76	QUANTITATIVE AGMCrLTUKAL A\AL}'M8
figure, and especially so if the solution is boi>d to fame flocoula-
tion of the colloid. Either acids or basos will dissolve* the pre-
cipitate; acids form soluble aluminium salts and bases form
soluble aluminates:
Al(OH), + 3HOI-* AK'1_3 + »H«(>;	(1)
A1(OH)_8 + NaOH -» XuAK), + 2H,O.	(2)
The possibility of the second reaction makes wroswiry the u«c
of ammonium hydroxide, rather than sodium or potiiHHimn
hydroxide, for the precipitation, as the excess of ammonia may
be removed by boiling the solution.
Determination of Aluminium.—Fill n weighing bottle with the powdered
sample of an aluminium salt. ( hum me the method tn he used in weighing
according to the nature of the substance and weigh two wimples of about
1 grn each into Pyrcx beakcra. Dissolve in RH» ec of water ami add
'dilute, recently filtered ammonium hydroxide, stirring until the liquid
is distinctly basic, as shown by a drop of methyl ml added to the solution.
Boil until the precipitate is coagulated and until thi* iwlor of ammonia above
the solution is faint. Boiling after the* odor has disappeared will fiiitst' «omc
of the precipitate to return to the Molutiou:
A1(OII)_8 -f :JXH_4C'1	- AK'l, -i- :tNH_3 -j- :Hii<).
Allow the precipitate to Hc.t.tlo and thfn filter fhnnigh p»pcr, UMiiig a
filter pump attached to a boll jar or filter ilmk mid plnnu^ u nupport-ing
cone of hardened paper or platinum in thc» futtm*!. W»]t with hot d»tilk»d
water containing 1 per ecmt of amtnontuin nitrate*, until tin* wanhingK arc
free from chlorides, shown by adding a drop of nitric nri*I mid it f«*w drop
of silver nitrate solution to a small amount of I In* wiwIiirigH naught in n tcwt
tube; also from sulphates, as shown by adding a drop of dilute hydrochloric
acid and a few drops of barium diloride, solution to (tnothi*r portion of the
washings. Suck the precipitate hh Hourly dry n« poHMiliit* iiful tnstmfi*r the
paper and precipitate to a porcelain or platinum rrurtble wliirli him been
ignited and weighed, folding the paper in the manner already lenrned.
Heat very gently in the covered erueihle until the ntomture i« vfilfttilixed,
then raise the temperature and burn the paper, iiirlifiiiig the rrtiril»li« and
placing the cover as in the case* of the ignition of the paper eontainmg cal-
cium oxalate. Wlie.n all of the c-arbmi iiiih be«*» tmnied, cover the crucible
and heat over the blast lamp or the hirgi* Mclcer btinirr fc»r ao iiiiinito.
Cool in the desiccator and weigh. Heat again for 10 tntnuteH, cool attci
weigh. If necessary repeat this proeeHH until the weight i« constant.
Calculate the per cent of aluminium in the nnU,
Aluminium oxide absorbs watei fn»m the iiir, reforming the hydrcixidc
with a corresponding gain in weight. On thin account the erueihle. m«l
oxide should be weighed rapidly.