A Rapid Spectrophotometric Method for the Determination
of Lead using 2,5- dimercapto-1, 3,4- thiodiazole (DMTD)
N. Imtiaz , M.Tahir Butt, and T. Shafiq.
Centre for Environmental Protection Studies,
PCSIR Laboratories Complex, Lahore- Pakistan
Abstract:
A new spectrophotometric method which is fairly selective, non extractive, is described
here for the determination of lead based upon its colour reaction with 2,5- dimercapto1, 3,4- thiodiazole (DMTD). According to this lead (II) can be determined within the
range from 1.0 to 50 ug per ml in a slightly acidic media (0.005 M HCl) , to give a
greenish yellow chelate complex, which has an absorption maximum at 370 nm. The
reaction proceed fastly and color remain stable. While no special conditions are
followed . The method is recommended for its routine use and is quite accurate,
precise, and sensitive. Effect of Foreign ions is also studied at 40 ug/ml of lead.
Keywords: Spectrophotometry, Lead determination, colour reaction, 2, 5- dimercapto1, 3, 4- thiadizole.
Introduction:
Lead is distributed widely in nature. It is found in many ores, galena, anglestic Cerosite,
storage batteries, alloys, cable sheaths, solder, X-ray equipments and radiation
shielding. The most widespread uses, which have led to environmental and health
problems, have been the use of lead in gasoline antiknock products and paint pigments
[1]. Lead is a serious cumulative body poison [2]. It enters our body system through
air, water and food. The toxicity of lead has been studied extensively [3]. Due to its
wide spread occurrence, mode of application as well as its high toxicity, lead is one of
the most important element in the human Environment. Variety of methods are
available in the literature for its determination. Trace amount of lead is usually
determined by the colorimetric methods with dithizone [4] tetraahydroxy quinone
method [5] and tetra hydroxy benzo quinone [6]. However the preparation of samples
and steps required to eliminate interferences make these methods rather time
consuming, tedious, and expensive.
Spectrophotometry is essentially a trace analysis technique and is one of the most
powerful tool in chemical analysis. 2,5 dimercapto-1, 3-4- thiadiazole (DMTD) has
been reported as a potentiometric reagent [6] and has not been used previously for the
spectrophotometric determination of metals. The method is based on the reaction of
non- absorbant DM TD in slightly, acidic solution (0.005 M HCl) with lead (II) to
produce a highly absorbent greenish yellow chelate product, followed by direct
measurement of the absorbance in aquous solution. Dimercaptothiodizol is known s a
specific reagent for lead and has not been used previously for the spectrophotometric
determination of metals.
1
In this paper attempt has been made to determine lead in a trace levels by
spectrophotometric technique which has obvious advantages of simplifying the
instrumentation and decreasing the cost per analysis. The method possesses to distinct
advantages over existing method [7] with respect to sensitivity, selectivity, simplicity,
acidity range, accuracy, precision and ease of operation. Linear calibration graph was
obtained for 1.0 to 50µg /ml of lead .The reagent blank do not show any absorbance.
Result and discussion:
The colour reaction between Lead (II) and DMTD has maximum absorbance at 370 nm.
Fig (1), hence all absorbance measurements were carried out at this wavelength. Since
the colour intensity remains constant at room temperature Fig (1II) all absorbance
measurement were made at room temperature (25  5oC).
The colour intensity is stable at pH range 2- 4.5 . Hence all absorbance measurements
were made at this pH. For all measurements, 1 ml 0.005 M Hydrochloric acid was added
to achieve the required pH range.
The result for the determination of lead are shown in table-1 which shows the reliability
of the method. The colour reaction is specific for lead, and no other cation or anion gives
any color with the colour producing reagent. The reaction between lead and
dimercapthiodiazol is very fast and colour developed within a few seconds and remains
stable for 24 hours Fig (IV). The method does not involve the use of rigid special
conditions.
2
Table-1: Spectrophotometric Determination of Lead with DMTD
Solution
Sr. No.
1234567891011-
Lead taken ug/ ml
Lead found ug/ ml
1.00
2.00
4.00
6.00
8.00
10.00
15.00
25.00
30.00
40.00
50.00
1.03
1.98
3.96
5.92
7.96
10.02
15.05
25.06
30.08
40.01
50.35
Error ug/ ml
+3.00
+0.02
+0.04
+0.08
+0.50
+0.2
+0.33
+0.24
+0.26
+0.25
+0.7
Table-2: Effect of Interfering Ions on Spectrophotometric Determination of
Lead with DMTD
Sr. No. Interfering ions
added ug/1ml
1
Barium
2
Zinc
3
Chromium
4
Bismuth
5
Cobalt
6
Tin
7
Iron
8
Copper
Lead taken
ug/ ml
40.00
40.00
40.00
40.00
40.00
40.00
40.00
40.0
Lead found
ug/ ml
40.05
40.02
40.07
40.05
40.06
40.04
41.0
41.5
Error
%
+ 0.12
+ 0.05
+ 0.17
+ 0.12
+ 0.15
+ 0.10
+ 2.5
+ 3.75
3
Experimental
Apparatus and instruments
All absorbance measurement for the determination of lead were made with UV - VIS
spctrophotmeter specord 200 with PC version (Germany). pH meter(Crison 2001
Spain) with combination of electrode were used for the measurement of pH
respectively. Graduated pipettes accurate to + 0.02 ml were used. All other volumetric
glass ware used of ‘A grade’ calibrated .
