Spectrophotometric Determination of Silver and Gold ...

Analyst, February 1995, Vol. 120

549

Spectrophotometric Determination of Silver and Gold with 5-(2,4-Dihydroxybenzy Iidene)rhodanine and Cationic Surfactants

F. M. El-Zawawy, M. F. El-Shahat, A. A. Mohamed and M. T. M. Zaki* Department of Chemistry, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt The synthesis, characteristics and analytical reactions of 5-(2,4-dihydroxybenzylidene)rhodanine(2,4-DHBR) with silver and gold are described. Maximum enhancement of the absorbance of the chelates is obtained in the presence of the cationic surfactants cetyltrimethylammoniumbromide (CTAB) and cetylpyridinium bromide (CPB), respectively. Each metal ion forms two ternary complexes depending on 2,4-DHBR concentration. The stoichiometriesof the complexes are 1 :1 :1 and 1 :2 :3 (Ag-2,4-DHBR-CTAB) and 1 :2 :3 and 1 :3 :4 (Au-2,4-DHBR-CPB). The complexes with higher reagent/metal ratio were used for the spectrophotometric determinationof silver and gold. In the pH ranges 9.2-10.6 and 9.S10.8, the molar absorptivitiesare 7.11 X 104 (Ag) and 8.45 x 104 (Au) dm3 mol-1 cm-1 at 547 and 558 nm, respectively. The methods adhere to Beer’s law for 0.13-1.83 pg cm-3 of silver and 0.162.24 pg cm-3 of gold and the corresponding sensitivitiesare 0.0015 and 0.0023 pg cm-2. The methods were used successfully to determine trace amounts of silver and gold in silicate rocks and a sulfide ore. Keywords: Silver determination; gold determination; spectrophotometry; 5-(2,4-dihydroxybenzylidene)rhodanine; surfactants

Introduction

The spectrophotometric determination of silver is usually preceded by reaction with dithizone,’ 4-(2-quinolylazo)phenol,2 2-[4-amino-3-(1,2,4-triazolylazo)]naphthol-4-sulfonate,3 4,4’-bis(dimethylamino)thiobenzophenone,4 ammonium 2-cyano-3-iminodithiobutyratesor ammonium (2’,3’-dihydroxypyridyl-4’-azo)benzene-4-arsonate6 and that of gold with dithizone,7 methiomeprazine hydrochloride ,8 dithiodiantipyrylmethane,g 2,2,2-trifluoroethyl xanthate,10 4-(2-quinolylazo)phenol,ll 4-(2-pyridylazo)resorcinoll2 or 4-(2-thiazoly1azo)resorcinol (TAR).13 However, most of these methods lack sensitivi ty , selectivity and/or reproducibility . Rhodanine derivatives have also found wide application in the determination of silver14-18 and gold. I s 2 3 However, many problems are associated with their use, including the solubility of reagents and chelates and the instability of the developed colo~rs.24~25 Consequently, a protective colloid is added to stabilize the pseudo-solution, or a water-miscible organic solvent is used throughout. Surfactants and micellar systems are currently used in the spectrophotometric determination of metals to solubilize reactants and products and to improve the sensitivity and * To whom correspondence should be addressed. Present address: Department of Chemistry, Faculty of Science, United Arab Emirates University, PO Box 17551 Al-Ain, United Arab Emirates. ~

selectivity of these methods.26.27 However, little attention has been directed to the determination of silver28J9 and gold30 in micellar media. In this work, 5-(2,4-dihydroxybenzylidene)rhodanine (2,4DHBR) was synthesized and tested as a possible chromogenic reagent for the determination of silver(1) and gold(m). The protolytic equilibria of the reagent were determined. The effects of some surfactants and protective colloids on the spectral characteristics of silver- and gold-2,4-DHBR chelates were investigated. Maximum sensitization of the colour reactions was obtained in the presence of cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) , respectively. The experimental variables affecting colour formation and the possible interference of co-existing ions were thoroughly studied. The optimum conditions established were incorporated in the recommended procedures, which were applied to determine silver and gold in silicate rocks and a sulfide ore. Experimental Apparatus

Absorbance measurements were obtained with a PerkinElmer Lambda 3B UV-VIS spectrophotometer using 1 cm matched quartz cells. An Orion Model 920A Ion-Analyzer equipped with a combination glass-calomel electrode was used for pH determinations. Reagents

