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CHEMICAL ANALYSIS 33 



their deliquescent character; therefore some operators add before 

 evaporation 2 or 3 c.c, accurately measured, of ^ sodium 

 carbonate, and subtract the weight (0*0053 gramme for each 

 c.c.) from the residue, but this should be called a " carbonated 

 residue." 



The ash can be determined (in porcelain or platinum) by 

 gently igniting and again weighing, but, from the presence of 

 ammonium salts and for other reasons, the " loss on ignition " 

 does not measure simply the organic matter. It is very difficult 

 to burn off all the carbon without volatilizing inorganic matter. 

 At Berlin the ash is moistened with distilled water, and again 

 evaporated and ignited, and the process is repeated four to six 

 times until the black particles have all been burnt away.^ 



Suspended matter can be estimated in five different ways : 



1. By Difference. — Well shake the sample, measure out a 

 portion, evaporate, dry, and weigh as above. Filter another 

 portion through paper (rejecting the first runnings), and 

 evaporate, dry, and weigh a measured portion of the filtrate. 

 The difference between the weights gives approximately the 

 suspended matter separated by filtration. But it is difficult to 

 dry to the same degree. 



2. A Swedish filter-paper of 12*5 millimetres diameter is 

 dried for half an hour at 100° C. in a weighing-bottle — a couple 

 of test-tubes sliding into one another answer very well — cooled 

 in the desiccator, and accurately weighed. A measured volume 

 of the shaken sample is filtered through, dried at 100° C, and 

 weighed. The difference gives the total suspended matter. 

 Gently incinerated in a platinum dish it gives the inorganic 

 suspended matter, which can be examined for sand, oxide of 

 iron, and other matters in the ordinary way. Many sewages 

 block filter-paper hopelessly ; hence the next processes. 



3. The sample is drawn through a hardened filter-paper by 

 means of an exhaust pump. If not allowed to get dry, the 

 solids can be transferred by a fine jet of water from a wash- 

 bottle to a weighed dish, evaporated, weighed, then burnt, and 

 again weighed as above. An advantage of this method is that 

 the suspended matters can be examined microscopically. It is 

 not, however, accurate as to weights. 



4. The Gooch crucible with prepared asbestos gives accurate 

 results in the minimum time with the use of a filter-pump, and 



^ Report of Royal Bureau of Sewage Disposal and Water Purification, Berlin, 

 1904. 



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