Abstract A catalytic method has been developed for the determination of microgram amounts of mercury (II) based on its catalytic effect on the rate of a ligand substitution reaction. The reaction studied involved the substitution of cyanide in hexacyanoferrate (II) by resacetophenone benzoic acid hydrazone (2&apos;, 4&apos;-dihydroxy propiophenone benzoic acid hydrazone) and the reaction was monitored spectrophotometrically. Satisfactory results were obtained for the determination of mercury (II) in the range 0.1086-0.9774 µg/ml. The relative standard deviation was less than 2.1%. The method detection limit and limit of quantification was found to be 0.041 µg/ml and 0.083 µg/ml respectively (n=7). The correlation coefficient for the determination was found to be 0.9989. The effect of foreign ions on the determination was studied

A Raghavendra

Guru Prasad

L K Ravindranath

V Seshagiri

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1 Chemical Sciences Journal, Volume 2010: CSJ-14 
A Catalytic Spectrophotometric Method for the Analytical Determination of 
Trace Amounts of Mercury (II) 
A Raghavendra Guru Prasad1*, V Seshagiri2, L K Ravindranath2 
1 ICFAI Foundation for Higher Education, Hyderabad, A.P., India 
2 Department of Chemistry, S K University, Anatapur, A.P., India 
 
*Correspondence to: A Raghavendra Guru Prasad, guruprasadar@yahoo.co.in 
Accepted: October 17, 2010; Published: December 23, 2010 
Abstract 
A catalytic method has been developed for the determination of microgram amounts of mercury (II) based on its catalytic effect on the rate of a 
ligand substitution reaction. The reaction studied involved the substitution of cyanide in hexacyanoferrate (II) by resacetophenone benzoic acid 
hydrazone (2’, 4’- dihydroxy propiophenone benzoic acid hydrazone) and the reaction was monitored spectrophotometrically. Satisfactory results 
were obtained for the determination of mercury (II) in the range 0.1086-0.9774 µg/ml. The relative standard deviation was less than 2.1%. The 
method detection limit and limit of quantification was found to be 0.041 µg/ml and 0.083 µg/ml respectively (n=7).  The correlation coefficient for 
the determination was found to be 0.9989. The effect of foreign ions on the determination was studied. 
Keywords: Mercury (II) Determination; Catalytic Method; Spectrophotometry. 
 
1. Introduction 
 
Mercury is highly toxic element and it can be easily absorbed by the animals and human beings [1]. Due to its high toxic nature, the 
determination of trace amounts of mercury in water samples is of great importance. Numerous catalytic methods [2, 3] for the 
determination of trace amounts of metals have been reported in the literature. Only few systems have been reported which involve 
ligand exchange reactions, e.g., hexacyanoferrate (II) is known [4, 5] to exchange cyanide ligands with chromogenic organic ligands. 
Many methods [6-14] for the determination of mercury (II) reported in the literature are very expensive and employ complicated 
procedure.  In this article, the authors report a simple catalytic spectrophotometric method for the determination of trace amounts 
of mercury (II). It was observed by the authors that, the colourless resacetophenone hydrazone (RBAH) reacts with yellow K4Fe(CN)6 
solution in appropriate buffer medium giving a reddish brown colored solution. The structure of RBAH is shown in Figure 1. 
 
OH
CH 2 CH 3
OH
C N NH CO
 
Figure 1: Structural formula of RBAH. 
 
The development of the color was extremely slow; it requires about 24 hours to attain the maximum intensity. The addition of small 
amounts of mercury (II) considerably accelerates the development of colour. No such colour was observed when the metal ion was 
added independently to K4Fe(CN)6 or RBAH, hence the colour formation was due to the ligand exchange reaction between K4Fe(CN)6 
and RBAH in the presence of mercury (II). 
 
The reddish brown solution in the presence of mercury (II) at pH 3 showed maximum absorbance at 430 nm. At this wavelength both 
K4Fe(CN)6 and RBAH solutions showed negligible absorbance. Therefore a catalytic method for the determination of mercury (II) at 
430 nm has been developed. Above facts are similar to that reported by Phull and Nigam [15]. 
 
 
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2 Research Article 
2. Methods 
 
2.1 Reagents 
 
All the reagents were analytical grade obtained from Merck. Double distilled water was used for all the experiments. Mercurous 
chloride solution and potassium ferrocyanide solutions are prepared by dissolving appropriate amounts of respective compounds in 
double distilled water. RBAH solution was prepared by dissolving appropriate amount of RBAH in dimethylformamide. Buffer 
solution of pH 1.0 – 3.0 was prepared by mixing appropriate quantities of 0.1M hydrochloric acid and 0.2M sodium acetate and that 
of pH 4.0 – 7.0 was prepared by mixing suitable quantities of 0.2M acetic acid and 0.2M sodium acetate. 
 
