By HERBERT JACKSON.
Having had occasion some short time ago to examine a hard water which
owed half its hardness to salts of magnesium, I noticed that the soap
test, applied in the usual way, gave a result which differed very much
from that obtained by the quantitative estimation of calcium and
magnesium. A perfectly normal lather was obtained when soap had been
added in quantities sufficient to neutralize 14° of hardness, whereas
the water contained salts of calcium and magnesium equivalent, on
Clark's scale, to a hardness of 27°.
Although I was aware that similar observations had been made before, I
thought that it might be useful to determine the conditions under
which the soap test could not be depended upon for reliable results.
I found with waters containing calcium or magnesium alone that,
whenever salts of either of these metals were in solution in
quantities sufficient to give 23° of hardness on Clark's scale, no
dependence could be placed upon the results given by the soap test. In
the case of waters containing salts of both calcium and magnesium, I
found that if the salts of the latter metal were in solution in
quantities sufficient to give more than 10° of hardness, no evidence
could be obtained of their presence so long as the salts of calcium in
the same water exceeded 6°; in such a case a perfect and permanent
lather was produced when soap had been added equivalent to 7° of
hardness.
If any water be diluted so as to reduce the proportions of the salts
of calcium and magnesium below those stated above, perfectly reliable
results will of course be obtained.
Instead of dilution I found that heating the water to about 70° C. was
sufficient to cause a complete reaction between the soap and the salts
of calcium and magnesium, even if these were present in far larger
quantities than any given here.
The experiments so far had all been made with a solution of Castile
soap of the strength suggested by Mr. Wanklyn in his book on "Water
Analysis." My attention was next directed to the use of any one of the
compounds of which such a soap is composed. I commenced with sodium
oleate, and found that by employing this substance in a moderately
pure condition, perfectly reliable results could be obtained in very
hard waters without the trouble of either diluting or heating. I was
unable to try sodium stearate directly because of the slight
solubility of this substance in cold water or dilute alcohol; but I
found that a mixture of sodium oleate and stearate behaved in exactly
the same manner as the Castile soap.
I am not prepared at present to state the exact reaction which takes
place between salts of calcium and magnesium and a compound soap
containing sodium oleate and stearate. I publish these results because
I have not noticed anywhere the fact that some waters show a greater
hardness with soap when their temperatures approach the boiling point
than they do at the average temperature of the air, it being, I
believe, the ordinary impression that cold water wastes more soap than
hot water before a good and useful lather can be obtained, whereas
with very many waters the case is quite the reverse. Neither am I
aware at present whether it is well known that the use of sodium
oleate unmixed with sodium stearate dispenses with the process of
dilution even in very hard waters.--_Chem. News._
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