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Spectrophotometric determination of mercury(II) as the ternary complex with rhodamine 6g and iodide
Analytica OElsevier
Chimica Acta, 84 (1976) 369--.375 Scientific Publishing Company, Amsterdam
-
Printed
in The Netherlands
SPECTROPHOTOMETRIC DETERMINATION OF MERCURY(I1) AS THE TERNARY COMPLEX WITH RHODAMINE 6G AND IODIDE
T. V. RAMAKRISIINA, Department (Received
of Chemistry. 10th November
G. ARAVAMUDAN Indian
Institute
and M. VIJAYAKUMAR of Technology,
hfadras
600 096 (India)
1975)
SUMMARY The formation of a pink-coloured product when rhodamine 6G is treated with tetrrriodomercurate(I1) is used to determine mercury (5-25 rg) in a final volume of 25 ml. The reaction occurs immediately over the pH range l-7 and, when the system is stabilized with gelatin, the absorbance remains unchanged at 575 nm for at least 24 h. The few interfering ions can be masked by the addition of appropriate reagent solutions. The method is simple and reliable and provides a molar absorptivity of 7.0 - 10’. 1 mole-’ cm-‘.
The determination of traces of mercury has attracted considerable attention owing to the toxicity of mercury and its compounds, and many calorimetric methods have been proposed [ 11. Methods based on ternary complex formation seem to provide the best alternative choice to the conventional method based on extraction with dithizone [i!] , as they are associated with superior sensitivity and selectivity, in addition to high stability and reproducibility. Amongst the ternary systems proposed, the method of Lebedeva [ 31 involving the reaction of tetrabromomercurate(I1) with methylenc green is most sensitive (cc,40 nm = 1.06 - 10” 1 mole-’ cm-‘) and selective since only gold(III), tellurium(III), antimony(V), lead(H) and cadmium(i1) are reported to interfere. IIowever, like most other ternary systems proposed for mercury, the method involves a selective extraction step because the species formed absorb at the same wavelength as the dye. Ilcnce there appeared to be a need for a rapid precise method with minimum manipulation, suitable for routine determination of traces of mercury. The survey of the various ternary systems proposed for mercury(II) suggested that the use of tetraiodomercurate(I1) as the primary complex instead of tetrabromomercurate(II), would improve the selectivity of the method, as it would extend the number of masking qents available to prevent any interferences. A detailed examination of the reaction of several cationic dyestuffs with mercury(I1) in the presence of potassium iodide showed that in weakly acidic solutions, rhodamine 6G offered the best possibilities. The colour reaction with tetraiodomercurate(I1) was distinctive, as the species formed was pink in contrast to the red colour of the reagent
370
solution under identical conditions. This indicated that the absorbanccs could be measured in the aqueous phase itself without any need for extraction. The nature of the reaction and the development of an analytical procedure were therefore examined in detail. EXPERIMENTAI.
Apparatus and reagents A Carl-Zeiss P&IQ-II spectrophotometer
with lo-mm quartz celIs, and a Knick pII meter with an Ingold combined glass-calomel eicctrode were used.
Merccrry(ll) solution (1 mg ml-‘). Dissolve 0.3385 g of mercury(I1) chloride in water and dilute to 250 ml. Dilute appropriate volumes of this stock solution with water to provide a 5-p-p-m. solution of mercury, as required. Buffered potassium iodide solution. Dissolve 5 g of potassium iodide and 5 g of potassium hydrogen phthatate in water, add a few crystak of sodium thiosulphatc
and dilute to 250 ml with water.
