CHAPTER-6
DETERMINATION OF ARSENIC IN
WATER
Determination of arsenic in water Chapter-6
6.0 Determination of arsenic in water
Different methods for determination of arsenic in water may be summarized as follows:
6.1 Field estimation
This is accomplished by Field Test Kit which indicates the presence or absence of
arsenic in water and it is an approximate method.
6.2 Laboratory Methods
U/V double beam Spectrophotometric Silver Diethyldithiocarbamate Method
Hydride generation Atomic Absorption Spectrophotometric Method
Electro thermal atomic absorption spectrometric method
Inductively coupled plasma (ICP) emission spectroscopy method
Inductively coupled plasma (ICP) mass spectroscopy method
Mercuric Bromide Stain Method
6.3 Other Methods
Photometer SQ 118-Arsenic of MERCK using DDTC Brucine solution as absorber
Molybdenum Blue Spectrometric Method By using Disposal Cartidges
By using Kit of MERK-ARSEN TEST code As-1.10026
6.4 U/V double beam Spectrophotometric Silver Diethyldithiocarbamate Method
Principal:
Total arsenic in groundwater comprises mostly of inorganic arsenites ( AsO33-) and
arsenates (AsO43-). Arsenates present in the water sample are first reduced quantitatively to
arsenite by potassium iodide (KI) solution. The entire arsenite is further reduced by zinc
and hydrochloric acid in situ, to arsine(AsH3), a colourless, volatile gas. The arsine
obtained in the process, is allowed to pass through a scrubber containing lead acetate
[(CH3COO)2Pb] solution where sulphurated hydrogen(H2S),if generated, is entirely arrested.
Arsine is then passed through a U-tube where it reacts with silver diethyldithiocarbamate
(C5H10NS2Ag) dissolved in morpholine-chloroform system, to produce an orange-red
complex which strongly absorbs at 520 nm in a properly calibrated spectrophotometer.
Since the intensity of the color of the complex is directly proportional to the concentration of
arsenic present in the water sample, the concentration can be determined by measuring the
absorbance of the complex with the help of a spectrophotometer.
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Determination of arsenic in water Chapter-6
Fig6.1 : Arsine generator and absorber assembly
Apparatus:
Arsine generator, scrubber and adsorption: A 200 ml three necked flask with a sidearm
through which the inert gas delivery tube reaching almost to the bottom of the flask is
inserted, a 24/40 female ground-glass joint to carry the scrubber, and ka second side arm
closed with a rubber septum, or preferably by a screw cap with a hole in its top for
insertion of a TFE-faced silicone septum. A magnetic string bar in the flask is placed.
Absorber tube should be fitted ( 20 ml capacity) to the scrubber and it should be filled
with silver diethyldithiocarbamate solution. Rubber or cork stoppers can not be used
because they may absorb arsine. The glass equipment should be cleaned with
concentrated nitric acid.
Fume bood: Apparatus in a well-ventilated hood with flask secured on top of a magnetic
stirrer should be used.
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Determination of arsenic in water Chapter-6
Photometric equipment: : Spectophotometer, for use at 520 nm with 1 cm clean dry cell.
Reagent
Concentrated Hydrochloric acid.
KI solution: Add 100 ml D/D water with 15 gm KI.
Lead acetate solution: dissolve 10 g Pb(CH3COO)2.3H2O in 100 ml water.
Zinc, 20-30 mesh, arsenic free.
Silver diethyldithiocarbamate solution: Dissolve 1 ml morpholine in 70 ml
chloroform,CHCl3. Add 0.3 g silver diethyldithiocarbamate, AgSCSN(C2H5)2.Shake in a
stoopered flask until most is dissolved. Dilute to 100 ml with chloroform. Filter and store
in a tightly closed brown bottle in a refrigerator.
Standard arsenite solution: Dissolve 0.1734 g NaAsO2 in water and dilute to 1000 ml with
water.
Standard arsenate solution: Dissolve 0.416g Na2HAsO4.7H2O in water and dilute to
1000ml. dilute 100ml to 100ml with water; dilute 10ml of this intermediate solution to
100ml; 1.00 ml = 100 µ g As.
Procedure
Treatment of sample: 35 ml sample was pipette out into a clean generator bottle, 5
ml HCl, 2 ml KI solution and 0.5 ml of Stannous chloride was added successively with
thorough mixing. 15 minutes was allowed for reduction of arsenic pentavalent to
trivalent state.
Preparation of scrubber and absorber: Glass wool in the scrubber was impregnated
with lead acetate solution. 5 ml Silver diethyldithiocarbamate solution was pipette into
the absorber tube.
Arsine generation and measurement: 3-4 gm Zinc was added to generator and scrubber-
absorber assembly was connected immediately. All connection were fitted tightly.
30minutes was allowed for complete evolution of arsine. The solution was directly
poured from absorber into a 1 cm cell and absorbance was measured 520 nm.
Preparation of standard curve: Portion of standard solution containing 0, 1.0, 2.0, 5.0,
10.0 µ g arsenic is treated as describe above. Absorbance vs. Concentration was
plotted.
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