Chemical and reagent
All chemicals used were of Analytical reagent grade and the highest purity available.
Double distilled de-ionized water was used throughout the experimentation. All glass
vessels were cleaned by soaking by acidified solutions of KMnO4 or K2 Cr2 O7,
following by washing with concentrated nitric acid and were rinsed several times with
distilled water and then over dried.
Colour producing reagent (DMDT Solution)
4.42 x 10 –3 M Dimercaptothiodiazol (Aldrich ACS grade) was made by dissolving it in
a known volume of distilled de-ionized water. More dilute solution of the reagent were
prepared as required.
Lead (II) Standard Solution
Stock standard solution of lead nitrate 4.83 x 10-3 M (Merck grade) was prepared by
dissolving 159.9 mg of lead nitrate in 100 ml double distilled deionized water. One ml
of dilute nitric aid was added to prevent Hydrolysis. More dilute standard solutions
were prepared by appropriate dilutions of aliquots from the stock solution with
deionized water when required.
Synthetic Mixture
A synthetic mixture of possible interfering inorganic ions such as Barium, Zinc,
Chromium and Bismuth were prepared from their equivalent grade water soluble salts.
In case of insoluble e substances special dissolution methods were adopted (Pal B.K. et
al., 1984).
Method
A volume of 1 ml neutral aqueous test solution containing 1 to 100 ug of lead in a 10
ml of calibrated flask was mixed with 1 ml DMTD reagent solution, followed by the
addition of 1 ml 0.005 M hydrochloric acid. The mixture was diluted to the mark with
deionized water. After one to two minutes, absorbance was measured at 370 nm against
a corresponding reagent blank which contains all the reagents except lead. Blank is run
along the sample under exactly the same conditions. The accuracy of the results was also
4
examined by preparing the synthetic mixture containing the known amount of pure
chemicals and analyzing it with reagent blank. The well known equation for
spectrophotometric analysis in very dilute solutions derived from bear,s law.The effect
of lead concentration was studied over 1 to 100 µg/ml.The absorbance was linear at
370nm upto 50µg/ml of lead solution . The calibration curve was prepared and color
reaction obeys Beers law.
Interference study:
The effect of 8 interfering ions on the determination of 40 ug/ml of lead (II) was
studied. They form strongly coloured complexes with DMTD at same pH and
wavelength. The criterion for interference (Bosch. C. et al., 1987) was an absorbance
value varying from the expected value of lead alone. Table-2 list of the percentage
error calculated for the lead ion determination in the presence of various interfering
ions.
The synthetic mixture containing lead and all the above mentioned foreign ions was
analysed. Table-2 shows that lead can be determined in solution in their presence with a
maximum error of 0.17% for 40 ug of its amount taken. Barium, Cobalt, Zinc, Tin,
Chromium and Bismuth do not interfere. While iron and copper interferes seriously with
this reagent, which can be climinated by making it with tartrates or thiocynide. During
interferences studies if a precipitate was formed, it was removed by centrifugation (Boch
C. et al.,).
“Fig.(1)” Absorption Spectra at 370 nm on Spectrophotometric
Determination of Lead with DMTD
Absorbance
Fig. II Pb(II) Calibration Curve
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
Concentration (ug/ml)
60
70
80
5
“Fig.(3)” Effect of temperature on spectrophotometric Determination with
DMTD.
“Fig. (4)” Stability of colour in hours on Spectrophotometric Determination
3
2.5
Absorbance
2
1.5
1
0.5
0
8
16
24
32
40
48
56
Number of hours
Lead with DMTD
Conclusion:
In this paper a new simple selective accurate, sensitive, convenient and practical
method with lead (II) - DMDT complex was developed. Therefore, it is
recommended for the routine use in the determination of lead at trace levels in
industrial, environmental and soil samples for continuous monitoring to establish
lead. The mechanism of colour reaction is not clear. The method was also tested
by analyzing several interfering mixtures and diverse ions (Table-2). Although
many sophisticated techniques such as AAS, ICP- AES, HPLC, ICP-MS are
available for the determination of lead in trace levels from many samples, yet
6
spectrophotometry is still a popular technique because of easy handling
maintenance and low cost of instrument. Hence, the precision and accuracy of the
method is excellent.
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8
The Head,
Centre for Environmental
Protection Studies,
PCSIR Labs. Complex,
Lahore
SUBJECT: Submission of Research Paper for Publication
Please forward my research paper for publication to the Chief Editor, Processing of the
Pakistan Academy of Sciences 3- Constitution Avenue G-5/2, Islamabad. The title of
research paper is “Spectrophotometer Determination of Lead using 2,5- dimercapto1, 3,4- thiodiazole (DMTD)”
Thank very much,
With best regards,
Yours sincerely,
9
( Mrs. Naz Imtiaz )
Senior Scientific Officer
Centre for Environmental
Protection Studies,
PCSIR Labs. Complex, Lahore-54600
16171819202122232425-
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