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methylene chloride portion in a 250-mL beaker and save for the EC determination (4.6.2.5).
Place the sodium carbonate extract in a 250-mL volumetric flask, and dilute it to the mark
with distilled water. Dilute a 15-mL aliquot of this solution to 100 mL with distilled water.
g.
Prepare a reference solution by diluting 100 mL of the 5 percent sodium carbonate solution
to 250 mL with distilled water and then further diluting a 15-mL aliquot of this to 100 mL
with distilled water.
h.
Determine the absorbance of the sample at 288 as previously described in step d. Refer to
the standard curve (step e) to obtain the concentration.
i.
Calculate percent resorcinol as follows:
5Η
Περχεντ ρεσ ρχινολ
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3Α
where:
H = Concentration (mg resorcinol per 100 mL solution)
A = Weight of sample in grams
4.6.2.5 EC. Determine the EC content as follows:
a.
Evaporate the methylene chloride layer from the resorcinol extraction (4.6.2.4) to dryness.
b.
Transfer the residue to a 250-mL flask (equipped with a stopper) using 50 mL of glacial
acetic acid.
c.
Add, by pipette, 25 mL of potassium bromate-bromide solution which has been prepared as
follows:
(1) Add 5.6 grams of potassium bromate, which has been dried at 100C for 2 hours, to a
beaker.
(2) Add 30 grams of potassium bromide.
(3) Add about 300 mL of distilled water to the beaker to dissolve the content, and dilute
the solution to 1 liter.
d.
Wet the stopper of the flask with a drop of 15 percent potassium iodide solution which has
been prepared by adding 15 grams of potassium iodide to 85 mL of distilled water.
e.
Add 5 mL of concentrated hydrochloric acid to the flask. Stopper the flask, swirl the
contents, and allow bromination to proceed for 1 minute.
f.
Immediately add 10 mL of 15 percent potassium iodide solution. Swirl the contents of the
flask.
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