Applications involving the iodide ion

MICROCHEMICAL

13, 664-670 ( 1968)

JOURNAL

Applications III. Determination Agents; Analysis

Involving

the Iodide

Ion

of Iodide in Presence of Some Complexing of Binary and Ternary Mixtures of Silver with Some Metal Ions

H. KHALIFA AND B. ATEYA Faculty

of Science,

Cairo

Uniuerdty,

Giza, U.A.R.

Received April 22, 1968 INTRODUCTION

The procedure using mercury( II) as back titrant for excess iodide was recently applied by Khalifa (4) to the micro- and semimicrodetermination of silver. In an attempt to eliminate interference from lead, iron ( III ) and copper ( 111) we used EDTA which forms stable complexes with these metal ions. Since the determination of these and other metal ions by back titrating excess EDTA (5) or CDTA (6) with mercury( II) in hexamine buffered media, is well established, it is the aim of the present work to ascertain the feasibility of determining silver plus any of these ions simultaneously, by a combined back titration procedure using hexamine buffer beyond the end point of excess iodide. The end point potential breaks indicating excess iodide and excess chelating agent correspond to the potential differences between each of the following two systems respectively, when coupled with the SEC, Hg/HgI, - Hg/( Hg-chelate)-, Hg/( Hg-chelate)2P - Hg/Hg( hexamine)2+ The present method provides a simple means for the simultaneous determination of two metal ions and hence avoids complications and time consumption ( 1,3,8). EXPERIMENTAL

The water The chemicals of cadmium, silver; iodide

METHOD

was always twice distilled from all glass equipment. were of the highest purity available. These were nitrates copper ( II ) , iron( III ) , lead, mercury ( II ) , nickel, and and dichromate of potassium; thiosulfate of sodium; 664

IODIDE

EDTA and CDTA; starch indicators.

IN

PRESENCE

hexamine,

OF

COMPLEXING

665

AGENTS

zinc oxide, Eriochrome

black T and

Solutions. The metal nitrate solutions (0.0513 M mercury( II), 0.1022 M cadium, 0.0248 M iron( III), 0.0525 M lead, 0.032 M nickel, 0.039 M copper( II), and 0.0498 M silver) were prepared and standardized as previously mentioned (4,5). The 0.0498 M iodide, 0.0496 M thiosulfate, 0.0455 M EDTA and 0.0433 M CDTA were prepared and standardized by well known procedures (4,6). Cell and equipment. The titration cell, pH- and potentiometer essentially the same as those described elsewhere (4).

are

Procedures (A). To study the effect of a complexing agent on the potential break detecting iodide, a mixture of known volumes of iodide and complexing agent solutions is diluted to about 50 ml, and titrated with a standard mercury( II) solution using the silver amalgam as indicator electrode. To determine the amount of EDTA or CDTA, 10 ml of 10% hexamine solution are added to the titration mixture, slightly after the end point detecting iodide and the titration is continued normally. Procedure (A) is also applied to calculate the extent of dissolution of mercuric iodide in a known volume of EDTA, by comparing the amounts of EDTA taken and found. The difference is to be taken into consideration when calculating the amount of a given metal ion. ( B ) . Analysis of binary and ternay mixtures. After adding to the solution containing silver plus one or two metal ions, a known excess of standard EDTA and iodide solutions and water up to about 50 ml, excess of both iodide and EDTA are back titrated as in (A). In the case of ternary mixtures an additional iodometric determination of iron( III) or copper( II) is necessary. RESULTS AND DISCUSSION

The data in Tables 1 and 2 indicate that in all cases the amount of iodide found is exactly the same as that taken. The data in Table 1 indicate that the potential breaks detecting both iodide and EDTA or CDTA decrease with increasing amounts of either EDTA or CDTA. However, in the case of EDTA the breaks are still large enough to detect the end points. Further, the amount

666

KHALIFA

AND AI'EYA

of mecuric iodide dissolved in EDTA or CDTA varies irregularly and hence a blank titration involving the same excess of iodide and EDTA is necessary whenever mixtures of metal ions are to be analyzed. TABLE SIMULTANEOUS DETERMINATION Iodide

