The Students' Guide in Quantitative Analysis - Free Ebooks Library

44
QUANTITATIVE ANALYSIS,
the Standard : one c.c. of the Na, CO, solution should ex-
actly Jiciitralize on« c.c. of the ILSO^ solution. In case oi
difficulties the exact standard of the acid should be deter-
mined gravimctrically by precipitating 10 or 20 c.c. with
BaClj, and calculating from tlie BaSO^ obtained the
amount of HjSO^ in one ex,
Carminic acid being stronger than carbonic acid, a solu-
tion of cochineal is sometimes substituted for litmus, in
which case boiling may be dispensed with. The dycstufi
tropaeoHne has recently been proposed as an indicator in
alkalimetry. Cf. BiK d. chem. Ges. XI, 460 (1878).
Dcci'mrnml Sohtiion of Acid.— Call the above normal
solution "No. r;" take 100 c.c. of No. t, put into a litre
flask, and dilute to one litre. Call this decUiormal soiution
" No. 2."
A.—Valuation of Soda Ash.
{Determination of Na^CO,.)
Place about 12 grms. powdered sample in a platinum
crucible or porcelain dish ; heat moderately for some min-
utes over a Bunsen burner, until all moisture is expelled ;
eool, weigh out exactly 10 grm, ; dissolve in water; dilute
to one-hall litre and mix well. Take out 50 c.c, solution
(which contains one grm, soda ash), and determine the
amount of normal acid needed to neutralize, adding litmus
»s before, and boiling to expel COj.
Suppose 50 c.c. solution soda ash required 15 c.c. stand-
ard acid thpn ^!f53 x ',1 x 100
^^^"
Tgrm, = 79-5 per cent, Na^CO^.
See Frcs., § 195, p. Og^. These results are only
appro.ximative and preliminary, and the operation must
be repeated, finishing with the deci-normal solution No.
2 .below Take another 50 c,c. of soda ash solution.;
run from a burette .2 c.c. of .solution "No. C and then