74

QUANTITATIVE AGIUCCLTI'RAL ASM.

white porcelain " spot plate." Drops of the* solut ion arc* removed
from time to time by means of a stirring rod and allowed to touch
the drops of ferricyanicle. So long as ferrous Iron is prrscnf. the
blue of ferrous ferrieyanlde is apparent on I hi' spot pint P. When
the last trace of iron has been oxidised there* is produwl on fhe
plate only the light brown ferric ferrieyanide. Thi»r<» being
nothing in the appearance of the solution of the* iron salt to
indicate the approach to the end point, the* tit rat ion is ni»rossurily
somewhat tedious unless a system m devised for rapid readings
Such a system is indicated in the next exereine.
Standard Solution.—The solution should hi* of Mirfi eoueenf ration that
1 cc is equivalent to 0.005 gm of iron. Calculate the weight «»f potawium
dichromatc necessary for 1000 cc of such it solution. If the will i* known
to be pure, weigh exactly the calculated weight and omit furl JUT *tandard-
ization. If it is not pure but its oxidizing power knmvn fn»»» previous
determinations, calculate the weight of impure wimple required and use thin
weight. If nothing is known of the purity UM* I per eent mi»r«« than the
weight of pure salt required for 1200 cc of solution and '«t;tnd;irdi/,e flu*
solution as directed below. In any cane* dissolve the weighed *>alt and dilute
to the proper volume. In cane tit-ration for Htandardi/af ion in f n lie omit fed
and direct weighing is to be made the ba«i.s for standardization, IfMiO en
of the solution should be accurately made a ltd poured into a dry battle.
{standardization, if this should be wfWHHtiry, in iieeomplinheil hy film-
tion against ferrous ammonium sulphate. Write nnd }*iilniiei* flu* i'i|iia-
tion for the oxidation of ferrous Kulplntte by potfi^sttm tiirhrtmtiifi* iii
presence of sulphuric acid, referring, if nwwHnry, t«» tlie i*f|urutton for flie
oxidation of the chloride, page 73. C'alriilnfe the weight of rn^fiilltxetl
ferrous ammonium sulphate necesnary to rednee ,'ff* er nf the litehroitintit
solution. Weigh four portions of exactly this weight i»t«i Hfiij-rr hrnkr*r* and
dissolve each, jwt before titrating, in 50 cc of reeenl Iy b*»ileil nml e<H*!**»l wiif i*r.
Prepare a 0.01-per cent solution of pottuwitim (WrieyiMiitii' mi!I plure n drop
in each of the depressions of a white porcelum wpnt |»Iiite, Atld to fltti
solution of ferrous ammonium sulphate fi re of dilute Nttlphurie iiriil, mid
titrate at once, as follows: To the firet solution iwld tin* *i!*'iir«»ifiiif<* Hu2uti«»n
5 cc at a time, stirring well after each addition, and hwf }»>* rftnoving n drop
by means of the stirring rod ami touching to n drop *»f |wifii^fiiii» fi*rri*
cyanide solution on the spot platen llie end point PI r^nehi'tl whni a
blue color is no longer produced on thfc plati*, after MnndntfC for J iitiiiiili*,
(Dust or reducing gases will interfere by reducing trw«*« «*f ferrir ei«I*irid<M
Titrate the second solution by adding 5 re. lew than the amount of *|p'Iiro-
mate solution used in the first, then adding I re «t, it time. 1'ilrnte tin?
third solution by adding 1 cc l<m Hum th« totul UM»iI in thn wi'itip^ then
adding 0.1 ee at a time. Titrate the fourth Hoiutton in thi* purin* nmrifH'r nii«l
take the average of the last two tit rations for pt'rrmtiH'nt ri-eortl. C 'alriilttf.i*