60 QUANTITATIVE AGRICULTURAL ANALYM8
* i
| ance. This means that it is usually better to add acid to haso in presence of
I : methyl orange or methyl red, and base to acid in pmsnirc of phonolphf hnloin.
< \ Methyl orange is the only indicator that can be used satisfactorily for c*ur-
i \ bonate solutions.
\ !
I : SULPHATES
5 Gravimetric, by Weighing Barium Sulphate. The basin for
'. \ ' this method is the following reaction:
| n : Bad.. + M2S()4 -> BaS04 + 2M( 51.
\ Solubility.—The solubility of barium sulphate in water is
! quite low. At 20°, 1000 cc of water will dissolve* about 0.002B
f , gm of the salt. This contains 0.00153 gm of barium and 0.00107
i gm of the sulphate radical. The precipitation of barium sulphate
i is made the basis for the determination of either barium or
* • sulphates. In either case it is necessary to maintain a slightly
,\ ' acid solution in order to avoid the possibility of precipitating
< other barium salts, such as carbonate, oxulute or phosphate, in
t case traces of these salts, or of their acids, an* present in the
i sample or in the reagents. A slight excess of hydrochloric: acid
; f is used for this purposes.
i \ Cry stallization.—Be cause of the ver> small solubility of barium
Aj , sulphate it precipitates almost instantaneously as the n agent
\\ 1 (a soluble sulphate or barium chloride) is added. On thin
account it usually forms relatively small ciystals and these* may
be so small as to pass through filters of ordinary density unless
care is given to the precipitation process. The best conditions
are provided by keeping the solution hot,, adding the reagent
drop-wise and stirring continuously. This is followed by a
process of digestion, which serves to enlarge the crystals already
formed, as explained on page 25.
Change of Weight of Barium Sulphate.—ronKicicrtihta care
must be exercised in burning the paper upon which barium
sulphate has been filtered and in subsequent ignition of the
precipitate to expel traces of moisture. If the temperature in
allowed to rise to too high a point barium sulphate* will
gradually decompose, yielding sulphur trioxide and kming
weight thereby:
BaS04 -> BaO + 80». (1)