Rhodamine 6G sc4ution (il.005 5%). Dissolve 0.05 g of the reagent (Chroma Geseltschaft, Schmid 5IcCo., Stuttgart) in water and dilute to 1 I. Procedure
Transfer a suitable aliquot (up to 10 ml) of the sample solution containing not more than 25 gg of mercury to a 25-mf volumetric flask. Add with mixing 5 ml of the buffered potassium iodide and 5 ml of the rhodamine 6G solution followed by 1 ml of 1 % (w/v) gelatin solution prepared in the usual way. Dilute the solution to the mark with distifled water, and measure the absorbance in IO-mm c&s at 575 nm against a reagent blank. Prepare a calibration curve for 5-25 pg of mercury by the above procedure. RESULTS AND DISCUSSION
Preliminary studies indicated that the reaction proceeded immediately when iodide was added to mercuryfii) solution before the addition of rhodamine 6G. The major difficuIty was the gradual precipitation of the compiex on standing, which made absorbance measurements difficult. The complex was colloidal in nature and stabilization was achieved by addition of the protective colloid gefatin, which successfully retarded precipitation of the complex even on Icaving overnight. As a matter of routine, gelatin was therefore added after the other reagents for stabilization purposes. Figure 1 shows the absorption spectra of rhodamine 6G with different molar proportions of mercury in the presence of excess of potassium iodide so!ution at pI1 4. It is evident that the interaction between fetraiodomercurate(II) and rhodamine 6G proceeds with a considerable bathochromic shift and
371
Fig. 1. Absorption spectra. (Total volume 25 ml, 4-cm cells) (A): 1 ml of 1 - lo-’ M rhodamine 6G and 5 ml of buffered potassium iodide solution with gelatin. (B-E): As in (A) with the addition of 0.33 ml (B), 0.50 ml (C), 1.0 ml (D) and 2.0 ml (E) of 1 - 10” M mercury solution_
that the complex
shows the reagent at 530 nm.
Effect
maxima!
absorption
at 575
nm, as against
that
of
of experimental variables pH was examined with a universal buffer solution over the pH range l-10. Measurements of the absorbances of the complex and the reagent blank showed that the colour system is independent of pH over the range 1-7. Below pH 1, iodine was liberated. Although the reaction proceeds over a wide pH range, the pH of the sample and the blank should not be widely different, as the blank absorbance varied slightly with change in pH. The pH was maintained at 4.0 in a!! subsequent investigations. To avoid the use of the complex universal buffer solution with its possible subsequent interferences, simple buffers for pH 4.0 were tested. Either potassium hydrogenphthalate or acetate buffer could be used without affecting the colour system; the buffer concentration was not important. Al! subsequent investigations were done with the hydrogenphthalate buffer, which was incorporated into the iodide solution for convenience. Two series of experiments were carried out to investigate the influence of the concentrations of the reagents on the development of the colour. In one, varying amounts of a 2 % solution of potassium iodide were added to a mixture of 4 ml of 5 p.p.m. mercury solution and 5 ml of 0.005 70 rhodamine 6G solution. The optimum amount was 5 ml, but a large excess of iodide scarcely affected the sensitivity. In the second series, varying amounts of rhodamine 6G were added with 5 ml of 2 70 potassium iodide solution. The colour development reached a maximum at 4 ml and then remained constant as the amount of rhodamine 6G was increased up to 10 ml. The optimal
372
The order of addition of the reagent solutions was not critical provided that gelatin was added after the other reagents. Prior addition of gelatin caused decreased absorbance. Beer’s law and precision
Beer’s law was obeyed over the range 5-25 c(g of mercury in a final volume of 25 ml. Under the conditions described, the molar absorptivity 7.0 - lo4 1 mole-’ cm-’ for mercury. A series of ten standard solutions containing 10 pg of mercury was analyzed; the standard deviation was 0.05 %.
is
Nature of the complex
With the large excess of iodide used, mercury(I1) forms tetraiodomercurate(II) rather than triiodomercurate(I1) (log & = 2.2); HgIz- then reacts with rhodamine 6G to form a neutral ternary complex which could be extracted into apolar solvents. The ratio of mercury to rhodamine 6G in the complex
in the presence of excess of iodide was established by the conventional continuous variations and mole ratio methods. The evidence shown by Fig. 2 (a, c) is indentical with the espected ratio of two molecules of rhodamine 6G for each ion of mercury; thus the complex formed has the empirical composition R,HgI,, (where R’ represents the rhodamine 6G cation). The ratio of mercury to iodide in the ternary complex was investigated in the presence of excess of rhodamine 6G. The plots (Fig. 2b, d) show evidence for the formation of the tri-iodomercurate(I1) anion; the definite 3:3 ratio of iodide to mercury suggests that the empirical composition of the complex formed under these conditions is RHgI,. This was confirmed by a slope ratio plot for the mole ratio of triiodomercury(I1) (formed by mixing stoichiometric amounts of mercury(II) and iodide) to rhodamine 6G. On the basis of all these results, it was concluded that the ternary complex formed at low concentrations of iodide has the empirical composition RIIgI, and that it is very weak in nature (see Fig. 2d, e). The more stable species with Hg:41:2R stoichiometry exists only at higher concentration of iodide (lOCJO-fold excess over mercury). In any case, the RHgIS and R,HgI, species had identical spectral characteristics. There is a significant shift in the wavelength of maximum absorption on complex formation (Fig. 1). Such shifts in similar ternary systems featuring bulky organic dyestuffs have been attributed by Babko and Pilipenko [4] to weak bonding between the amine nitrogen atom of the dye and the metal ion. However, in the present complex, mercury(H) in tetraiodomercurate( II) anion is already coordinately saturated, so that RzHgL should probably be considered as an ion-pair. Interference
studies
The effect of various cations and anions on the determination of mercury was examined. In these tests, 10 r.lg of mercury and 1 mg of the foreign ion were used. The ions examined are listed in Table 1.