All chemicals were of analytical-reagent grade. Surfactants and protective colloids were used as received. Doubly distilled water was used throughout. 2,4-DHBR was synthesized according to the general method of Julian and Strugis,3l by adding 10 g of fused sodium acetate to 100 cm3 of glacial acetic acid containing 0.1 mol each of rhodanine and 2,4-dihydroxybenzaldehyde.The mixture was refluxed for 2 h with occasional shaking. 2,4-DHBR was separated by cooling. The whole mass was poured into 1 dm3 of cold water. The solid was separated and crystallized several times from acetic acid. The yield was 82%. The purity of the reagent was checked by thin-layer chromatography with benzene-light petroleum (b.p. 40-60 “C) (1 + 4 v/v) as eluent. A single orange-yellow band was obtained for the pure product. Results of elemental analysis confirmed the purity of 2,4-DHBR (calculated, C 47.43, H 2.77, N 5.53; found, C 47.3, H 2.7, N 5.5%). The solid reagent was kept for as long as 18 months without any change in structure, as evidenced from IR, UV and NMR spectroscopic measurements. An ethanolic solution of 2,4-DHBR (5 X 10-3 mol dm-3) was prepared by dissolving 0.3163 g in warm

550


ethanol and diluting to 250 cm3 with the same solvent. This solution was stable for more than 3 months. Silver(]) stock standard solution (0.01 mol dm-3) was prepared by dissolving 0.8494 g of silver nitrate (dried at 110 "C) in water containing 1 cm3 of nitric acid and diluting to 500 cm3 in a calibrated flask. Gold(in) stock standard solution (1 mg cm-3) was prepared by dissolving 0.5 g of gold in aqua regia by warming, evaporating the solution to dryness and dissolving the residue in hydrochloric acid, evaporating the solution to half its volume, cooling and diluting with water to 500 cm3 in a calibrated flask. Cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) were prepared as 0.01 mol dm-3 solutions. Borax buffer of pH 10 was prepared by adjusting the pH of 0.05 mol dm-3 sodium tetraborate solution with dilute sodium hydroxide solution. Recommended Procedures

Transfer an aliquot of the sample solution containing not more than 45 pg of silver(1) or 56 pg of gold(m) into a 25 cm3 calibrated flask. Add 0.5 cm3 of 2,4-DHBR solution and 5 cm3 of CTAB or CPB, respectively. Dilute to volume with borax buffer. Leave at room temperature for 5 min and measure the absorbance at 547 or 558 nm, respectively, against a blank. Determine the silver or gold concentration using calibration graphs prepared in the same manner. Results and Discussion

absorption band, in acidic medium, with a wavelength maximum at 413 nm, which shows a bathochromic shift to 460 and 512 nm in slightly and strongly basic media, respectively. Choice of Surfactant

The effect of some surfactants (cationic, anionic and nonionic) and protective colloids on the spectral characteristics of the binary silver- and gold-2,4-DHBR chelates was studied. Surfactants were used at concentrations above their critical micellization concentration (c.m.c.). The results are given in Table 2. Maximum enhancement of the absorbance of the chelates is obtained in the presence of cationic surfactants. CTAB and CPB are the optimum sensitizers for the colour reactions of silver and gold, respectively. Absorption Spectra

The absorption spectra of the binary and sensitized silver- and gold-2,4-DHBR chelates, along with their reagent blank in the presence of surfactants, are shown in Fig. 2. All the spectral curves were recorded at pH 10. The binary chelates have absorption maxima at 557 and 560 nm, respectively (curves C and E). Addition of CTAB to the silver complex cause a marked enhancement of the absorbance with a hypsochromic shift of 10 nm (curve D). Moreover, in the presence of CPB, the absorbance of the gold complex is more than doubled (curve F).

Table 1 Determination of the ionization constants of 2,4-DHBR

Several rhodanine derivatives were synthesized and tested as reagents for silver(1) and gold(m): 5-(4-chlorobenzylidene)rhodanine, 3-(5-~hlorobenzylidene)rhodanine,5-(3-chloro-2hydroxybenzylidene)rhodanine, 5-(3,4-diethoxybenzylidene)rhodanine, 5-(4-hydroxy-3-methoxybenzylidene)rhodanine and 5-(2,4-dihydroxybenzylidene)rhodanine (2,4DHBR). The study revealed that the first five reagents have no advantage over 5-(4-dimethylaminobenzylidene)rhodanine in the determination of silver(1) and gold(u1). However, the colour reactions of 2,4-DHBR are more sensitive in basic media. Therefore, 2,4-DHBR was selected for further studies.

