550 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



During the washing 300 c.c. of water are heated to boiling and excess 

 (30-50 c.c.) of *2N permanganate is added from a burette. The asbestos 

 and the precipitate are dropped into this solution and the remainder washed 

 in. Sufficient sulphuric acid (20 c.c. of 25 per cent.) is added, the mixture 

 well stirred and gently boiled for about 5 minutes until the yellow precipitate 

 is completely decomposed. Manganese hydroxide separates out. Standard 

 oxalic acid is run in from a burette until it has dissolved and the liquid is 

 clear and of a very faint pink colour. The excess of oxalic acid is titrated 

 with -2N permanganate. The difference between the amounts of oxalic acid and 

 permanganate gives the amount of permanganate used, 

 i c.c. -2N KMnO 4 = 0*001714 gm. K 2 O. 



It is advisable to standardise the permanganate against potassium chloride ; 

 a concentration of ptiaianganate, so that i c.c. = o - oo2 gm. K 2 O is more con- 

 venient than exactly '2N. 31-6 c.c. of this strength of permanganate = 10 

 cc. of i per cent. KC1. 



Acidity. 



In order to estimate the acidity a sufficient amount of alkali of 

 known strength is added from a burette until neutralisation is produced, 

 as indicated by the colour reaction of phenolphthalein, which has been 

 previously added. It must be remembered that different indicators 

 give different results. Congo red and methyl orange are only affected 

 by strong acids, whereas litmus and phenolphthalein are affected by 

 weak acids, such as carbonic acid. The two former do not react with 

 urine. 



Procedure. 



(a) 2$ c.c. of urine are measured out with a pipette into a flask 

 or beaker and diluted with 50-100 c.c. of water. About 6 drops of 

 phenolphthalein solution are added and 'iN sodium hydroxide is run 

 in from a burette until the solution has a permanent and distinct pink 

 colour. Owing to the presence of calcium salts in the urine the end 

 point is difficult to see. 



() To overcome the difficulty Folin recommends that the titration 

 be carried out in the presence of neutral potassium oxalate : 



25 c.c. of urine are diluted with an equal volume of water, 15 gm. 

 of finely powdered potassium oxalate and 4-5 drops of phenol- 

 phthalein are added. The solution is shaken for 1-2 minutes to dis- 

 solve most of the oxalate and whilst the solution is still cold from the 

 effect of the oxalate, it is titrated with p iN alkali until a permanent pink 

 tint remains. 



This neutral solution is used in the estimation of " ammonia " (p. 



550- 



As urine contains a mixture of acids, the result is not expressed in 

 terms of any particular acid, but in terms of -iN acid either per 100 c.c. 

 of urine or per 24 hours' quantity (diluted to 2000 c.c.), thus : 



Suppose 25 c.c. urine required 4-5 c.c. of *iN alkali, the acidity is 

 1 8 c.c. per 100 c.c. or 360 c.c. *i N per 24 hours' quantity. 