2.2 Apparatus 
 
Absorbance measurements were made on an ELICO UV-Visible Spectrophotometer SL-164. An ELICO Digital pH meter C-212 was 
used for pH measurements. 
 
 
3. Results and Discussion 
 
3.1 Procedure  
 
Suitable aliquots of buffer solution, RBAH solution, Hg (II) solution and distilled water were taken in conical flask. An aliquot of 
ferrocyanide solution was added from the pipette into the conical flask. A stopwatch was started when the pipette is half-empty. A 
known volume of the reaction mixture was withdrawn from the conical flask and absorbance was measured at known intervals of 
time (5 minutes) at 430 nm. The initial rates were calculated from absorbance-time curves [16]. 
 
3.2 Optimization of reaction conditions 
 
3.2.1 pH  
The change of initial rate with pH is shown in the Figure 2. The rate was maximum at pH 3. The decrease in rate at higher pH may be 
due to the shortage of protons needed to regenerate the catalytic species and or due to the decrease in the concentration of Hg2+ as 
a result of hydrolytic precipitation as the hydroxide. The rate is reduced at low pH values because of protonation of Fe(CN)6
–4. 
Therefore,   pH 3 was chosen for the determination of mercury (II). 
3.2.2 RBAH concentration 
 With the optimum pH fixed at 3, ferrocyanide concentration and mercury (II) concentration at 4×10–3M and 1×10–6M respectively, 
the concentration of  RBAH was varied from 0.2×10–5 to 1.6×10–5  M. The linear plot of initial rate Vs RBAH is shown in the Figure 3. 
Keeping in view the fact that, the reaction hardly occurs at lower RBAH concentrations and higher concentrations are not advisable 
because of the limited solubility of the reagent in the reaction medium, a concentration of 1.0×10–5 M was fixed for further studies. 
3.2.3 Ferrocyanide concentration 
With the other conditions as before, the concentration of ferrocyanide was varied between 0.4×10–3 to 6.0×10–3 M. The linear plot of 
initial rate vs ferrocyanide concentration is shown in the Figure 4. At lower ferrocyanide concentrations the reaction does not occur, 
whereas at higher concentrations, forward rate is inversely proportional to the concentration of cyanide ion (15). A concentration of 
4×10–3M was fixed for subsequent studies. 
3.3 Determination of Hg2+ 
Different aliquots of buffer solution of pH 3, RBAH solution of concentration 1×10–4 M, a known aliquot of mercury solution  of 
concentration 2 ×10–5 M and requisite volume of distilled water were taken in a glass stoppered conical flask, stirred well and kept in 
a thermostat at 27±10C.  Known aliquot of the thermostated K4[Fe(CN)6] solution of concentration 4×10
–2M was added to the conical 
 
 
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3 Chemical Sciences Journal, Volume 2010: CSJ-14 
flask. A stop watch was started when the pipette delivering K4[Fe(CN)6] solution was half empty. A known aliquot of the reaction 
mixture was withdrawn from the conical flask and absorbance was measured at known intervals of time at 430 nm against a blank 
containing a mixture of a buffer solution of pH 3, RBAH and K4[Fe(CN)6] solutions. The initial rates were calculated from absorbance-
time curves [16]. 
3.4 Method validation 
3.4.1 Linear range and sensitivity 
Plot of absorbance Vs concentration was linear in the range   0.08 to 0.72 µg/ml and is shown in the Figure 5. The absorbance at 
shorter and longer times also gave linear relationships, 30 minutes was found to be optimum for the determination. The least square 
calibration equation was A = 7.1977 C + 1.3472 (A is absorbance and C is concentration in µg mL-1) which was of the straight-line 
equation form Y = mX + c and has a regression coefficient 0.9989.  
3.4.2 Selectivity 
 The proposed method was found to be highly selective in a solution of pH 3 at 430 nm. The effect of foreign cations and anions on 
the determination was studied and the tolerance limits were established; the results are presented in the Table 1.  
3.4.3 Accuracy and precision 
 The reproducibility of the proposed method was tested by performing replicate determinations. The results given in Table 2 reveal 
the excellent intermediate precision (%RSD < 1.90) and accuracy (%RE ≤ 2.50) of the results. 
3.4.4 Robustness and ruggedness  
To check the ruggedness, analysis was performed by three different chemists; and on three different spectrophotometers by the 
same chemist. The robustness and the ruggedness were checked at three different mercury (II) levels. The intermediate precision, 
expressed as percent RSD, which is a measure of robustness and ruggedness was within the acceptable limits as shown in the Table. 
3. 
3.5 Application to the real samples 
The proposed method was applied for the determination of Hg (II) in several samples. The samples were spiked with several known 
amounts of Hg (II). The results reported as the average values from seven sample measurements are shown in the Table 4. These 
results showed the applicability of the method in various types of environmental water samples. 
4. Conclusion 
The proposed method is simple, highly reproducible, sensitive and selective for the determination of trace amounts of mercury (II). 
A notable advantage of this method is that the Hg (II) determination is free from interference of almost all cations and anions found 
in the water samples. 
 