m
373
(b)
s z 4” 0 5 01 L-.--u 0.1. 02
07+&T _ Mole of fraction of Mercury @igI(Hg*RCl~]
$04 2 f..,
04
05
oc
0.3
--
08
t 06
03
Cc)
k3 -
1
1 02 03 Mole fractwn of Mercury [Hg/Wg*KI)]
04
-
-(d)
_.5
*I-
to.2 s zs ‘0 0.1 t 4:
0 E__I
3
4
5
6
7 8 KI(ml)
9 --
1011
23456709
Rhodamrnc 6G UnO -
::
3= 0.2 f sol.
LT.
*-L._ 1
4 2 3 Rhodammc 6G (ml) --c
!
Fig. 2. Composition studies. (Total volume 25 ml; l-cm cells at 575 nm.) (a, b) Continuous variation plots. (a) Total concentration of mercury and rhodamine 6G = 10 ml of 10’ M solution. Buffered potassium iodide (5 ml) and gelatin (1 ml) added. (b) Total Rhodamine 6C (10 ml concentration of mercury and iodide = 10 ml of 10 * M solution. of 10’ M) and gelatin (1 ml) added. (c, d. e) Mole ratio plots. (c) 2 ml of lo-’ M mercury + x ml of 10’ M rhodamine 6G + 5 ml of buffered iodide + gelatin (1 ml). (d) 2 ml of lo-’ M mercury + x ml of lo-’ iodide + 10 ml of lo-’ M rhodamine 6G in 0.05 M hydrogenphthalate + gelatin (1 ml). (e) 2 ml of lo--’ M mercury + 10 ml of lo-’ M iodide in 0.05 M hydrogenphthalate t x ml of lo-’ M rhodamine 6G + gelatin (1 ml). Ionic strength 0.2.
374 TABLE
1
interference studies ,-__. ..--
-_.-_-
..-.
.------
Group I II III IV V VI VII VIII
Li”, Cu’+, Ag’ Mg’ l, Ca“, Si*, Ba”, Zn”, Cd” B,O;-, BOj., t\l’+, Cc’: ‘I’l”, Tl’, La”, Th” Sn”‘, Sn’*, Pb”’ NIiz, Sb’*. Bi”. AsO;, AsO,, VO;, PO:-, NO:. NO; W+, SOj-, S,Ot; SO:‘-, SeOi-, TeOi-, Cr,Of-, MoOi *, Mn” +, F-, Cl-, Br-. ClO;, IO; Fe2*, Fe’*, Co’*, Nil’, Pd”, Pt.*
MisceIIancous: Thiocyanate, citrate, oxdate ---.---._ .---_ ..--_
_ -_
WO:-
___
Under the conditions established, the interference encountered included (a) those that oxidized iodide to iodine: Cu*+, Cr,O:-, Fe3’ and IO;; (b) those which reduced mercury(I1) to lower oxidation states: Sn”’ and AsO;; (c) those that precipitated out as their iodides or hydrous oxides: Ag’, Pb’+, Bi’+, Tl+ and Cu’+; and (d) those that formed coloured species similar to that formed by mercu~{II): I’d’+, Pt?’ and Cd”+. The deleterious effects of Cu’+, Pb’+, Bi3+, Cd” and Fe3’ were overcome by the addition of 2 ml of 0.05 M EDTA solution. The addition of hydrazinc sulphate reduced the higher oxidation state of chromium so that it no longer interfered; interferences of Sn” and AsO; were prevented by oxidation with bromine water. Sulphite addition overcame the interference of Pt”‘, and the addition of ammonia similarly eliminated the effect of l?d’+. The interference of silver(I) could not be removed by the addition of cyanide, which would mask mercury itself; the interference of silver was due to its precipitation as iodide, and could be avoided by centrifuging the precipitate before the addition of rhodamine 6G. Thus the relatively few interferences that were encountered were easily eliminated by properly conditioning the solution and under these conditions the sensitivity of the procedure remained unaffected. Conclusion The reaction between rhodamine 6G and tetraiodomercurate(I1) provides a reliable means of determining traces of mercury. The method, although less sensitive (E~,~ - = 7.0 - lo4 1 mole-’ cm-‘) than that based on the reaction between tetrabromomercurate(I1) and methylene green (eGqOnm = 1.06 * 10’ 1 mole-’ cm-‘), compares favourably with the standard method based on dithizone (E.VK,Nn = 3.2 - lo4 1 mole-’ cm-‘). Also, the absorptiometric finish does not involve an extractive separation, so that the proposed method is simple and rapid. The method has also the advantage of virtual freedom from interferences of many extraneous ions and therefore should
