If the sample is not entirely uniform it must be thoroughly mixed
before weighing out, after all the lumps are broken up, best with a
mortar and pestle. Then 26.048 grammes are weighed out on the balance
in the tared German silver dish furnished for this purpose. Care must
be taken that the operations of mixing and weighing out are not unduly
prolonged, otherwise the sample may easily suffer considerable loss of
moisture, especially in a warm room. The portion of sugar weighed out
is washed by means of a jet from a wash bottle into a 100 c.c. flask,
the dish being well rinsed three or four times and the rinsings added
to the contents of the flask. The water used must be either distilled
water or clear water which has been found to have no optical activity.
After the dish has been thoroughly rinsed, enough water is added to
bring the contents of the flask to about 80 c.c. and it is gently
rotated until all the sugar has dissolved. The flask should be held by
the neck with the thumb and finger, and the bulb not handled during
this operation. Care must be taken that no particle of the sugar or
solution is lost. To determine if all the sugar is dissolved, the
flask is held above the level of the eye, in which position any
undissolved crystals can be easily seen at the bottom. The character
of the solution is now observed. If it be colorless or of a very light
straw color, and not opalescent, so that it will give a clear
transparent liquid on filtration through paper, the volume is made up
directly with water to the 100 c.c. mark on the flask. Most sugar
solutions, however, will require the addition of a clarifying or
decolorizing agent in order to render them sufficiently clear and
colorless to polarize. In such case, before making up to the mark, a
saturated solution of subacetate of lead is added.
The quantity of this agent required will vary according to the quality
of the sugar; for sugar which has been grained in the strike pan and
washed in the centrifugals, from 3 to 15 drops will be required; for
sugar grained in the strike pan but not well washed in the
centrifugals, that is, sugar intended for refining purposes, from 15
to 30 drops will be required; for sugar not grained in the strike pan,
that is, "wagon" or "string sugar," "second sugar," etc., from 1 to 3
c.c. will be required. After adding the solution of subacetate of lead
the flask must be gently shaken, so as to mix it with the sugar
solution. If the proper amount has been added, the precipitate will
usually subside rapidly, but if not, the operator may judge of the
completeness of the precipitation by holding the flask above the level
of the eye and allowing an additional drop of subacetate of lead to
flow down the side of the flask into the solution; if this drop leaves
a clear track along the glass through the solution it indicates that
the precipitation is complete; if, on the other hand, all traces of
the drop are lost on entering the solution, it indicates that an
additional small quantity of the subacetate of lead is required. The
operator must learn by experience the point where the addition should
cease; a decided excess of subacetate of lead solution should never be
used.
The use of subacetate of lead should, in all cases, be followed by the
addition of "alumina cream" (aluminic hydrate suspended in water)[2]
in about double the volume of the subacetate solution used, for the
purpose of completing the clarification, precipitating excess of lead,
and facilitating filtration. In many cases of high grade sugars,
especially beet sugars, the use of alumina alone will be sufficient
for clarification without the previous addition of subacetate of lead.
[Footnote 2: Prepared as follows: Shake up powdered commercial
alum with water at ordinary temperature until a saturated
solution is obtained. Set aside a little of the solution, and to
the residue add ammonia, little by little, stirring between
additions, until the mixture is alkaline to litmus paper. Then
drop in additions of the portion left aside, until the mixture
is just acid to litmus paper. By this procedure a cream of
aluminum hydroxide is obtained suspended in a solution of
ammonium sulphate, the presence of which is not at all
detrimental for sugar work when added after subacetate of lead,
the ammonium sulphate precipitating whatever excess of lead may
be present.]
The solution is now made up to the mark by the addition of distilled
water in the following manner. The flask, grasped by the neck between
the thumb and finger, is held before the operator in an upright
position, so that the mark is at the level of the eye, and distilled
water is added drop by drop from a siphon bottle or wash bottle, until
the lowest point of the curve or meniscus formed by the surface of the
liquid just touches the mark. If bubbles hinder the operation, they
may be broken up by adding a single drop of ether, or a spray from an
ether atomizer, before making up to the mark. The mouth of the flask
is now tightly closed with the thumb, and the contents of the flask
are thoroughly mixed by turning and shaking. The entire solution is
now poured upon the filter, using for this purpose a funnel large
enough to contain all the 100 c.c. at once, and a watch glass is
placed over the funnel during filtration to prevent a concentration of
the solution by evaporation.
