250 c.c. of the filtrate, representing a
quarter of the sample taken, is then titrated with standard acid. The
standard acid is made by diluting 250 c.c. of the normal acid to 1
litre. The c.c. of acid used multiplied by 2 gives the percentage. A
correction must be made to counteract the effect of impurities in the
baryta as well as errors inherent in the process. This is small, and its
amount is determined by an experiment with 3.55 grams of pure sodium
sulphate.
EXAMINATION OF COMMON SALT.
~Moisture.~--Powder and weigh up 10 grams of the sample into a platinum
dish. Dry in a water oven for an hour, and afterwards heat to bare
redness over a Bunsen burner. Cool, and weigh. The loss gives the water.
~Chlorine.~--Weigh up two separate lots of 1 gram each; dissolve in 100
c.c. of water, and determine the chlorine by titrating with the standard
silver nitrate solution, using chromate of potash as indicator. See
_Chlorine_.
~Insoluble Matter.~--Dissolve 10 grams of the salt in water with the
help of a little hydrochloric acid. Filter off the sediment, wash,
ignite, and weigh. This residue is chiefly sand. Dilute the nitrate to
500 c.c.
~Lime.~--Take 250 c.c. of the filtrate, render ammoniacal and add
ammonium oxalate; wash, dry, and ignite the precipitate. Weigh as lime
(CaO).
~Magnesia.
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