375
be of value in trace analysis for mercury in biological and pharmaceutical samp!es, plant materials and industrial effluents. One of us (M. V.) is grateful to the Council of Scientific and Industrial Research, New Delhi, for financial support. The assistance of Mr. I. R. K. Raju in the initial stages of the investigation is gratefully acknowledged. REFERENCES 1 For a review, see S. Chilov, Talanta, 22 (1975) 205. 2 E. B. Sandell, Calorimetric Determination of Traces of Metals, Interscience. New York, 1959. 3 S. P. A. Lebedeva, Arm. Khim. Zh., 25 (1972) 303. 4 A. Babko and A. Pilipenko, Photometric Analysis, MIR. Moscow, 1971. pp. 342-376.
Chimica Acta, 84 (1976) 369--.375 Scientific Publishing Company, Amsterdam
-
Printed
in The Netherlands
SPECTROPHOTOMETRIC DETERMINATION OF MERCURY(I1) AS THE TERNARY COMPLEX WITH RHODAMINE 6G AND IODIDE
T. V. RAMAKRISIINA, Department (Received
of Chemistry. 10th November
G. ARAVAMUDAN Indian
Institute
and M. VIJAYAKUMAR of Technology,
hfadras
600 096 (India)
1975)
SUMMARY The formation of a pink-coloured product when rhodamine 6G is treated with tetrrriodomercurate(I1) is used to determine mercury (5-25 rg) in a final volume of 25 ml. The reaction occurs immediately over the pH range l-7 and, when the system is stabilized with gelatin, the absorbance remains unchanged at 575 nm for at least 24 h. The few interfering ions can be masked by the addition of appropriate reagent solutions. The method is simple and reliable and provides a molar absorptivity of 7.0 - 10’. 1 mole-’ cm-‘.
The determination of traces of mercury has attracted considerable attention owing to the toxicity of mercury and its compounds, and many calorimetric methods have been proposed [ 11. Methods based on ternary complex formation seem to provide the best alternative choice to the conventional method based on extraction with dithizone [i!] , as they are associated with superior sensitivity and selectivity, in addition to high stability and reproducibility. Amongst the ternary systems proposed, the method of Lebedeva [ 31 involving the reaction of tetrabromomercurate(I1) with methylenc green is most sensitive (cc,40 nm = 1.06 - 10” 1 mole-’ cm-‘) and selective since only gold(III), tellurium(III), antimony(V), lead(H) and cadmium(i1) are reported to interfere. IIowever, like most other ternary systems proposed for mercury, the method involves a selective extraction step because the species formed absorb at the same wavelength as the dye. Ilcnce there appeared to be a need for a rapid precise method with minimum manipulation, suitable for routine determination of traces of mercury. The survey of the various ternary systems proposed for mercury(II) suggested that the use of tetraiodomercurate(I1) as the primary complex instead of tetrabromomercurate(II), would improve the selectivity of the method, as it would extend the number of masking qents available to prevent any interferences. A detailed examination of the reaction of several cationic dyestuffs with mercury(I1) in the presence of potassium iodide showed that in weakly acidic solutions, rhodamine 6G offered the best possibilities. The colour reaction with tetraiodomercurate(I1) was distinctive, as the species formed was pink in contrast to the red colour of the reagent
370
solution under identical conditions. This indicated that the absorbanccs could be measured in the aqueous phase itself without any need for extraction. The nature of the reaction and the development of an analytical procedure were therefore examined in detail. EXPERIMENTAI.
