11. DETERMINATION OF WATER CONTENT
a. Determination by the Bidweil-Sterling Method.
The most convenient method for the determination of water in essential oils, oleoresins,
and drugs is by the water determination apparatus of Bidwell and Sterling.160
The sample to be tested is distilled in this apparatus
with a liquid immiscible with water, such as toluene. The special trap collects
and measures the condensed water, the excess solvent overflowing and returning
to the still.
Procedure: Connect the apparatus as shown in Diagram 4.16.
Introduce into the 500 cc. flask, sufficient material, accurately weighed, to
yield from 2 to 4 cc. of water. Add about 200 cc. of toluene to the flask and also
fill the receiving trap with
REFRACTIVE INDICES* OP ETHYL ALCOHOL-WATER
* These values are based on the Reference Tables
of "Methods of Analysis of the A.O.A.C.," 4th Ed. (1935), 663-670.
toluene, poured through the top of the
condenser. Heat the flask gently by means of a Bunsen burner or electric hot-plate
until the toluene begins to boil. Distill at a rate of about 2 drops per sec. until
most of the water has passed over. Then increase the rate of distillation to about
4 drops per sec. When no further increase in collected water is observed,
continue the distillation for an additional 15 min. Permit the apparatus to cool.
When the water and toluene have separated completely, read the volume of water,
and calculate the percentage present in the substance.
If the condenser and moisture trap have been thoroughly
cleaned with chromic acid cleaning solution, the tendency of droplets to adhere
is greatly minimized. Should such droplets of water be observed on the sides of
the condenser, they may be forced down by brushing the inner tube of the
condenser with a small brush previously saturated with toluene.
A convenient method for detecting the
presence of dissolved water in essential oils, such as rose and bay, has been described
under "Solubility," see p. 252.
b. Determination
by Karl Fischer Method. For the
determination of mere traces of water, the method employing the Karl Fischer water
titration reagent will prove exceptionally sensitive.
The Karl Fischer water titration reagent161 is
a solution of iodine, sulfur dioxide, and pyridine in methyl alcohol. The
method depends on the oxidation of sulfur dioxide by iodine in the presence of water
to form sulfuric and hydriodic acid.
S02 + 2I + 2H2O
-> H2SO4 + 2HI
The reaction is conducted in the presence of pyridine
which acts as an acid acceptor, thus enabling the reaction to go to completion.
The end point is indicated by a color change from yellow to reddish-brown, the latter
being produced by the free iodine in the reagent when an excess of the reagent is
added.
DIAGRAM 4.16.
Apparatus for the determination of water.
The method is applicable to a large number of
organic and inorganic compounds, both liquid and solid. The exact limitations of
the method have not been determined, but it can probably be used on all organic
and inorganic compounds that do not react with the reagent and that are not naturally
colored red or brown. It is known to be applicable to organic compounds such as
hydrocarbons, alcohols, esters, carboxylic acids (except formic), halogen derivatives
of hydrocarbons, phenols, nitro compounds, amines, and heterocyclic compounds. It
is not applicable to aldehydes and ketones, nor to reducing compounds which react
readily with iodine in the cold. The active hydrogen in primary and secondary
amines must be blocked by solution in glacial acetic acid before titrating with
the reagent.
Liquids are dissolved in a mutual solvent for
both the sample and the reagent before titrating. Solids may be analyzed by pulverizing
and dissolving or suspending in dry methyl alcohol. It is not essential that
the material be soluble in methyl alcohol, as the hygroscopic nature of both
methyl alcohol and the reagent will act to extract the water from the sample.
The solvent used in preparing the sample for analysis
will contain some moisture, hence a blank titration must be made using the same
volume of solvent and the same size flask, as the moisture in the air space is an
integral part of the blank. To check the end point, breathe into the flask and the
end point will disappear, but an additional drop or two of the reagent should bring
back the reddish-brown color. The choice of solvents is wide : methyl alcohol, dioxane,
glacial acetic acid, chloroform, etc.
When attempting new applications of the method,
i.e., with new or unknown compounds, the reactivity of the compound with the
Fischer reagent must first be determined. If the compound is inert toward the reagent,
the method is applicable. Also, the reagent is so avidly hygroscopic that it will
dehydrate hydrated compounds. The degree of such dehydration (number of mols of
water reacting with the reagent) must be determined beforehand.
All apparatus must be thoroughly dried and every
precaution must be made to exclude atmospheric moisture during the titration. The
titration is carried out in a small flask (125 cc. Erlenmeyer) and taken to completion
rapidly. This method will detect, in general, 0.0005 g. of water, equivalent to
0.005 per cent when using a 10 g. sample.
Procedure: Pipette 10 cc. of methyl alcohol into each of
three 125 cc. glass stoppered Erlenmeyer flasks, which should be kept stoppered
as much as possible. Weigh accurately from a weighing pipette about 0.1 g. of distilled
water into each flask. Titrate with the Karl Fischer reagent to the color change
(the color should change from a straw yellow to a reddish-brown when the end point
is reached). At the same time run a blank on the methyl alcohol. Calculate the water
equivalent of the reagent by means of the following formula :
E = w/(A
– B)
where: E
= water equivalent of the reagent (in grams of water per cc.) ;
w = weight in grams of water
used ;
A = cc. of reagent used for the
determination ;
B = cc. of reagent used for the blank.
Into a 125 cc. Erlenmeyer flask weigh a sufficiently
large sample of the material to be tested to yield approximately 0.1 g. of water.
Add 10 cc. of methyl alcohol and titrate. Run a blank at the same time on the alcohol.
The water content may be calculated from the following formula:
Percentage of water = 100(A-B)E/w
where: A
= cc. of reagent used for the determination;
B = cc. of reagent used for the
blank ;
E = water equivalent of the
reagent ;
w = weight of sample in grams.
It is necessary to standardize the reagent daily.
The Karl Fischer water titration reagent may be
purchased from chemical supply houses, or may be prepared in the following way :
Place 1 liter of dry methyl alcohol and 400 cc.
of pyridine in a 2 liter reservoir of an automatic burette. Add 127 g. of iodine,
stopper the bottle and swirl until the iodine is completely dissolved. Cool the
bottle in a salt-ice mixture for one-half hour and then add 100 g. of sulfur
dioxide,162 weighing by difference on a balance. The resulting
solution is very hygroscopic and must be kept stoppered as much as possible, Then
remove the bottle from the ice bath and insert the siphon and burette unit. Thoroughly
grease the ground glass joint between the bottle and burette to give an airtight
seal. Fit a calcium chloride drying tube to the opening at the top of the burette
and between the bottle and hand aspirator, which is used to fill the burette. The
tip of the burette is fitted with a 2-hole rubber stopper which fits the neck of
the 125 cc. Erlenmeyer flask. Protect the tip of the burette when not in use.
It is best to age the solution for two to four
days before using so that the variation in standardizing from day to day will be
minimized.
---------------------------------
160 J. Ind. Eng. Chem. 17 (1925),
147.
161 Angew. Chem. 48 (1935),
394. water.
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