30

QUANTITATIVE jr,7<vrr///V7,M/.

Even in cases where the burning paper ha> no redwing action
upon the precipitate it in still desirable to complete the ruin-
bustion of the paper at a comparatively low temperature, 77//.s-
is a matter that is too often ignored //// flic ,v/////^///.^ <'rys-
talline precipitates that are ordinarily regarded ji* infusible will
often undergo softening at the sharp corners of I hi* crystals.
This causes a certain sticking together which n*MiIts in flu* endo-
sure of a small amount of carbon in such a. way as to make it*
oxidation extremely difficult, If the paper containing flu*

I1

FIG. 11. — Correct portion
of crucible for oxidation.

"f

6
InrMir*-«.» j«

M.-l «f M,.,|4

precipitate is heated to a high temperature at tin* vn i

it is often almost Impossible to make* if whit**. < IIP- »»f tin* IH^H!

examples of this action is in the ignition of mague-Mum aiit!iir>»

nium phosphate to convcsri it into inatfneMtim pyr«»

Premature heating of thin HuhHtnnc^ f<» very high f^f

will frequently result in a black or gray material that cannot.

whitened by long ignition.

Decomposition in the Crucible. ............ After oxidation nf tin* pnf

is completed the temperature is raiml in order fi*
completely any volatile impurities thai limy rciaaiti iirnl to fimw*
whatever decomposition IK (Icvired. Hince ffxidaiifin in no l«*ngi*r
an object the crucible in placed in an upright potion and tin*
cover is placed over the* top. This given mi opportunity for
the flame to bear directly on the boll-out of flu* eniriM**
the precipitate lies. The CJOVPT ulno largely pri'vi-ntf*
heat due to convection currents of air within tin* crueibli*,

Crucibles.— Porcelain cruciblen of lii'irfi gradi iiuiv !»»• 11
most work, in cases where the precipitate i* nut to hi*
Alundum already han been mentioned in connection with !i
crucibles. When any compound or mixture i* I** l*e

nf

for