Apparatus and reagents A Carl-Zeiss P&IQ-II spectrophotometer
with lo-mm quartz celIs, and a Knick pII meter with an Ingold combined glass-calomel eicctrode were used.
Merccrry(ll) solution (1 mg ml-‘). Dissolve 0.3385 g of mercury(I1) chloride in water and dilute to 250 ml. Dilute appropriate volumes of this stock solution with water to provide a 5-p-p-m. solution of mercury, as required. Buffered potassium iodide solution. Dissolve 5 g of potassium iodide and 5 g of potassium hydrogen phthatate in water, add a few crystak of sodium thiosulphatc
and dilute to 250 ml with water.
Rhodamine 6G sc4ution (il.005 5%). Dissolve 0.05 g of the reagent (Chroma Geseltschaft, Schmid 5IcCo., Stuttgart) in water and dilute to 1 I. Procedure
Transfer a suitable aliquot (up to 10 ml) of the sample solution containing not more than 25 gg of mercury to a 25-mf volumetric flask. Add with mixing 5 ml of the buffered potassium iodide and 5 ml of the rhodamine 6G solution followed by 1 ml of 1 % (w/v) gelatin solution prepared in the usual way. Dilute the solution to the mark with distifled water, and measure the absorbance in IO-mm c&s at 575 nm against a reagent blank. Prepare a calibration curve for 5-25 pg of mercury by the above procedure. RESULTS AND DISCUSSION
Preliminary studies indicated that the reaction proceeded immediately when iodide was added to mercuryfii) solution before the addition of rhodamine 6G. The major difficuIty was the gradual precipitation of the compiex on standing, which made absorbance measurements difficult. The complex was colloidal in nature and stabilization was achieved by addition of the protective colloid gefatin, which successfully retarded precipitation of the complex even on Icaving overnight. As a matter of routine, gelatin was therefore added after the other reagents for stabilization purposes. Figure 1 shows the absorption spectra of rhodamine 6G with different molar proportions of mercury in the presence of excess of potassium iodide so!ution at pI1 4. It is evident that the interaction between fetraiodomercurate(II) and rhodamine 6G proceeds with a considerable bathochromic shift and
371
Fig. 1. Absorption spectra. (Total volume 25 ml, 4-cm cells) (A): 1 ml of 1 - lo-’ M rhodamine 6G and 5 ml of buffered potassium iodide solution with gelatin. (B-E): As in (A) with the addition of 0.33 ml (B), 0.50 ml (C), 1.0 ml (D) and 2.0 ml (E) of 1 - 10” M mercury solution_
that the complex
shows the reagent at 530 nm.
Effect
maxima!
