70 QUANTITATIVE AdltH't'l.Tt'KM. .1 A AL 1 ,s7,S'

IRON

Iron is sometimes determined gruvimetrieally by precipitating
ferric hydroxide by ammonium hydroxide, igniting aii<{ we;-1 *
this aw ferric oxide:

'; ft)

2Fe(OH)3 • > I«V2«>3 4- :t!U», (2)

However ferric hydroxide is a colloidal Mib>tanei* whirl} adsorbs
soluble Halts with great tenacity, so that it i> diffinilt ii> purify
any but a small precipitate by washing. A!M» during ignition
on paper, reduction of the oxide is liable tn o«vur. Fur these
reasons volumetric methods are ustially prejrrabl**. Standard
solutions of permanganates or of dirhromate* are .^uifable for
this purpose.

Volumetric, by Permanganate.- In acid Mention* potnssitiiu
permanganate oxidizes ferrous salt** by the frtHmving reactions:

KMn04 + 5F(K ?IS + HlfC 1 -- K( 1 -4 Mn< lv *

:>rvri * in;,n: f|j

2KMn04+ 10FeS04 + 8H2S<>4 * K7SU4 f 2MnSo4 »

SKivS* *i' ! SH (l '. S2)

The equations show that the* added aciil j»Ia\> a fielinite part in
the reactions and if an insufficient amount i* pre*i«nf tbe Hoiution
will become basic and a precipitate will form, coiisi^ting of
hydratcd manganeKe dioxide and ba-sir iron >*alf?*:

3Fe(Jl2 + KMn()4 + (// + 2)H3< * - * 3!"VI lr« HI -f-

H7o -f- KoH. f;i

Aside from the trouble* ex!*eri<»ncc«i in n*mling I fit* i-ni
caused by the appearance of the precipitate, it in H*Hi from
Kq. (3), above, that mungnnfw m reilureii In tbi* apparent
valence of 4 instead of 2, HO thiit fhr» iron jn*r rent euitttot l«»
calculated when bot.h reaclions faki* pliiri*.
la presence of hydrochloric iirid won* (H*rtuaiiKaimt<* ttiurt
the theoretical amount may be UH»df with lifierntion of HWW*
free chlorine. Tim action may l«* itlnifmf ent iiely pre%*enti*«l by
tnc addition of rna«Kun«HC! sulphate to fin* Miltitioii, < »i*!ii'rnl!y
phosphoric acid also is added to prevrnl f h«- hy«IniI>>i.H t»f fi*rrir