Indian Journal of Chemical Technology Vol. 7, September 2000, pp. 223-226

Sorption behaviour of certain metal ions in normal-phase and reversed-phase TLC using layers of silica-zirconium tungstophosphate gels

Ali Mohammad* & Eram Iraqi

Department of Applied Chemistry, Z H College of Engineering and Technology, Aligarh Muslim University, Aligarh 202 002, India

Received 28 December 1999; accepted 20 June 2000

Normal-phase and reversed-phase thin layer chromatographic techniques using mixed layers consisting of silica and zirconium tungstophosphate gels have been utilized to examine the sorption behaviour of some metal ions. Tri-Il-butyl phosphate at concentration levels 0. 1 -2 M has been used to impregnate the layer material. Acidic solvent system �ontaining formic acid were used as mobile phase. Several metal ions from their two-component mixtures have been clearly separated.

In 1 958, Winchesterl first combined the favourable features of high- molecular weight extractants with a chromatographic technique and separated a number of rare earth elements using di (2-ethyl hexyl) phosphoric acid loaded on alumina as stationary phase and dilute HCl as the mobile phase. S ince then, the technique of reversed-phase chromatography (or reversed-phase extraction chromatography) has become very popular. Several papers have been published

2-6 on reversed-phase thin layer chromatography (RPTLC) of metal ions using long­chain aliphatic amines, substituted quarternary ammonium salts, heterocyclic amines and neutral organophosphorous compounds as impregnants of the stationary phase . Solutions of strong monobasic acids or their alkali metal salts are general ly selected as the mobile phase.

Tri-n-butyl phosphate (TBP), one of the most versatile extractants has received considerable attention in analytical separation chemistry because of its capabil ity to extract most of the elements of the periodic table under suitable conditions of extraction. Since its first use of the separation of Th and V from impurities?, TBP has been used extensively as extractant to accompl ish analytical scale separations of actinides, lanthanides and Pt- group elements . However, its use for the separation of transition metals has been l imited.

From literature,TBP impregnated layers of sil ica gel , polyvinyl chloride and cellulose have been used to separate various metal ions8-IO with little work on the use of layers prepared from mixtures of inorganic ion-exchanger gel and silica gel in the presence of

*For correspondence

TBP and crown ethers as impregnants or eluents In the analysis of transition metal ions II.

The present study is an attempt to examine the use of silica- inorganic ion exchanger gel in RPTLC of certain metal ions. For this purpose, zirconium (IV) tungstophosphate ion exchanger in gel form was mixed with silica gel G. The resultant mixture was used as layer material after i mpregnation with TBP at different concentration levels to separate metal ions using HCI or formic acid containing solvent systems as eluents. Efforts were also made to explore the possible use of crown ethers as impregnants or as eluents in inorganic TLC

Experimental Procedure Materials-Zirconium oxychloride, sodium

tungstate, orthophosphoric acid (85% purity), tri-n­butyl phosphate (98% purity) were procured from CDH, India. Benzo-15-crown-5 and dibenzo -18-crown-6 were of Aldrich, USA . Al l other chemicals of Analytical Reagent grade were also from CDH, India.

Test solutions and detection reagents-l % solutions of nitrates, chlorides or sulphates of cations were prepared in demineralized water. Ammonium sulphide for Pb2+, Ag+, Tl+ and B i3+; potassium ferrocyanide for Fe

3+, Cu2+ and U02

2+; dithizone in carbon tetrachloride for Cd2+ and Zn

2+ and alcohol ic dimethylglyoxime for Ni

2+ and C02+ were used as

detection reagents. Synthesis of ion-exchanger gel-Zicronium(IV)

tungstophosphate was prepared by mixing 0.1 M solutions of zirconium oxychloride, sodium tungstate and orthophosphoric acid in different volume ratios. The optimum volume ratio for better gel formation

224 INDIAN 1. CHEM. TECHNOL., SEPTEMBER 2000

was found to be I: I: I. The pH of the resultant gel was maintained around I by adding concentrated HN03 with constant stirring. The gel thus obtained was kept for 24 h at room temperature before use.