absorption
at 575
nm, as against
that
of
of experimental variables pH was examined with a universal buffer solution over the pH range l-10. Measurements of the absorbances of the complex and the reagent blank showed that the colour system is independent of pH over the range 1-7. Below pH 1, iodine was liberated. Although the reaction proceeds over a wide pH range, the pH of the sample and the blank should not be widely different, as the blank absorbance varied slightly with change in pH. The pH was maintained at 4.0 in a!! subsequent investigations. To avoid the use of the complex universal buffer solution with its possible subsequent interferences, simple buffers for pH 4.0 were tested. Either potassium hydrogenphthalate or acetate buffer could be used without affecting the colour system; the buffer concentration was not important. Al! subsequent investigations were done with the hydrogenphthalate buffer, which was incorporated into the iodide solution for convenience. Two series of experiments were carried out to investigate the influence of the concentrations of the reagents on the development of the colour. In one, varying amounts of a 2 % solution of potassium iodide were added to a mixture of 4 ml of 5 p.p.m. mercury solution and 5 ml of 0.005 70 rhodamine 6G solution. The optimum amount was 5 ml, but a large excess of iodide scarcely affected the sensitivity. In the second series, varying amounts of rhodamine 6G were added with 5 ml of 2 70 potassium iodide solution. The colour development reached a maximum at 4 ml and then remained constant as the amount of rhodamine 6G was increased up to 10 ml. The optimal
372
The order of addition of the reagent solutions was not critical provided that gelatin was added after the other reagents. Prior addition of gelatin caused decreased absorbance. Beer’s law and precision
Beer’s law was obeyed over the range 5-25 c(g of mercury in a final volume of 25 ml. Under the conditions described, the molar absorptivity 7.0 - lo4 1 mole-’ cm-’ for mercury. A series of ten standard solutions containing 10 pg of mercury was analyzed; the standard deviation was 0.05 %.
is
Nature of the complex
With the large excess of iodide used, mercury(I1) forms tetraiodomercurate(II) rather than triiodomercurate(I1) (log & = 2.2); HgIz- then reacts with rhodamine 6G to form a neutral ternary complex which could be extracted into apolar solvents. The ratio of mercury to rhodamine 6G in the complex
in the presence of excess of iodide was established by the conventional continuous variations and mole ratio methods. The evidence shown by Fig. 2 (a, c) is indentical with the espected ratio of two molecules of rhodamine 6G for each ion of mercury; thus the complex formed has the empirical composition R,HgI,, (where R’ represents the rhodamine 6G cation). The ratio of mercury to iodide in the ternary complex was investigated in the presence of excess of rhodamine 6G. The plots (Fig. 2b, d) show evidence for the formation of the tri-iodomercurate(I1) anion; the definite 3:3 ratio of iodide to mercury suggests that the empirical composition of the complex formed under these conditions is RHgI,. This was confirmed by a slope ratio plot for the mole ratio of triiodomercury(I1) (formed by mixing stoichiometric amounts of mercury(II) and iodide) to rhodamine 6G. On the basis of all these results, it was concluded that the ternary complex formed at low concentrations of iodide has the empirical composition RIIgI, and that it is very weak in nature (see Fig. 2d, e). The more stable species with Hg:41:2R stoichiometry exists only at higher concentration of iodide (lOCJO-fold excess over mercury). In any case, the RHgIS and R,HgI, species had identical spectral characteristics. There is a significant shift in the wavelength of maximum absorption on complex formation (Fig. 1). Such shifts in similar ternary systems featuring bulky organic dyestuffs have been attributed by Babko and Pilipenko [4] to weak bonding between the amine nitrogen atom of the dye and the metal ion. However, in the present complex, mercury(H) in tetraiodomercurate( II) anion is already coordinately saturated, so that RzHgL should probably be considered as an ion-pair. Interference
studies
The effect of various cations and anions on the determination of mercury was examined. In these tests, 10 r.lg of mercury and 1 mg of the foreign ion were used. The ions examined are listed in Table 1.
m
373
(b)
s z 4” 0 5 01 L-.--u 0.1. 02
07+&T _ Mole of fraction of Mercury @igI(Hg*RCl~]
$04 2 f..,
04
05
oc
0.3
--
08
t 06
03
Cc)
k3 -
1
1 02 03 Mole fractwn of Mercury [Hg/Wg*KI)]
04
-
-(d)
_.5
*I-
to.2 s zs ‘0 0.1 t 4:
0 E__I
3
4
5
6
7 8 KI(ml)
9 --
1011
23456709
Rhodamrnc 6G UnO -
::
3= 0.2 f sol.
LT.
*-L._ 1
4 2 3 Rhodammc 6G (ml) --c
!
