6. Bromine standards. Add measured quantities of a standard solution of
a bromide to the liquid in each of the small flasks from which the
iodine has been removed. Add to each 5 cc. of purified carbon bisulfide,
and proceed exactly as with the sample.
_Procedure._—Evaporate to dryness an aliquot portion of the alkaline
filtrate. Dissolve the residue in 2 or 3 cc. of water, and add enough
absolute alcohol to make the percentage of alcohol about 90. Boil and
filter and repeat the extraction of the residue with alcohol once or
twice. Add 2 or 3 drops of sodium hydroxide to the combined alcoholic
filtrates and evaporate to dryness. Dissolve the residue in 2 or 3 cc.
of water and repeat the extraction with alcohol and the filtration. Add
a drop of sodium hydroxide to the filtrate and evaporate it to dryness.
Dissolve the residue in a little water. Acidify this solution with
dilute sulfuric acid, adding 3 or 4 drops excess, and transfer it to a
small flask. Add 4 drops of the solution of potassium nitrite or sodium
nitrite and about 5 cc. of carbon bisulfide. Shake the mixture until all
the iodine is extracted. Separate the acid solution from the carbon
bisulfide by filtration. Wash the flask, filter, and contents with cold
distilled water, and transfer the carbon bisulfide containing the iodine
in solution to Nessler tubes by means of about 5 cc. of pure carbon
bisulfide. In washing the filter, dilute the contents of the tube to a
definite volume, usually 12 or 15 cc., and compare the color with that
of known amounts of iodine dissolved in carbon bisulfide in other tubes.
Transfer to a small flask the sample from which the iodine has been
removed. Add saturated chlorine water, 1 cc. at a time, shaking after
each addition until all the bromine is freed. Care must be taken not to
add much more chlorine than that necessary to free the bromine, since an
excess of reagent may form a bromochloride that spoils the color
reaction. Separate the water solution from the carbon bisulfide by
filtration through a moistened filter, wash the contents of the filter
two or three times with water, and then transfer them to a Nessler tube
by means of about 1 cc. of carbon bisulfide. Repeat this extraction of
the filtrate twice, using 3 cc. of carbon bisulfide each time. The
combined carbon bisulfide extracts usually amount to 11.5 to 12 cc. Add
enough carbon bisulfide to the tubes to bring them to a definite volume,
usually 12 to 15 cc., and compare the sample with the standards. If much
bromine is present it is not always completely extracted by the amounts
of carbon bisulfide recommended. If the extraction is incomplete,
therefore, make one or two extra extractions with carbon bisulfide,
transfer the extracts to another tube, and compare the color with that
of the standards.