( mmoles )

mV/O.l

1

OF Iouu~ EDTA

ml

Taken

Found

titrant

Taken

0.294 0.294 0.294 0.294 0.294 0.294

0.294 0.294 0.294 0.294 0.294 0.294

262 263 211 187 161 147

0.00435 0.00906 0.02265 0.04530 0.09060 0.13590

0.294 0.294 0.294 0.294 0.294 0.294

0.294 0.294 0.294 0.294 -

291 242 202 123 55 23

0.00433 0.02165 0.04330 0.08660 0.12990 0.17320

Taken 0.05460 0.06825 0.09100 0.012285 0.18200 0.22750 0.27300 Gmmoles b mmoles

( mmoles ) Found 0.054890 0.06820 0.08977 0.12260 0.18100 0.22499 0.26757 of iodidex0.0996. of iodide=O.l992.

mV/O.l ml iodide titration 212 176 167 162 146 142 130

( mmoles)

0.00420 0.00800 0.02250 0.04430 0.08950 0.13500

mVjO.1

ml

titrant 217 223 161 146 132 107

( mmoles ) 0.00400 0.02100 0.04200 0.08600 -

199 273 194 185 164 172

2

SIMULTANEOUS DETEHMINATION

EDTA

OR CDTA

Found

CDTA

TABLE

ANU EDTA

OF IODIDE AND EDTA

EDTA Taken 0.04550 0.09100 0.12285 0.13650 0.18200 0.22750 0.27300

(mmoles) Found 0.04565'> 0.09044 0.12158 0.13543 0.18160 0.22264 0.26932

mV/O. 1 ml iodide titration 205 166 172 170 135 100 119

IODIDE

Ix

PHESENCE OF COMPLEXING

AGENTS

667

TABLE 3 ANALYSIS OF BINARY MIXTIJHES Silver

(mg)

Metal Found

Taken

5.372 10.745 16.551 21.923 5.3724 10.655 16.505 21.755 5.297 21.166 16.550 10.745 5.328 10.689 16.001 23.852 5.3724 10.745 2.686

5.372 10.745 16.344 21.792 5.372 10.745 16.344 21.792 5.372 21.166 16.344 10.745 5.372 10.745 16.117 23.852 5.372 10.745 2.686

Found

Taken 34.460 22.974 11.487 5.743 43.388 32.541 21.694 10.847 15.029 11.372 9.393 5.636 9.932 7.349 4.966 2.483 6.925 8.310 9.695

(mg)

Cd

PI>

Xi

cu

Fc

34.437 23.186 11.758 5.741 43.4125 33.110 22.004 10.878 14.736 11.372 9.325 5.678 9.925 7.493 4.846 2.516 7.010 8.260 9.738

The presence of EDTA or CDTA together with mercuric iodide leads to its partial dissolution. It is well known that a precipitate such as HgI, tends to dissolve in a solution containing the deprotonated anion (X)4- of a chelating agent, if SI?gIz K(ngxjz- exceeds unity, where SHgIz is the solubility product of mercuric iodide and K,,x,,- is the stability constant of the mercury chelate. In the light of the values of 10-l” ( SHg12) (7), 10z2 (K(HgEDTA) *-) and 10z4 (K( Hg-CDTA)‘-) (6), one must expect the reaction HgI, + X4- + (HgX)*- + 21- to proceed forward to a larger extent in CDTA than in EDTA which was verified experimentally, a reason why EDTA is preferred to CDTA. The iodide ions liberated in the above reaction do not appear as an increment in its content, as the backward reaction is enhanced by the continuous removal of X4ions by mercuric ions used as titrant. When applying procedure (B) EDTA must be added before iodide to the solution of metal ions, since it decreases the oxidation potentials l

668

KHALIFA

AND

TABLE

ATEYA

4

ANALYSIS OF TERNARY MIXTURES Silver (mg) Taken

Found

5.372 10.745 18.344 21.792 21.792 13.485 8.091 10.788 5.372 18.879 21.576 10.788 5.372 10.745 16.117 5.372 10.745 16.117 10.745 13.011 15.602

5.372 10.745 16.551 21.923 21.923 13.485 8.091 10.788 5.372 19.027 21.490 10.867 5.372 10.637 16.327 5.378 10.745 16.117 10.627 12.970 15.644

Metal (mg) Taken 1.385 Fe 2.770 4.155 5.540 8.310 6.925 5.540 4.155 5.540 8.310 6.925 8.310 4.829 Cu 9.658 7.243 9.658 7.243 4.829 4.829 9.658 7.243

Metal (mg) Found 1.385 2.770 4.155 5.540 8.310 6.925 5.540 4.155 5.540 8.310 6.925 8.310 4.829 9.658 7.243 9.658 7.243 4.829 4.829 9.658 7.243

Taken 34.460 22.974 11.487 5.742 10.847 21.694 32.531 43.388 15.029 11.372 7.515 5.686 11.835 5.918 8.876 32.541 21.694 10.847 3.757 5.636 7.415