Method Potentiometric Spectrophotometric

K2

H3L

PK3 9.05 9.08

PK4 10.85 10.81

0.5

Assuming the reagent to be H3L, five species are involved in the acid dissociation behaviour. The equilibria between these species can be written as K1

PK2 6.77 6.87

0.6

Characteristics of 2,4-DHBR

H4L+

PK1 2.32 -

K4

K3

H2L-

HL2-

L3-

g

0.4

C

5 9

0.3

where H4L+ is 0

0.2

\\

C-N-H H O ~ C H = C , s zI C =\ S 0.1

I

OH

H

The respective ionization constants were determined in ethanol-water (1 l), maintaining the ionic strength at 0.2 with potassium nitrate, by spectrophotometric32 and potentiometric33 methods (Table 1). K 1 ,K2, K3 and K4 are assigned to the deprotonation of SH, NH, p-OH and o-OH, respectively. Fig. 1 shows the influence of pH on the absorption spectra of the reagent. The spectra reveal the presence of an

+

320

360

400

440

480

520

560

600

Wavelengthhrn

Fig. 1 Effect of pH on the spectral characteristics of 2,4-DHBR (1.2 X 10-5 mol dm-3). Curve: 1, pH = 2.3; 2, pH = 4.5; 3, pH = 6.7; 4,

pH = 7.0; 5, pH = 7.35; 6, pH = 7.75; 7, pH = 8.1; 8, pH = 8.25; 9, pH = 8.45; 10, pH = 8.81; 11, pH = 9.25; 12, pH = 9.45; 13, pH = 10.0; 14, pH = 10.55; 15, pH = 10.8; 16, pH = 11.1; 17, pH = 11.29; 18, pH = 11.55; 19, pH = 12.1; 20. pH = 12.5; and 21, pH = 12.7.

55 1


Optimum Experimental Conditions

The influence of pH on the absorbance of the sensitized silverand gold-2,4-DHBR complexes was studied over the pH range 2-11. The pH was adjusted to the desired value using dilute nitric acid or sodium hydroxide solution. Maximum and constant absorbances of the complexes were obtained in the pH ranges 9.2-10.6 and 9.3-10.8, respectively. In the optimum pH ranges, no differences in absorbance were noticed when borax buffer was used. Consequently, pH 10 adjusted with borax buffer was adopted in the procedures. The effect of reagent concentration was investigated, at the optimum pH, by measuring the absorbance of a solution containing 0.85 or 1.72 ppm of silver or gold, 2 x 10-3 rnol dm-3 of surfactant and various amounts of 2,4-DHBR ranging between 2 X 10-5 and 4 X 10-4 rnol dm-3, at 547 or 558 nm, respectively. Full colour development was obtained in the presence of 3 6 x 10-5 and 3 8 x 10-5 rnol dm-3 of reagent, respectively. In the procedures, 0.5 cm3 of reagent (1 x 10-4 rnol dm-3) was utilized. The function of surfactants in the colour reactions was studied. The binary silver chelate was solubilized in the presence of 22.4 X 10-4 rnol dm-3 of CTAB and the corresponding CPB concentration for the gold complex was 33.2 x 10-4 rnol dm-3. Moreover, the absorbance of the sensitized complexes was maximum for CTAB and CPB concentrations of 3 4 X 10-4 and 26.2 x 10-4 rnol dm-3, respectively. It is apparent that sensitization of the colour reactions occurs at surfactant concentrations well below their c.m.c.s. The c.m.c.s of CTAB and CPB have been reported as 9.5 x 10-4 and 1.02 x 10-3 rnol dm-3, respectively.34 Consequently, the surfactants interact with the binary chelates to form true ternary complexes.35 The ternary complex colours are instantaneously established. Moreover, the absorbance of the silver complex is stable for more than 3 days, whereas the corresponding gold complex is stable for 2 h, after which the stability slightly decreases with time.

