Hach DR/700 Procedures Manual page 550

SILVER,
continued
Caution: Poisonous hydrogen cyanide gas is generated during this
digestion.
Always wear safety glasses and use a safety shield, or operate the
Digesdahl within a closed fume hood. Additional safety precautions are
given in General Digesdahl Digestion (Section 1).
a) Add an appropriate size sample to the 100-mL volumetric flask of the
Digesdahl. Add several boiling chips to prevent bumping.
Note: The final concentration of the sample (after dilution to 100 mL)
should be between 0 and 0.5 mg/L. Larger dilutions may be necessary
for electroplating baths and silver strike solutions. This must be
determined experimentally. The maximum sample size is 25 mL.
Several 25-mL aliquots may be digested in succession to concentrate a
very dilute sample. Do not exceed the maximum sample volume.
b) Turn on the water aspirator and check to be sure there is suction in the
fractionating head.
c) Add 3 mL of concentrated sulfuric acid to the sample in the
volumetric flask. Immediately place the head on the volumetric flask.
Never use less than 3 mL of acid.
d) Place the volumetric flask in the heater. Turn the temperature dial to
440 °C (825 °F). After the sample begins to char or the sulfuric acid
reflux line becomes visible, wait 3-5 minutes. Visually confirm the
presence of acid in the flask before adding hydrogen peroxide.
e) Add 10 mL of 50% hydrogen peroxide to the sample via the capillary
funnel on the fractionating head.
f) 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. The sample must not 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 i.
57-44