152 QUANTITATIVE AGRH'ULTl'HAL AXALr,s*/,S'
(d) Metallic Mvrcury, Mercuric Oj-.idc or (-n^n'r Xul.p/utic.-.....Mercuric
oxide should be that prepared in the wet way but not from mercuric nitrate.
(«) Potassium Sulphide Solution.—Dissolve at the rate of 40 gm for
each liter of solution. Commercial potassium sulphide is used. This
solution is not required unless mercury or mercuric oxide is to be used as
the catalyzer.
(/) Sodium Hydroxide Solution.—A saturated solution (55 gm per 100
cc of water), free from nitrates and containing as little carbonate as possible.
((/) Methyl Red Solution.-— Dissolve 1 gm of methyl red in UK) cc of
V/f 95-per cent alcohol. This is the solution ordinarily used in volumetric
>' I! *
i/ 'v I analysis. Add very dilute acid or base to make exactly neutral.
»fjjjl If the approximate per cent of nitrogen in the sample is known, calcu-
. *''^ late the weight that will yield ammonia equivalent to about 35 ec of the
*', standard acid. If nothing is known of the nitrogen content use about
", 2 grn of sample. (For this method the sample must contain no nitrates,
MJ nitrites, or nitro-compounds. This is ordinarily true with feeds.) Place
•il two weighed samples in 500-cc Kjeldahl digestion flasks, holding the latter
;/ in a vertical position to prevent the sample from sticking to the sides of the
'[t neck, which should be dry. Weigh 1 gm of sugar into each of two other
'$, flasks and treat the same as the feed sample*. Add about 0.7 gin of
/$' mercuric oxide or of mercury, or 0.3 gm of copper sulphate, also 25 cc
of concentrated sulphuric acid. Incline the flask in a hood or with the
neck inserted into a lead-pipe ventilator and heat gently until the violence
of the reactions has moderated, then gradually raise the temperatures until
the acid is boiling. The flask may be heated without protection by a
gauze if it is of Pyrex or similar resistance glass and if if is placed over
a hole in a stand of sheet iron in such a mariner that the flame*, cannot
come into contact with the sides of the flask above the liquid.
Digest by gently boiling until the solution is nearly colorless (blue if
copper sulphate has been used). This may occur after a short time or the
digestion may require several hours. Finally remove the? flame and at once
drop into the flask small quantities of powdered potassium permanganate
until the solution acquires a green or purple tint whicth persists after shaking.
Allow the flask to stand until cool. (Do not and under a tap.) C itrefully
add 200 cc of distilled water and mix by rotating the flank. Add about
0.5 gm of crushed porcelain and 25 cc of potai-wium sulphide solution (<"),
shaking as the latter is added. (If cupric sulphate has been used OH a cataly-
zer, omit the potassium sulphide solution.)
Have the connections with a tin condenser ready and have 50 cc of stand-
ard acid measured into a 250-cc flask into which the delivery tube (of
glass) dips. Most laboratories in whicth much work of this kind is clone* will
be equipped with a special form of apparatus for carrying on several distil-
lations at once. Such an apparatus an is shown in Fig. 44 will be found
convenient for individual work. The flask should bo in a vertical position
and some kind of trap should be used to prevent spray from being curried
over by the steam. The delivery tube should be capable of being detached
from the condenser for the purpose of cleaning and mining it. The entire