Stoichiometry of the Complexes

The molar ratio and continuous variation methods were used to determine the metal-to-reagent ratio in the complexes at pH 10. The study revealed that each metal ion forms two complexes, depending on 2,4-DHBR concentration (Figs. 3 and 4). The complexes of ratios 1 : 1 and 1 : 2 (Ag : 2,4-DHBR) have absorption maxima at 560 and 547 nm, respectively, whereas the corresponding 1:2 and 1 :3 (Au : reagent) complexes absorb at 585 and 558 nm. The metal complex-to-surfactant ratios in the ternary complexes were determined in an analogous study by the 0.6

D

480

500

520

540

560

580

600

620

Wavelengthhm

Fig. 2 Absorption spectra of silver and gold binary and ternary complexes and their reagent blank. [Ag] = 1.4 x rnol dm-3 (7.8 X 10-6 rnol dm-3 for the ternary complex), [Au] = 4.7 x 10-6 rnol dm-3, [2,4-DHBR] = 1 x rnol dm-3 (7.2 x 10-6 rnol dm-3 for curves A and B) and [CTAB] = [CPB] = 2 X 10-3 rnol dm-3 at pH 10. Curve A, 2,4-DHBR versus buffer; B, 2,4-DHBR + surfactant versus blank; C, silver chelate in aqueous medium; D, sensitized silver complex; E , gold chelate in aqueous medium; and F, sensitized gold complex.

Table 2 Influence of some surfactants and protective colloids on the spectral characteristics of silver- and gold-2,4-DHBR chelates. Conditions: [Ag+] = 7.87 x 10--6 rnol dm-3; [Au”] = 8.12 x 10-6 rnol dm-3; [2,4-DHBR] = 1 X mol dm-3; [surfactant] = 2 X 10-3 rnol dm-3 (0.4% for non-ionic surfactants); protective colloid, 0.2% m/v; pH = 10; 1 cm cells

Ag-2,4-DHBR chelate Au-2,4-DHBR chelate Surfactant or protective E x 10-41 E x 10-41 Absorbance dm3 mol-1 cm-1 colloid* Absorbance dm3 mol-l cm-l hmaX./nm hmax./nm TYPe 0.343 0.117 4.22 None 557 1.49 560 0.650 Cationic 8.00 CTAB 0.559 7.11 553 547 0.686 0.497 Cationic 8.45 550 6.32 558 CPB 0.627 7.72 0.518 Cationic CPC 560 6.57 558 0.459 0.461 Cationic 5.65 Zephiramine 550 5.86 558 0.263 Non-ionic 3.24 Triton X-100 0.140 1.78 555 545 3.10 0.252 0.114 Non-ionic Emulsifier S 558 1.45 545 0.347 4.27 0.093 1.18 555 Non-ionic Tween 20 563 0.213 Non-ionic 2.62 Tween 40 0.124 1.58 550 550 0.213 Non-ionic 2.62 0.103 1.31 555 Tween 60 560 0.235 2.89 0.109 1.39 552 Non-ionic 560 Tween 80 2.41 0.196 0.71 545 Anionic 0.060 542 SLS 0.179 Anionic 2.20 0.66 550 0.052 567 SAS 0.414 5.10 Protective colloidal 0.067 0.85 555 558 PVP 0.206 2.54 0.078 0.99 555 555 Protective colloidal PVA 0.330 4 ..06 Protective colloidal 0.098 Gelati ne 1.25 545 565 * CTAB, cetyltrimethylammonium bromide; CPB, cetylpyridinium bromide; CPC, cetylpyridinium chloride; zephiramine, benzyldimethyltetradecylammonium chloride; Triton X-100, polyoxyethylene-p-rerr-octylphenol; Tween 40, polyoxyethylene sorbitan monopalmitate; Tween 60, polyoxyethylene sorbitan monostearate; Tween 80, polyoxyethylene sorbitan monooleate; SLS, sodium lauryl sulfate; SAS, sodium alkylbenzene sulfonate; PVP. polyvinylpyrrolidone; PVA, poly(viny1 alcohol).

552


molar ratio method, by increasing the CTAB or CPB concentration whilst maintaining a constant concentration of 1: l(2) or 1 : 2(3) silver- or gold-2,4-DHBR complex. The study confirmed the formation of 1 : 1 : 1 and 1 : 2 : 3 (Ag-2,4DHBR-CTAB) and 1: 2 :3 and 1:3 : 4 (Au-2,4-DHBRCPB) complexes. Calibration and Precision

The proposed methods adhere to Beer's law over the concentration ranges 0.13-1.83 and 0.16-2.24 ppm of silver and gold at 547 and 558 nm, respectively. The corresponding molar absorptivities,as determined by a least-squaresfit for 15 results each, are 7.11 x 104 and 8.45 x lo4 dm3 mol-1 cm-1, respectively. Sandell's sensitivities are 0.0015 pg cm-2 and 0.0023 pg cm-2, respectively.