Competing Interests 
The authors declare that they have no competing interests. 
Authors’ Contributions 
 
LKR and VS supervised the project and preparation of manuscript. ARGP did the actual work, prepared the manuscript and 
communicated to the journal.  
 
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References 
 
[1]  O’Neil P, 1995. Mercury in Environmental Chemistry, 2nd ed., Chapman and Hall, London.  
[2]  Petkovic BB, Simonovic RM, Micic RJ, 2007. Determination of ultramicroquantities of Au(III)  by its catalytic effect on 4-
hydroxyl coumarone oxidation with potassium permanganate. Journal of Analytical Chemistry, 62: 697-700. 
[3]  Qin W, Guohua C, Jinhu Y, Bin D, 2003. catalytic kinetic spectrophotometric determination of trace amounts of silver in triton 
x-100 micellar medium. Analytical Letters, 36: 627-640. 
[4]  Gadia MK, Mehra MC, 1978.  Analytical reactions of substituted cyanoferrates: 2. Pentacyanoamminoferrate (II) in catalytic 
spectrophotometric determination of sub parts per million amounts of Ag+, Au3+, and Hg2+ in solution. Microchemical Journal, 
23: 278-284. 
[5]  Mohan Reddy KPRR, Govind Chowdary P, Krishna Reddy V, Raveendra Reddy P, 2007. Catalytic-kinetic determination of 
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[6]  Fischer R, Rapsomankis S, Andreae MO, 1993. Determination of methylmercury in fish samples using GC/AA and sodium 
tetraethylborate derivatization. Analytical Chemistry, 65: 763-766. 
[7]  Huang W, Zhang S, 2002. Determination of mercury at a dithizone-modified glassy carbon electrode by anodic stripping 
voltametry. Analytical Sciences, 18: 187-189. 
[8]  Svarc-Gajic J, Suturovic Z, Marjanovic N, Kravic S, 2006. Determination of mercury by chronopotentiometric stripping analysis 
using glassy carbon vessel as a working electrode. Electroanalysis, 18: 513-516. 
[9]  Hosseini MS, Moghaddam HH, 2005. Sensitized extraction spectrophotometric determination of Hg(II) with dithizone after its 
flotation as ion-associate using iodide and ferroin. Talanta, 67: 555-559. 
[10]  Padmaja P, Balasubramanian PN, Ramakrishna TV, 1994. Chemical enhancement method for the determination of mercury by 
spectrophotometry after iodide extraction. Talanta, 41: 255-260. 
[11]  Rajesh N, Gurulakshmanan G, 2008. Solid phase extraction and spectrophotometric determination of mercury by adsorption 
of its diphenylthiocarbazone complex on an alumina column. Spectrochimica Acta, Part A, 69: 391-394. 
[12]  Resano M, Gelaude I, Dams R, Vanhaecke F, 2005. Solid sampling electrothermal vaporization inductively coupled plasma 
mass spectrometry for the direct determination of Hg in different materials using isotope dilution with a gaseous phase for 
calibration. Spectrochimica Acta, Part B, Atomic Spectroscopy, 60: 319-326. 
[13]  Schintzer G, Soubelet A, Chafey C, 1995. Comparison of open and closed focused microwave digestions in view of total 
mercury determination by cold vapour atomic absorption spectrometry. Mikrochimica Acta, 119: 199-209. 
[14]  Shafawi A, Ebdon L, Foulkes M, Stockwell P, Corns W, 1999. Determination of total mercury in hydrocarbons and natural gas 
condensates by Atomic Fluorescence Spectroscopy. Analyst, 124: 185-189. 
[15]   Phull M, Nigam PC, 1981. Kinetics and mechanism of mercury(II)-catalysed replacement of cyanide in hexacyanoferrate(II) by 
p-nitrosodiphenylamine. Talanta, 28: 591-597. 
[16]  Gupta VK, 1974. Kinetics and mechanism of oxidation of p-substituted anilines by peroxydisulfate ion in acetic acid-water 
medium. Reaction Kinetics and Catalysis Letters, 27: 207-211. 
 
