Silver
Silver
Silver_PP_Other_CLR_Eng_Ody.fm Page 5 of 6
3. Add 3 mL of concentrated sulfuric acid to the sample in the volumetric flask.
Immediately place the head on the digestion flask. Never use less than 3 mL
of acid.
4. Place the digestion flask on the heater. Turn the temperature dial to
440 °C (825 °F).
5. After the sample begins to char or the sulfuric acid reflux line becomes visible,
wait 3–5 minutes.
6. Visually confirm the presence of acid in the flask before adding hydrogen
peroxide!
7. Add 10 mL of 50% hydrogen peroxide to the sample via the capillary funnel
in the fractionating head.
8. After the hydrogen peroxide has boiled off, heat the sample until heavy white
sulfuric acid fumes are present. Continue heating and reduce the sample
volume to near dryness. Do not let the sample go completely dry at any time.
Note: If the sample goes to dryness, turn the Digesdahl off and cool completely. Add water to flask
before handling. Repeat digestion from the beginning.
Note: If only thiosulfate is present in the sample, proceed to step 1 of the Colorimetric procedure.
9. Add another 3 mL of sulfuric acid via the capillary funnel.
10. 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.
11. After digestion is complete and all the hydrogen peroxide is 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. Cool to room
temperature.
12. Slowly add about 25 mL of deionized water to the cooled flask.
13. Add 2 drops of 1 g/L Phenolphthalein Indicator Solution. Add 2 drops of
1 g/L Thymolphthalein Indicator Solution.
14. Using sodium hydroxide, adjust the pH of the solution to 9–10. 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 end point is approached.
15. Filter turbid digestates. Quantitatively transfer the filtrate (or unfiltered
sample) to a clean 100-mL volumetric flask. Dilute to the mark with deionized
water. The sample is ready for analysis.
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