Hach DR/700 Procedures Manual page 352

ARSENIC,
continued
a) Prepare a 10.0-mg/L arsenic working standard by pipetting 1.00 mL
of Arsenic Standard Solution, 1000 mg/L As, into a 100-mL volumetric
flask. Dilute to volume with demineralized water. Cap and invert 10
times to mix.
b) Prepare a 0.20 -mg/L arsenic working solution by using a 5-mL
volumetric pipet to put 5.0 mL of working standard into a 250-mL
volumetric flask. Dilute to volume with demineralized water. Cap the
flask and invert 10 times to mix.
c) Perform Steps 1-18 of the arsenic procedure using the 0.20 mg/L
arsenic standard.
d) Perform the instructions in the "Calibration Using Two Prepared
Standards" section of the DR/700 Instrument Manual (paragraph 3.2.4.1).
For Standard 1, use the blank from the arsenic procedure and make the
display read 00.00 mg/L.
For Standard 2, use the prepared sample from the arsenic procedure and
make the display read 00.20 mg/L.
SAMPLING AND STORAGE
Collect samples in acid washed glass or plastic bottles. Adjust the pH to
2 or less with nitric acid (about 2 mL per liter). Preserved samples may
be stored up to six months at room temperature. Correct the test result
for volume additions; see Sampling and Storage, Volume Additions,
(Section I) for more information.
INTERFERENCES
Antimony salts may interfere with color development.
SUMMARY OF METHOD
Arsenic is reduced to arsine gas by a mixture of zinc, stannous chloride,
potassium iodide and hydrochloric acid in a specially equipped
distillation apparatus. The arsine is passed through a scrubber
containing cotton saturated with lead acetate and then into an absorber
tube containing silver diethyldithiocarbamate in pyridine. The arsenic
reacts to form a red complex which is read colorimetrically. This
procedure requires a manual calibration.
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