Fig. 2. Composition studies. (Total volume 25 ml; l-cm cells at 575 nm.) (a, b) Continuous variation plots. (a) Total concentration of mercury and rhodamine 6G = 10 ml of 10’ M solution. Buffered potassium iodide (5 ml) and gelatin (1 ml) added. (b) Total Rhodamine 6C (10 ml concentration of mercury and iodide = 10 ml of 10 * M solution. of 10’ M) and gelatin (1 ml) added. (c, d. e) Mole ratio plots. (c) 2 ml of lo-’ M mercury + x ml of 10’ M rhodamine 6G + 5 ml of buffered iodide + gelatin (1 ml). (d) 2 ml of lo-’ M mercury + x ml of lo-’ iodide + 10 ml of lo-’ M rhodamine 6G in 0.05 M hydrogenphthalate + gelatin (1 ml). (e) 2 ml of lo--’ M mercury + 10 ml of lo-’ M iodide in 0.05 M hydrogenphthalate t x ml of lo-’ M rhodamine 6G + gelatin (1 ml). Ionic strength 0.2.
374 TABLE
1
interference studies ,-__. ..--
-_.-_-
..-.
.------
Group I II III IV V VI VII VIII
Li”, Cu’+, Ag’ Mg’ l, Ca“, Si*, Ba”, Zn”, Cd” B,O;-, BOj., t\l’+, Cc’: ‘I’l”, Tl’, La”, Th” Sn”‘, Sn’*, Pb”’ NIiz, Sb’*. Bi”. AsO;, AsO,, VO;, PO:-, NO:. NO; W+, SOj-, S,Ot; SO:‘-, SeOi-, TeOi-, Cr,Of-, MoOi *, Mn” +, F-, Cl-, Br-. ClO;, IO; Fe2*, Fe’*, Co’*, Nil’, Pd”, Pt.*
MisceIIancous: Thiocyanate, citrate, oxdate ---.---._ .---_ ..--_
_ -_
WO:-
___
Under the conditions established, the interference encountered included (a) those that oxidized iodide to iodine: Cu*+, Cr,O:-, Fe3’ and IO;; (b) those which reduced mercury(I1) to lower oxidation states: Sn”’ and AsO;; (c) those that precipitated out as their iodides or hydrous oxides: Ag’, Pb’+, Bi’+, Tl+ and Cu’+; and (d) those that formed coloured species similar to that formed by mercu~{II): I’d’+, Pt?’ and Cd”+. The deleterious effects of Cu’+, Pb’+, Bi3+, Cd” and Fe3’ were overcome by the addition of 2 ml of 0.05 M EDTA solution. The addition of hydrazinc sulphate reduced the higher oxidation state of chromium so that it no longer interfered; interferences of Sn” and AsO; were prevented by oxidation with bromine water. Sulphite addition overcame the interference of Pt”‘, and the addition of ammonia similarly eliminated the effect of l?d’+. The interference of silver(I) could not be removed by the addition of cyanide, which would mask mercury itself; the interference of silver was due to its precipitation as iodide, and could be avoided by centrifuging the precipitate before the addition of rhodamine 6G. Thus the relatively few interferences that were encountered were easily eliminated by properly conditioning the solution and under these conditions the sensitivity of the procedure remained unaffected. Conclusion The reaction between rhodamine 6G and tetraiodomercurate(I1) provides a reliable means of determining traces of mercury. The method, although less sensitive (E~,~ - = 7.0 - lo4 1 mole-’ cm-‘) than that based on the reaction between tetrabromomercurate(I1) and methylene green (eGqOnm = 1.06 * 10’ 1 mole-’ cm-‘), compares favourably with the standard method based on dithizone (E.VK,Nn = 3.2 - lo4 1 mole-’ cm-‘). Also, the absorptiometric finish does not involve an extractive separation, so that the proposed method is simple and rapid. The method has also the advantage of virtual freedom from interferences of many extraneous ions and therefore should
375
be of value in trace analysis for mercury in biological and pharmaceutical samp!es, plant materials and industrial effluents. One of us (M. V.) is grateful to the Council of Scientific and Industrial Research, New Delhi, for financial support. The assistance of Mr. I. R. K. Raju in the initial stages of the investigation is gratefully acknowledged. REFERENCES 1 For a review, see S. Chilov, Talanta, 22 (1975) 205. 2 E. B. Sandell, Calorimetric Determination of Traces of Metals, Interscience. New York, 1959. 3 S. P. A. Lebedeva, Arm. Khim. Zh., 25 (1972) 303. 4 A. Babko and A. Pilipenko, Photometric Analysis, MIR. Moscow, 1971. pp. 342-376.