DISINFECTANTS, ETC. 137 



Ferrous Sulphate. Reverse the above process, using 

 a standard solution of permanganate : K 2 Mn 2 O 8 -f- 

 ioFeSO 4 . 7 H 2 + 8 H 2 SO 4 = K 2 SO 4 + -2 MnSO 4 -f 

 5 Fe 2 (SO 4)3+ I5H 2 O. The standard solutions can be 

 made any .suitable strengths, but as we have already in 

 use a solution of permanganate, 3-95 grm. to I litre, it can 

 be used for the ferrous sulphate : i c.c. = 0-03475 grm. 

 FeSO 4 7H 2 O. Similarly, standard ferrous sulphate solu- 

 tion 34-75 grm. per litre, i c.c. = 3-95 mgr. permanganate. 



Carbolic Acid, or phenol, C 6 H 5 OH, is detected 

 qualitatively : (i) By its odour ; (2) Add a few drops of 

 ferric chloride a violet coloration discharged by acetic 

 acid (distinction from salicylic acid) ; (3) Bromine water 

 gives a yellowish- white precipitate of tribromophenol, 

 even in very dilute solutions ; (4) Add one-fourth volume 

 of ammonia and then a few drops of dilute bleaching- 

 powder solution a blue colour develops. 



Quantitatively, carbolic acid is estimated by the process 

 of Koppeschaar. The phenol is precipitated as tri-bromo- 

 phenol by the addition of excess of bromine solution. The 

 overplus of bromine is determined by adding potassium 

 iodide from which the bromine displaces iodine, and the 

 amount of the latter is found by titration with N/io 

 sodium thiosulphate solution. 



C 6 H 5 -OH + 3 Br 2 = C 6 H 2 Br 3 -OH + 3 HBr. 

 Br + 2KI = 2KBr + I. 



2Na 2 S 2 O 3 + I 2 = Na 2 S 4 O e 



The bromine solution used may be N/io, or can have its 

 strength estimated at the time in terms of iodine and 

 sodium thiosulphate. 



Process. Weigh out 1*556 grm. of the sample, and 

 dissolve in sufficient water to make 1000 c.c. Take 25 c.c. 

 of dilution (= 0-0389 grm. of specimen) in a glass-stop- 

 pered bottle ; add 30 c.c. N/io bromine solution ; 5 c.c. of 

 strong hydrochloric acid ; and 5 c.c. of potassium iodide 

 solution (20 per cent weight in volume). Stopper bottle 

 quickly and shake well. Remove stopper and wash neck 

 and stopper, allowing washings to run into bottle. Add 

 i c.c. of chloroform and shake mixture. Titrate with N/io 