 
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5 Chemical Sciences Journal, Volume 2010: CSJ-14 
 
 
Figure 2: Effect of pH on initial rate. 
 
[K4Fe(CN)6] = 4 × 10
–3M; [RBAH] = 1 × 10–5M; [Hg(II)] = 1 × 10–6M; λmax = 430 nm. 
 
 
 
 
 
 
 
 
Figure 3:  Effect of RBAH concentration on initial rate. 
 
[K4Fe(CN)6] = 4 × 10
–3M; [Hg(II)] = 1 × 10–6M; pH = 3; λmax = 430 nm. 
 
 
 
0
1
2
3
4
5
6
0 1 2 3 4 5 6 7 8
In
tia
l r
at
e×
10
4 
s
pH
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5
In
iti
al
 r
at
e 
×
10
4
s
[RBAH] × 105 M–1
 
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6 Research Article 
 
 
 
 
 
Figure 4:  Effect of ferrocyanide concentration on initial rate. 
 
[RBAH] = 1 × 10–5M; [Hg(II)] = 1 × 10–6M; pH = 3; λmax = 430 nm. 
 
 
 
 
 
 
 
 
Figure 5:  Determination mercury (II). 
 
[K4Fe(CN)6] = 4 × 10
–3M; [RBAH] = 1 × 10–5M; pH = 3; λmax = 430 nm. 
0
2
4
6
8
10
0 2 4 6 8 10
In
iti
al
 r
at
e 
×
10
4 
s
[ferrocyanide] × 103 M–1
0
1.5
3
4.5
6
7.5
9
0 0.2 0.4 0.6 0.8 1
In
it
ia
l r
at
e 
×
10
4
s
[Hg(II)] in µg/ml
 
 
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7 Chemical Sciences Journal, Volume 2010: CSJ-14 
Table 1: Effect of foreign ions on the determination of mercury (II); [Hg(II)] = 0.543 µg/ml. 
 
Foreign ion Tolerance limit*  
(µg/ml) 
Oxalate 2.963 
Nitrate 0.416 
Sulphate 1.211 
Tartrate 2.457 
Phosphate 1.216 
Fluoride 1.121 
chloride 1.218 
Bromide 9.409 
Iodide 1.113 
Calcium 13.273 
Magnesium 11.273 
Cadmium 7.562 
Tungsten 10.821 
Zinc 5.618 
Copper 3.825 
Nickel 1.727 
Palladium 1.822 
Lead 11.712 
Barium 7.218 
Tellurium 9.125 
Platinum 12.120 
 
* Average of seven determinations. 
 
 
 
 
 
Table 2: Accuracy and precision. 
 
 
Added 
µg/ml 
Precision and Accuracy 
Founda 
µg/ml 
RSD 
% 
Recoveryb 
% 
0.200 0.205±0.157 
 
1.827 
 
102.500 
0.400 0.409 ± 0.114 
 
1.335 102.250 
0.600 0.611±0.073 
 
0.856 101.830 
 
 
 
aThe tabulated value of t at 95% confidence level is 2.26; RSD. Relative standard deviation;  bRecovery = 100% + [(Found – Added) / Added] x 
100%. 
 
 
 
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Table 3: Robustness and ruggedness. 
 
[Hg(II)] studied 
µg mL-1 
Robustness 
(RSD %) 
Ruggedness (RSD %) 
Reaction time 
(n = 3)* 
Inter analysts 
(n = 3) 
Inter instruments 
(n = 3) 
0.200 1.634 1.992 2.021 
0.400 1.421 1.002 1.274 
0.600 1.120 1.335 0.997 
 
* The stop clock was started after 2 min, 3 min and 4 min of the addition of K4[Fe(CN)6] solution in to the reaction mixture. 
 
 
 
 
 
 
Table 4: Results of mercury (II) determination in spiked tap water. 
 
Sample Added 
(µg/ml) 
Found 
(µg/ml) 
SE 
µg/ml 
Recovery 
% 
Tap water 0.600 0.611 ± 0.073 0.030 101.830 
0.400 0.409 ± 0.114 0.046 102.250 
0.200 0.205 ± 0.157 0.063 102.500 
 
RE = relative error; SE = standard error. 
 
 
 


A Catalytic Spectrophotometric Method for the Analytical Determination of Trace Amounts of Mercury (II)

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A Catalytic Spectrophotometric Method for the Analytical Determination of Trace Amounts of Mercury (II)

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