The funnel and vessel used to receive the filtrate must be perfectly
dry. The first portion of the filtrate, about 20 to 30 c.c., should be
rejected entirely, as its concentration may be affected by a previous
hygroscopic moisture content of the filter paper. It may also be
necessary to return subsequent portions to the filter until the liquid
passes through perfectly clear.
If a satisfactory clarification has not been obtained, the entire
operation must be repeated, since only with solutions that are
entirely clear and bright can accurate polarimetric observations be
made.
When a sufficient quantity of the clear liquid has passed through the
filter, the 200 mm. observation tube is filled with it. The 100 mm.
tube should never be used except in rare cases, when notwithstanding
all the means used to effect the proper decolorization of the
solution, it is still too dark to polarize in the 200 mm. tube. In
such cases the shorter tube may be used, and its reading multiplied by
two. The zero deviation must then be determined and applied to the
product. This will give the reading which would have been obtained if
a 200 mm. tube could have been used, and it only remains to apply the
correction determined by the use of the control plate as previously
described.
Example:
Solution reads in 100 mm. tube 47.0
Multiplied by 2 2.0
----
Product 94.0
Zero reads plus 0.3 0.3
----
Solution would read in 200 mm. tube 93.7
Reading of control plate 90.4
Sugar value of control plate 90.5
----
Instrument too low by 0.1
Add 0.1 to 93.7
----
Correct polarization of solution 93.8
Before filling the tube it must either be thoroughly dried by pushing
a plug of filter paper through it, or it must be rinsed several times
with the solution itself. The cover glasses must also be clean and
dry, and without serious defects or scratches. Unnecessary warming of
the tube by the hand during filling should be avoided; it is closed at
one end with the screw cap and cover glass, and grasped by the other
end with the thumb and finger. The solution is poured into it until
its curved surface projects slightly above the opening, the air
bubbles allowed time to rise, and the cover glass pushed horizontally
over the end of the tube in such a manner that the excess of liquid is
carried over the side, leaving the cover glass exactly closing the
tube with no air bubbles beneath it, and with no portion of the liquid
upon its upper surface. If this result is not attained, the operation
must be repeated, the cover glass being rubbed clean and dry, and the
solution again brought up over the end by adding a few more drops. The
cover glass being in position, the tube is closed by screwing on the
cap. The greatest care must be observed in screwing down the caps that
they do not press too tightly upon the cover glasses; by such pressure
the glasses themselves may become optically active, and cause
erroneous readings when placed in the instrument. It should therefore
be ascertained that the rubber washers are in position over the cover
glasses, and the caps should be screwed on lightly. It must also be
remembered that a cover glass, once compressed, may part with its
acquired optical activity very slowly, and some time must be allowed
to elapse before it is used again.
The polariscopic reading may now be taken, an observation on the 90°
control plate having been made immediately before as previously
described. Then without altering the position of the instrument
relative to the light, or changing the character of the latter in any
way, the tube filled with the sugar solution is substituted for the
control plate. The telescope is adjusted, if necessary, so as to give
a sharply defined field, which must appear round and clear. (This
condition must be fulfilled before the observation is performed, as it
is essential to accuracy.) The milled head is turned until the neutral
point is found, and the reading is taken exactly as previously
described, the operation repeated five or six times, the average taken
with the rejection of aberrant readings, the average figure corrected
for the deviation shown by the control observation from the sugar
value of the control plate at the temperature of observation as given
in the table, and the result taken as the polarization of the sugar.
When a series of successive polarizations is made under the same
conditions as regards temperature, position of the instrument with
relation to the high intensity, of the light, etc., the control
observation need not be made before each polarization, one such
observation being sufficient for the entire series. The control must
be repeated at least once an hour, however, and oftener when the
operator has reason to think that any of the factors indicated above
have been altered, for any such alteration of conditions may change
the zero point of the instrument.
In the polarization of the quartz plates, as also in the polarization
of very white sugars, difficulty may be experienced in obtaining a
complete correspondence of both halves of the field. With a little
practice this may be overcome and the neutral point found, but when it
cannot, the ordinary telescope of the instrument may be replaced by
another, which is furnished with the polariscope and which carries a
yellow plate. This removes the difficulty and renders it possible,
even for one not well accustomed to the instrument, to set it at the
exact point of neutrality.