Effect of Co-existing Ions

The influence of foreign ions on the determination of silver and gold was examined using the recommended procedures. The tolerance limit was taken as the ratio of foreign ion to that being determined that causes a f 2 % error in the absorbance value. Several metal ions interfered with the determination of silver. Consequently, some masking agents were utilized to improve the selectivity of the developed method. The results obtained are given in Table 3. However, halides and cyanide interfered. Common masking agents slightly improved the tolerance limits for foreign ions in the determination of gold (Table 3). Thus, the Mg-EDTA complex (log /3 = 8.6)36 was prepared and tested as a masking agent for interfering metal ions. The study was conducted by adjusting the pH of Mg-EDTA solution to 5.3 to facilitate the replacement reaction. The masking solution was added to an aliquot containing gold(n1) plus the foreign ion. After 2 min, the reagent and surfactant

0.5

-

0.5

0.6 ,p-xx;

'

558 nm

0.4

0.4

0.4 0.2 0

585 nm

5 0.3

a

e 5: 9

0.4

1''

-e"

0.3 -

I

0

2 13

I

I

6

2 4 [DHBRy[Au3+]

a

0.2

0.2 -

0.1

0.1

3.4

3.2

558 nm

585 nrn

0-

n

540

560

500

580

540

580

Mnm

7Jnm

Fig. 3 Influence of 2.4-DHBR concentration on the formation of silver ternary complexes. (a) Spectral curves of the complexes; (b) molar ratio graphs; ( c ) Job's continuous variation graph.

620 0

0.2

0.4

0.6

0.8

[Au3+V{[Au3+]+[ DHBR]}

Fig. 4 Influence of 2.4-DHBR concentration on the formation of gold ternary complexes. ( a ) Spectral curve of the complexes: ( b ) molar ratio graph; (c) Job's continuous variation graph.

Table 3 Effect of foreign ions on the determination of silver(i) and gold(ii1) as the ternary complexes. [Ag+] = 4.8 X [Au3+] = 4.4 X 10-6 rnol dm-3

Ternary silver complex method

rnol dm-';

Ternary gold complex method Tolerance limit ([ionl/[Agl) 8 000 5000 lo00

Tolerance limit ([ion]/Au]) 10 000 5 000 3000 1000 so0 300

Foreign ion* Foreign ion Tartrate, citrate. malonate. C1H202, tartrate, citrate, FBa2+.Ca2+. Mg2+, F-, Br-. IBa2+, Ca2+,Co2+ (b), Mg2+. Mn2+, Ni2+, Zn2+ (b) NH,OH*HCI, NH2NHZ.H2SOJ Cd2+ (b), Cu2+(b), Mo6+, W6+ Sn2+(a), Ti4+ (a), NH20H.HCI 750 EDTA, oxalate, POX3AP+ (b), Be2+(b), Fe3+ (b), Pb2+ (b) Als+ Zn2+, SCN500 Be'+, Ni2+ Pd2+(c), Pt4+ (c) 250 Bi3++,Cr3+, Hg2+ (d), La?+,Pr3+, SG+, Sm3++, BP+, Cr3+,Fe3+,La3+. Mn2+,Mo6+. Pr3+, Sm3+.Th4+,Ti4+,Vs+, U6+ Th4+,Vs+, U6+, Zr4+ 150 200 * Masking agents: (a) 0.3 ml of 1 rnol 1-1 sodium fluoride; (b) 1.5 cm3 of EDTA-citrate masking solution (prepared by mixing equal volumes of 0.1 rnol dm-3 EDTA and 1.2 rnol dm-3 sodium citrate); (c) 1 cm? of 0.25 mol dm-3 dimethylglyoxime; (d) 2 ml of 0.1 rnol dm-3 sodium diethy Idit hiocarbamate.

