wide test-tube. Into another clean tube measure 10 cc. of 

 the acid magnesium sulphate and boil. Whilst vigorously boiling 

 pour the solution on to the paper and allow it to stand for 30 minutes. 



3. Removal of the silver chloride. To the tube, still containing 

 the paper, add 2 cc. of N/ioo silver nitrate and the measured amount 

 of kieselgur. Grease the rim of the tube with a 

 smear of vaseline and shake vigorously. Filter 

 through p a 6 cc. Gooch crucible into a 125 cc. 

 filtering"~flask connected to a water pump (see 

 fig- 33)-||The bottom of the crucible is covered 

 with a piece of filter paper cut a trifle larger than 

 the bottom. The paper is then washed with a 

 little water which is sucked through. Care must 

 be taken to see that all the perforations of the 

 crucible are covered. Empty the flask and before 

 turning on the pressure fill the crucible with the 

 mixture in the tube, being careful to get as much 

 kieselgur as possible into the crucible. Allow a 

 few drops to filter through before turning on the 

 pressure. Filtration is rapid and the nitrate 

 is usually quite clear. If it is cloudy it must be 

 refiltered. 



Fig. 33. Gooch 

 crucible and fil- 

 tering apparatus 

 for micro-analysis 

 of chlorides. 



4. Washing the paper. When the whole of the fluid has been 

 filtered add 5 cc. of distilled water to the tube, shake vigorously, 

 pour it into the crucible and filter. Repeat this operation once 

 more. 



5. Titration of the silver. To the fluid in the flask add 2 cc. 

 of the iodide-iodate solution and a few drops of the starch solution. 

 Titrate against a white ground with N/ioo silver nitrate from 

 a icroburette. The blue colour gradually fades as the silver 

 iodide is formed and then sharply disappears, leaving a yellowish 

 green solution. 



6. Calculation of results. 2 cc. of N/ioo silver = 2 cc. of the 

 iodide-iodate solution. The volume of N/ioo silver required to effect 

 the disappearance of the blue colour is the volume of silver that has 



