Hach DR/700 Procedures Manual page 551

SILVER,
continued
g) Add another 3 mL of sulfuric acid via the capillary funnel. Add
another 5 mL of hydrogen peroxide. Check the solution for digestion
completion. If digestion is not complete, continue adding hydrogen
peroxide in 5 to 10 mL portions. Several portions may be necessary.
Note: Digestion is complete when the digestate is colorless or the color of
the digestate does not change upon addition of hydrogen peroxide. Also, a
completely digested sample will not foam.
h) After digestion is complete and all the hydrogen peroxide has boiled off,
reduce the volume of the digestate to near dryness. Do not allow the
sample to become completely dry. Remove the flask from the heater. Wait
until the flask cools to room temperature.
i) Slowly add about 25 mL of demineralized water to the cooled flask.
j) Add two drops of Phenolphthalein Indicator Solution, 1 g/L, and two
drops of Thymolphthalein Indicator Solution, 1 g/L.
k) Using sodium hydroxide, adjust the pH of the solution to between 9 and
10. The solution will be pink in this pH range.
Note: A purple color indicates a pH greater than 10. If this occurs, add a
drop of sulfuric acid and 2 drops of each indicator; repeat pH adjustment.
Initially, use 50% sodium hydroxide, then 1 N sodium hydroxide as the
endpoint is approached.
l) Filter turbid digestates. Quantitatively transfer the filtrate (or unfiltered
sample) to a clean 100-mL volumetric flask. Dilute to the mark with
demineralized water. The sample is now ready for analysis by the
colorimetric method.
SUMMARY OF METHOD
Silver ions in basic solution (pH 9 to 10) react with cadion 2B to form a
green to brownish to red-purple silver-cadion 2B complex. The sodium
thiosulfate acts as a decolorizing agent for the blank. The contents of one
Silver 1 Powder Pillow and the contents of one Silver 2 Reagent Powder
Pillow incorporate the buffer, indicator and masking agents. No organic
solvent extractions are required. This colorimetric method is easier to
perform than the traditional dithizone method and has fewer interferences.
It may also be used for electroplating and silver strike solutions.
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