.


solutions were added and the pH was re-adjusted to 10. Up to a 2000-fold molar excess of Ba", Ca" and Sr", a 5000-fold molar excess of Cd", Cu" and Zn", a 4000-fold molar excess of Mn", Ni", Pb" and Vv, a 2000-fold molar excess of Be", Co", Fe"', La1L1, MoV1,Prl" and Sm"', a 1000-fold molar excess of A P , Bill1, Sc"' and Th'" and a 400-fold molar excess of Cr"', UIV, WV1and Zrl" did not interfere in the determination of 4.4 x 10-6 rnol 1-1 of gold(n1). However, Hg", Pd" and PtIVseriously interfered. In the presence of 8 mmol 1-1 sodium diethyldithiocarbamate, however, up to a 250-fold molar excess of Hg" did not interfere, and 10 mmol 1-1 dimethylglyoxime masked a 300-fold molar excess of Pd" and PP" Finally, the presence of reducing agents such as hydroxylamine hydrochloride and hydrazine sulfate did not alter the spectral characteristics of the ternary gold complex, whereas traces of an oxidizing agent (hydrogen peroxide) depressed the absorbance of the complex, indicating that excess of

553

2,4-DHBR reduces the metal ion to the MI form before complexation. This is in agreement with the findings of Borissova et a f .16 for the reaction of gold(ii1) with dimethylaminobenzylidenerhodanine, which is a complex process involving the reduction of gold(ii1) to gold(i), followed by chelate formation. Applications

The proposed methods were applied to the determination of silver and gold in a sulfide ore and for the determination of gold in silicate rocks. The ore was obtained from Um-Samuiki, in the south central eastern desert of Egypt, and the rocks from El-Daghbag, in the central eastern desert of Egypt. The procedures of Jeffery and Hutchison37 for sample decomposition were followed. Silver38 and gold39 were separated from matrix elements with a strong anion exchanger, using column and batch techniques, respectively. The results

Table 4 Comparison with other methods for silver determination

Reagent Dithizone

PH u p to 4 mot dm-3 H2S04 4-(2-Quinolylazo)phenol 9.2 2-[4-Amino-3-(1,2,4-triazolylazo)]naphthol-4sulfonate 9-1 1 4,4'-Bis(dimethylamino)thiobenzophenone 3

2-Cyano-3-iminodithiobutyrate 2' ,3'-Dihydroxypyridyl-4'-azobenzene-4arsonate p-Dieth ylaminobenzylidenerhodanine p-Dimeth ylaminobenzylidenerhodanine

5-(4-Hydroxybenzylidene)rhodanine 5-[4-(2-methyl-3-hydroxy5-hydroxymethyl)pyridylene]rhodanine 5-(2,4-Dihydroxybenzylidene)rhodanine

Lax.lnm 462

4-6

E x 10-41 dm3 rnol - 1 Sensitivityl pg cm-2 Remarks cm-1 0.003 Extraction with carbon 3.1 tetrachloride

Ref.

1

530

8.3

0.001

Cu, Co, Fe, Ni and Pd interfere

580 520

2.1 9.3

0.005 0.001

565

1.3

0.008

535 490

3.0 2.2

0.004 0.005

3 Extraction with isoamyl alcohol; noble metals interfere 4 Hg interferes 5 Complex formation in strongly alkaline media 6 Low solubility of complex 14

0.05 rnol dm-3 HN03 0.05 rnol dm-3 HN03 Citrate buffer

580

2.0

0.005

490

1.5

0.007

8.2 10

530 547

1.5 7.1

0.007 0.002

2

Long path length cuvettes are recommended 16 Cu, Hg and noble metals interfere 17 Pt metals interfere

18 This work

Remarks Extraction with CHCL

Ref. 7 8

Table 5 Comparison with other methods for gold determination E

x 10-41

1 mol-1

Sensitivityl

Lax.lnm 420 630 370

cm-1 2.8 1.3 3.5

pg cm-2

0.007 0.015 0.006

Reagent Dithizone Methiomeprazine hydrochloride Dithiodiantipyrylmethane

PH

2,2,2-Trifluoroethylxanthate 4-(2-Pyridylazo)resorcinol

2-1 1 2.5

452 540

0.1 8.3

1.97 0.002

4-(2-Thiazolylazo)resorcinol

1.5

520

1.5

0.013

5-(4-Dimethylaminobenzylidene)rhodanine

0.12 rnol dm-3 HCI

500

5-(4-Dimethylaminobenzylidene)rhodanine

3

515

3.8

0.005

515 422 446 418 420 558

3.9 0.9 0.9 1.1 1.1 8.5

0.005