Movement of Water in Irrigated Soils 
641 
Mar. i, 1924 
CALCIUM 
The fact that calcium forms an insoluble compound when combined 
with oxalic acid is the basis of one group of calcium determinations. The 
precipitated calcium oxalate may be separated by filtering and the 
quantity of calcium weighed as the oxid after burning or the quantity of 
combined oxalic acid determined volumetrically by the use of potassium 
permanganate. 
Another reaction of calcium is also used to some extent. This is its 
reaction with soap. Both calcium and magnesium form insoluble com¬ 
pounds with soap, and consequently it is possible to estimate the quantity 
of calcium and magnesium there is in a solution by using a standard soap 
solution added a little at a time. The solution being tested is shaken in a 
bottle after each addition of the soap solution. When all the calcium and 
magnesium have been precipitated by the soap, the solution quickly 
forms a lather when shaken. 
MAGNESIUM 
The gravimetric determination of magnesium is based on the fact that 
this base forms an insoluble precipitate with phosphoric acid. When all 
the calcium has been precipitated from a solution by the use of oxalic 
acid, as mentioned above, the addition of sodium phosphate results in 
the formation of magnesium phosphate which may be filtered off and 
weighed. 
The magnesium content of a solution may be estimated by first deter¬ 
mining the total content of calcium and magnesium, as with the soap 
solution; the calcium content may then be determined on another sample 
of the solution and the magnesium estimated by difference. 
SODIUM AND POTASSIUM 
The,accurate gravimetric determination of sodium and potassium in 
solution is rendered difficult by the fact that most of the compounds of 
these bases are soluble. The usual method of procedure is to separate 
out the other bases and also remove all the sulphate from the solution, 
replacing it with chlorin to combine with the sodium and potassium and 
then evaporate to dryness and weigh the residue as the chlorids of the two 
bases. If it is desired to determine the potassium separately from the 
sodium the residue is redissolved and the potassium is precipitated with 
platinic chlorid. Many water analysts do not undertake to make direct 
determinations of sodium and potassium because the processes are 
difficult and tedious. Having determined the total salts or the totals of 
the acids and of the calcium and magnesium in the solution they estimate 
the sodium by difference or by the requirement of the acids after elimi¬ 
nating the quantity of the acids required to combine with the calcium and 
magnesium. 
sulphate 
The sulphate content of the solution may be determined by the addition 
to it of an excess of barium chlorid. This results in the formation of 
the insoluble barium sulphate which may be filtered off and weighed. 
The freshly precipitated barium sulphate forms a white turbidity in the 
solution, so that a fair estimate of the quantity of sulphate may be made 
by comparing this turbidity with known standards. 
