546 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Calcium and Magnesium. 



Only small amounts of calcium and magnesium are present in urine so 

 that large quantities of urine must be used in their analysis. Two methods 

 are used for their estimation : 



(a) 200 c.c. of urine are made alkaline with ammonia; the precipitate, 

 consisting of calcium and magnesium phosphates, is filtered off, washed and 

 dissolved in the smallest quantity of dilute hydrochloric acid. Sodium acetate 

 is added to the solution and the calcium is precipitated as oxalate by adding 

 ammonium oxalate. The precipitate is filtered off, washed and dried at 100; 

 the precipitate and filter paper are heated in a weighed crucible until of con- 

 stant weight. The increase in weight gives the amount of calcium oxide. 



The filtrate and washings from the calcium oxalate, to which a few drops 

 of sodium phosphate solution may be added, are made alkaline with 

 ammonia. Ammonium magnesium phosphate l is precipitated and converted 

 into pyrophosphate as in the gravimetric estimation of inorganic phosphates 



(P- 545)- 



(b) 200 c.c. of urine are evaporated in portions of 25 or 50 c.c. with 10 

 c.c. of sulphuric acid contained in a 200 c.c. conical Jena glass flask and 

 oxidised by adding nitric acid as described under estimation of total phosphates 

 (p. 545). When the oxidation is complete about 10 c.c. of water are added 

 and the mixture again evaporated so as to remove nitric acid as completely 

 as possible. To the remainder about 40 c.c. of water are added and 1-2 

 volumes of alcohol. The mixture is warmed on a water-bath and allowed to 

 stand until it is cold. Calcium sulphate is precipitated ; it is filtered off on 

 a weighed Gooch crucible, washed and dried at 100 or at a red heat. 



The filtrate and washings from the calcium sulphate are evaporated to 

 remove alcohol and treated with concentrated nitric acid to remove organic 

 matter as above. The acid remainder is diluted with water and made 

 alkaline with ammonia. Ammonium magnesium phosphate is precipitated 

 and treated as above. 



(c) Cahen and Hurtley 2 found that the incinerated residue of urine or 

 tissues was most suitably dissolved in phosphoric acid. The calcium is pre- 

 cipitated from this solution as oxalate and estimated volumetrically or gravi- 

 metrically. 



100 c.c. of urine are evaporated on a water- bath, dried at 120 and 

 incinerated. 100 gm. of heart, muscle, etc., less of an aorta rich in calcium, 

 are dried and incinerated. 



10 c.c. of phosphoric acid of sp. gr. 1*2 (i vol. syrupy phosphoric acid to 

 3 vols. water), or 20 c.c. if much calcium be present, are added to the residue. 

 The calcium dissolves completely on warming. The solution is filtered, diluted 

 with 50 or 100 c.c. of water and treated with 5 times the necessary quantity 

 of oxalic acid solution (37*8 gm. per litre), e.g. if *2 gm. of calcium be present 

 as calcium carbonate, 100 c.c. would be necessary. Calcium oxalate is rapidly 

 precipitated in a crystalline state, but completion of the precipitation only 

 occurs on adding an equivalent volume of ammonia (10*2 gm. NH 3 per litre) ; 

 it must be added slowly from a burette with shaking, if magnesium be present. 

 Calcium oxalate is not precipitated in a su- table condition if ammonium 

 oxalate be used. The calcium oxalate is filtered off on a small conical 

 Buchner funnel, washed 7 times with cold water, returned together with filter 

 paper to the vessel in which it was precipitated and titrated with permanganate 

 solution after adding 10 c.c. of dilute sulphuric acid (i : i) and heating to 

 nearly boiling. The sulphuric acid usually requires a separate titration and 

 this figure is deducted from the previous figure. 



The gravimetric estimation is effected as under (a). The magnesium in 

 the filtrate is precipitated on adding concentrated ammonia to the filtrate and 

 estimated as pyrophosphate. 



1 Minute quantities of ferric phosphate are present in this precipitate, but may 

 neglected. 



2 Biochem. J., 1916, 10, 308. 



