ZINC OXIDE 231carbonate in 500 cc. of water and filter. Pour the filtrate into the
purified ZnSO 4 solution. Note if there is any effervescence. The
precipitate should be pure white. Stir or shake to aid the escape of
carbon dioxide. Fill the bottle to the top with water, stir, and
let stand until the white precipitate has settled to one-third or
one-fourth the volume of the bottle. This will take about 30
minutes. Draw off the clear solution and wash the remaining
precipitate by decantation until it is calculated that it is con-
taminated with less than 0.1 per cent of the soluble sodium sulphate
present at first (see Note 5 (6), page 10). Finally, transfer the
sludge to a large, ordinary filter (Note 4 (c), page 7), and allow it
to drain over night. It should now be in the form of a jelly-like
cake which holds its shape. Without removing it from the filter
lift the latter from the funnel, unfold it without tearing, spread it
flat on paper towels, and leave it on the steam table until the ma-
terial is dry. Do not try to remove the material from the filter
until it is absolutely dry; it will then flake off clean when the paper
is bent. Heat the basic zinc carbonate in the 4-inch porcelain dish
gently with a flame 2 inches high which is held in the hand and
played over the bottom of the dish. The material falls to an
impalpable powder which seethes, or appears to boil, as long as
gases are being expelled. Avoid heating the dish to any faintest
sign of visible redness. The product is faintly yellow when hot
but it should become pure white when cold. Test the product by
wetting 0.1 gram with 2 cc. of water and adding a few drops of
6N HC1. The zinc oxide should dissolve, and there should be no
trace of effervescence. Put up the preparation in a 2-ounce
cork-stoppered bottle.
QUESTIONS- Why could not the precipitate of basic zinc carbonate have
been advantageously freed from the solution by means of a suction
filter? - Which is more readily decomposed by heat, calcium car-
bonate or zinc carbonate? Which then is the more strongly basic,
calcium oxide or zinc oxide? - To a solution of zinc sulphate add a solution of sodium
hydroxide, drop by drop, until the precipitate first formed re-
dissolves. How is zinc hydroxide similar to aluminum hydroxide