34 PUBLIC HEALTH CHEMISTRY 



solution, every c.c. of which used equals 0-00008 grm. O, 

 or 0-055825 c.c. oxygen. Starch solution is used for the 

 end reaction. The amount of oxygen found is present in 

 the capacity of the bottle, less the 2 c.c. of solutions added. 

 The result is returned in parts by weight per 100,000, or in 

 cubic centimetres per litre. 



1. 2MnCl 2 + 4NaOH = 4NaCl + 2Mn(OH) 2 . 



2. 2Mn(OH) 2 + O + H 2 O = 2Mn(OH) 3 . 



3. 2Mn(OH) 3 + 6HC1 = 2MnCl 3 + 6H 2 O. 



4. 2MnCl 3 + 2KI = 2MnCl 2 + 2KC1 + I 2 . 



The process must be done rapidly. Nitrites liberate 

 iodine and so vitiate the result, increasing it. Much 

 organic matter interferes with the method, as it absorbs 

 the liberated iodine, thus diminishing the result. Rapid 

 working diminishes the latter interference. 



The amount of dissolved oxygen in a water is influenced 

 mainly by temperature, being less in summer and more in 

 winter. Ordinary tap water in this country contains on 

 an average 7 c.c. per litre, which is about i part by weight 

 in 100,000. Water is saturated at 5 C, 10 C, 15 C, and 

 20 C. respectively, by 8-68 c.c., 7-77 c.c., 6-96 c.c., and 

 6-28 c.c. per litre. 



Suphuretted Hydrogen. This is estimated by titration 

 with N /ioo iodine, which is decolorized by the H 2 S ; thus 

 H 2 S + I 2 = 2HI + S. 



PROCESS. Take 10 c.c. N/ioo iodine in a white porcelain 

 basin. Fill a burette with sample, and add to basin until 

 colour is gone, using starch for end reaction. The N/ioo 

 iodine is made as above, and is standardized against N/ioo 

 thiosulphate solution. Every c.c. of N/ioo I equals i c.c. of 

 N/ioo H 2 S. But N/i H 2 S is 17 grm. per litre, therefore 

 i c.c. N/i = 0-017 rm -. an d I c - c - N/ioo = 0-00017 grm. 

 H 2 S, or 0-17 mgr. Hence the number of c.c. of N/iool 

 used x 0-17, gives the number of mgr. of H 2 S in the amount 

 of sample run in from burette to decolorize the iodine. 



HARDNESS (TOTAL, TEMPORARY, OR PERMANENT). 



Hardness is due to the presence in a water of metallic 

 salts which form insoluble compounds with the fatty acids 

 usually present in soap. A soap is the oleate, stearate, 



