3. INTRODUCTION FOR ARSENIC
Arsenic is steel grey, very brittle, crystalline in
nature and oxidizes on rapid heating to arsenous
oxide with an odor of garlic. Arsenic exists as
inorganic and organic compounds.
In the environment, it combines with oxygen,
chlorine and sulfur to form inorganic arsenic
compounds.
Arsenic also combines with carbon and hydrogen
to form organo-arsenic compounds in animals
and plants.
Inorganic arsenic compounds are mainly used to
preserve wood, and organic arsenic compounds
are used as pesticides – primarily for cotton crop
(Carapella, 1973; Calvert, 1975).
4. INTRODUCTION FOR ARSENIC
Arsenic monitoring is utmost important nowadays.
Palmer (2001) reported atomic spectroscopy is the most
widely-used method for the arsenic determination.
Atomic spectroscopy involves use of the absorption
characteristic of metals (Andreae, 1977; Christian and
Feldman, 1970; Chu et al., 1972; Clement et al., 1973;
Fishman and Spencer, 1977).
For arsenic analysis, detection limits are required to be
very low (≤1 ppb). It can be achieved only by state-of-
the art, latest versions of equipment like the atomic
absorption spectrometer or ICP.
5. Spectrometers are available in most water quality
laboratories; however, arsenic analysis in drinking water
is quite difficult with such equipment due to the required
low detection limits for arsenic.
6. DETERMINATION OF ARSENIC
The numerous procedures available for the determination
minute amounts of arsenic, only one will be described,
i.e. the molybdenum method.
It possess great sensitivity and precision.
And it is readily applied colorimetrically or
spectrophotometrically.
7. METHODOLOGY FOR ARSENIC:
Molybdenum blue method: when arsenic, as arsenate, is
treated with ammonium molybdate solution and the resulting
heteropolymolybdoarsenate is reduced with hydrazinium
sulphate or with tin (II) chloride a blue soluble complex
‘molybdenum blue’ is formed.
The molybdenum is present in a lower oxidation state.
The stable blue color has a maximum absorption at about
840nm and shows no appreciable change in 24 hours.
Another method of isolation involves volatilisation of arsenic
as arsine by the action of zinc in hydrochloric or sulphuric
acid solution.
The arsine which is evolved may be absorbed in a sodium
hydrogen carbonate solution of iodine.
The absorption apparatus should be so designed that the arsine
is completely absorbed.
8. REAGENTS WHICH ARE USED IN ARSENIC?
Potassium iodide solution: Dissolve 15g of solid
in 100mL water.
Tin (II) chloride solution: Dissolve 40g hydrated
tin (II) chloride in 100 mL concentrated
hydrochloric acid.
Iodine-potassium iodide solution: Dissolve 0.25g
iodine in a small volume of water containing 0.4 g
potassium iodide, and dilute to 100mL.
Sodium hydrogencarbonate solution: Dissolve
4.2g of the solid in 100mL water.
9. PROCEDURE FOR ARSENIC
The arsenic must be in the arsenic (III) state; this
may be secured by first distilling in all glass
apparatus with conc hydrochloric acid.
A
B
C
D
10. • A=Pyrex solution
• B=tube is loosely packed with cotton wool soaked in lead acetate
solution
• C=capillary tube with 0.5mm internal diameter
• D=absorption tube.
Transfer an aliquot portion of the arsenate
solution, having a volume of 25mL and
containing not more than 20ug of arsenic, to the
50mL Pyrex evolution vessel A and add
sufficient hcl acid to make the total volume
present in the solution 5-6 mL, followed by 2mL
of the potassium iodide solution and 0.5 mL of
the tin(II) chloride solution.
Allow to stand at room temp for 20-30 minutes
to permit the complete reduction of arsenate.
11. Rapidly add 2.0g of zinc to the vessel A, immediately insert the
stopper, and allow the gases to bubble through the solution for
30minutes.
At the end of this time the solution in D should still contain some
iodine. Disconnect the delivery tube C and leave in the absorption
tube.
Construct the calibration curve by taking 0,2.5,5.0,7.5 and 10.0ug As
(for a final volume of 10mL), mixing with iodine –iodide –
hydrogencrabonate solution, adding molybdate-hydrazinium sulphate-
disulphite, and heating to 95-100°C
Calibration curve for arsenic analysis by UV- Visible
spectrophotometric technique
12. INTRODUCTION FOR LEAD
Lead is a chemical element that is assigned
the symbol Pb (from the Latin plumbum) and
the atomic number 82. It is a heavy metal that
is denser than most common materials.