Found cd

Pb

Ni

cd

Pb

Ni

34.437 22.963 11.611 5.741 10.839 22.003 32.760 43.200 14.685 11.086 7.507 5.690 11.836 5.924 8.899 32.195 21.578 10.888 3.682 5.781 7.390

(2) of the ferric-ferrous and cupric-cuprous systems rendering impossible the oxidation of iodide by ferric or cupric ions. Further strong chelation with lead ions prevents precipitation of lead iodide. In order to account for the lower potential end point breaks obtained in presence of either EDTA or CDTA we calculated the electrode potentials E1 just before, and Ez just after the end point using Nemst equation, El = E’& + 0.0296 log [Hgl*+] where [Hgr “f ] = 2.8 x 10m30mole/liter as computed from solubility of HgIz of 8.8 X 10-’ mole/liter (7), instability constant values of HgIz and Hg-EDTA of 10H2” (7) and 10W2*, respectively, and a concentration of 10-” mole/liter of EDTA corresponding to 1 ml of 0.05 M EDTA in total volume of 50 ml. Ez = E& + 0.0296 log [Hgz*+]

IODIDE

IN

PRESENCE

OF

COMPLEXING

AGENTS

669

where [Hgz”] = 1.1 X lo-“:: mole/liter as computed from a concentration of lO-” M Hg-EDTA corresponding to 0.1 ml of 0.05 it4 Hg?’ in total volume of 50 ml containing 0.9 ml of 0.05 M EDTA. The difference between E, and E, amounts to 195 mV which lies in good agreement with the experimental value of 187 mV obtained in presence of 1 ml of 0.05 M EDTA (Table 1). Using the instability constant value of lo-” for Hg-CDTA (6) the same value of 195 mV is obtained in presence of 1 ml of about 0.05 &f CDTA which coincides fairly well with the experimental value of 202 mV (Table 1). With few exceptions the data in Table 3 show that procedure (B) is quite reliable for the simultaneous determination of the constituents of such type of mixtures. The data in Table 4 show that the procedure described for analysis of ternary mixtures of the above type is extremely reliable. SUMMARY The effect of some complexing agents on the potential end point breaks in titrations of iodide with mercury( II) is studied. Iodide and complex ions ( EDTA and CDTA) are determined simultaneously. The slight errors involved, are attributed to the solvent action of EDTA or CDTA on the precipitated mercuric iodide. In almost all cases the end points indicating iodide are stoichiometric, though the Lwrresponding inflections are smaller than those obtained in absence of complexing agents; the decrement is dependent on the concentration of the complexing agent. Trials to analyze simultaneously binary and ternary mixtures of silver plus a variety of cations, which form stable complexes with EDTA were successful. A correction procedure is adopted to minimize the observed errors. REFERENCES 1. 2.

J. 4.

5.

AMIN, A. M., Microvolumetric determination of silver and cwpper in coinage. Chemist-Analyst 44, 17 (1955). BELCH-, R., GIBBONS, D., AND WEST, T. S., The effect of EDTA on the ferrous/ferric and cuprous/cupric systems. Anal. Chem. Acta 12, 107 (1955). EFIDY, L., RADY, GY., AND GIMESI, O., Analysis of silver lead alloys. Acta Chim. Ad. Sci. Hung. 32, 151 (1962). KHALIFA, H., AND ATEYA, B., Applications involving the iodide ion. I. A New Potentiometric method for the micro- /and semimicrodetermination of silver. Analysis of binary and ternary mixtures. Micro&m. J. 12, 440 (1967). KHALIFA, H., AND KHATEH, M., Back titration with mercuric nitrate in urotropine buffered media. Estimation of alkaline-earth and some heavy metals. Analysis of quatemary mixtures. J. Chem. U. A. R. 10, 123 ( 1987).

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7.

S.

KHALfFA

AND

ATEYA

H., Studies on the reaction between mercury(I1) and CDTA; estimation of metal ions and analysis of cation mixtures. Z. Anal. Chem. 203, 161 (1964). KOHLRAIJSCH, F., AND HOSE, F., In “Comprehensive Treatise on Inorganic and Theoretical Chemistry” (J. W. Mellor, ed.), Vol. 4, p. 911. Longmans Green, New York, 1946. MUKHERJI, A. K., AND DEY, A. K., Complex citrates of metals in inorganic analysis. II. Separation and estimation of Ag and Pb in a mixture. 2. Anal. Chem. 145, 93 ( 1955). &iALIFA,