Lead is soft and malleable , and has a relatively
low melting point.
When freshly cut, lead is bluish-white; it tarnishes to
a dull gray color when exposed to air.
Lead has the highest atomic number of any stable
element and concludes three major decay chains of
heavier elements.
13. lead and lead oxides react with acids and bases,
and it tends to form covalent bonds.
Lead is easily extracted from its ores; prehistoric
people in Western Asia knew of it. Galena
In the late 19th century, lead's toxicity was
recognized, and its use has since been phased out
of many applications.
Lead is a neurotoxin that accumulates in soft
tissues and bones, damages the nervous system,
and causes blood disorders.
It is particularly problematic in children: even if
blood levels are promptly normalized with
treatment, permanent brain damage may result.
14. DETERMINATION OF LEAD BY DITHIZONE METHOD
For the determination of small amounts of lead
(0.005-0.25) advantage is taken of the fact that when
a sulphide is added to a solution containing lead ions
a brown colour, due to the formation of colloidal
lead sulphide, is produced.
Dithizone is a violet-black solid which is insoluble
in water, soluble in dilute ammonia solution, and
also soluble in chloroform and in carbon
tetrachloride to yield green solutions.
It is an excellent reagent for the determination of
small quantities of many metals.
15. REAGENTS WHICH ARE USED IN LEAD?
Dithizone reagent: Dissolve 5mg of the solid in 100mL
of chloroform.
PROCEDURE: Place 10.0mL of the working lead solution in
a 250mL separatory funnel, add 75mL of the ammonia-
cyanide-sulphite solution and then addition of dilute HCL
acid adjust pH of the solution to 9.5 (pH-meter).Now add
7.5mL of the dithizone reagent to the separatory funnel,
followed by a further 17.5mL of chloroform.
Shake for 1 minute, allow the layers to separate, then remove
the chloroform layer.
Repeat the procedure with 5.0mL, 7.5mL and 15.0mL
portions of the working lead solution and then with 10mL of
the test solution.
16. MERCURY
Mercury is a chemical element with
symbol Hg and atomic number 80.
It is commonly known as quicksilver and was
formerly named hydrargyrum.
A heavy, silvery d-block element, mercury is
the only metallic element that is liquid
at standard conditions for temperature and
pressure.
Mercury was found in Egyptian tombs that date
from 1500 BC. Mercury remains in use in
scientific research applications and in amalgam
for dental restoration in some locales. It is used
in fluorescent lighting.
17. Mercury (II) thiocyanate method: The determination of
trace amounts of chloride ion depends upon the
displacement of thiocyanate ion from mercury(II)
thiocyanate by chloride ion; in the presence of iron(III)
ion a highly colored iron(III) thiocyanate complex is
formed.
PROCEDURE: Place a 20mL aliquot of the chloride
solution in a 25mL graduated flask, add 2.0mL
ammonium iron(III) sulphate [Fe(NH4)(SO4)2,12H2O] in
9M nitric acid, followed by 2.0mL of a saturated solution
of mercury(II) thiocyanate in ethanol.
The amount of chloride ion in the sample corresponds to
the difference between the two absorbances and is
obtained from a calibration curve.
18. Construct a calibration curve using a standard sodium
chloride solution containing 10ugCl-
mL -1. Plot absorbance against micrograms of chloride ion.
19. CONCLUSION
The following procedure has been recommended by
the Analytical Methods committee of the Society
for Analytical Chemistry for determination of small
amounts of arsenic in organic matter.
Organic matter is destroyed by wet oxidation , and
the arsenic, after extractioin with dietyhlammonium
diethyldithiocarbonate in chloroform, is converted
into the arsenomolybdate complex.
The latter is reduced by means of hydrazinium
sulphate to a molybdenum blue complex and
determined spectrophotometrically at 840nm and
referred to a calibration graph in the usual manner.
20. REFERNCES
1. J Mendham, R.C.Denney,et.al;Vogel’s Textbook of
Quantitative chemical analysis;Pg.No:-680-700.
2. Takeru Higuchi, Einar Brochmann,et.al
Pharmarmaceutical Analysis;Pg.No:-738-744.
3. M.A.Augelli,R.A.A.Munoz,et.al;Article of analytical
procedures for total mercury determination;Pg.No:-
579-584.
4. https://en.wikipedia.org/wiki/mercury_(element)
5. https://en.wikipedia.org/wiki/lead.