Chromatography Preparation of thin layer plates-The ion­

exchanger gel was mixed with silica gel G in different ratios SUGh as 3: I, 1: 1 and 1:3 to obtain a slurry. This slurry was coated on thin glass plates (ISx3 em) with the help of an applicator (Toshinwal) to give a layer of 0.2S mm thickness. The plates were air dried first and then kept at lOO± SoC for 1 h in an electrically controlled oven. The plates were cooled to room temperature in a closed chamber before use. The various stationary and mobile phases used are listed below:

Stationary Phase

S, = Plain silica gel G. S2 = S, + Ion-exchanger gel (1: I) wlw

S3 = S,+Ion-exchanger gel (I:3) and (3:I) wlw

S4 = S2 Impregnated with 0.1-2 M TBP in acetone.

S5 = S2 Impregnated with 1M TBP in acetone.

S6 = S3 Impregnated with 1M TBP in acetone.

Mobile Phase

M, = 0.1, O.S, I and 2M tri-n-butylphosphate

M2 = 0.1 % Benzo-IS-crown-S in methanol.

MJ = 0.1 % Dibenzo 18 crown-6 in a mixture of 1,4-dioxane and formic acid in 1: 1 ratio.

M4 == Water-HCI-acetone (10:0:90, 10:90:0, 0: 10:90, 0:90: 10, 90: 1 0:0, 90:0: 10, SO:SO:O, SO:O:SO, O:SO:SO, 40:30:30, 30:40:30, v/v/v).

Ms = I, 10 and 26.SM formic acid.

M6 = Water-formic acid-acetone (0: 10:90, 8:2:90,10:0:90, 2:8:90, v/v/v).

M7 = 1M Sodium formate- l M KI (I :4, 4: 1, 3:2, I: I, v/v).

Mg = 1M Sodium formate-1M formic acid (1:1).

Procedure-The TLC plates coated with mixed layer material were first run in methanol to remove excess acid and then the plates were run in the desired concentration of TBP in acetone for impregnation. This process of running the plate in the impregnant is called as the indirect impregnation method. The acetone from the TBP impregnated plates was completely removed by drying plates at SO°C for I h. The test solutions of metal ions (3-S �lL) were spotted on to the TLC plates (S,-S6) with the help of a micropipette. The spots were dried at room temperature before development . The plates were developed in the chosen mobile phase by the ascending technique with a solvent run upto 10 em in all cases. After development, the plates were air dried and the cations were detected by spraying the plates with appropriate chromogenic reagents. The values of RL (RF of the leading front) and RT (RF of the trailing front) were determined for each spot from which the RF values of the cations were calculated.

Table I-Separation of some metal ions achieved under different experimental conditions by normal-phase TLC

Stationary Phase

lon-exchange gel +

Silica gel G (I: I)

Mobile Phase

Water + HCI + Acetone

(0 + 10 + 90)

Water + HCI + Acetone

(8 + 2 + 90)

Water + HCI + Acetone

( 1 0 + 0 + 90)

Water + Formic Acid +

Acetone (8 + 2 + 90)

Water + Formic Acid +

Acetone (2+ 8+90)

Separations (RF) Cu2+(0.90)_Pb2+(0), U02

2+(0. 90)-Pb2+(0),

Cd2+(0.80)-Pb2+(0), Co2+(0.70)-Pb2+(0)

Co2+(0.80)-n+(O). Cd2+(O.80)-TI+(O),

Cu2+(0. 70)-TI+(O), U02 2+(0.70)-TnO)

Cd2+(0.7S)-Cu2+(O), Cd2+(0.7S)-U022+(0. IS)

Cd2+(0.7S)-Pb2+(0), Cd2+(0. 7S)-TI+(O),

Ni2+(0.70)-Cu2+(0), Ni2+(0.70)-UO/+(0. IS)

Ni2+(0.70)-Pb2+(0), Ni2+(0.70)-TI+(O)

Co2+(0. 1 0)-Cu2+(0), Co2+( 1 .0)-Ag+(0.2S),

Co2+( 1 .0)-Pb2+(0), Co2+( 1 .0)-TI+(O),

Co2+( 1 .0)-Bi3+(0), Cd2+( 1 .0)-Ag+(0.25)

Cd2+( 1 .0)-Pb2+(0), Cd2+( 1 .0)-TI+(O), Cd2+( 1 .Q)-Bi3+(0)

Cd2+( 1 .0)-Co2+(0. 1 0), Cd2+( 1 .0)-Cu2+(0),

Cd2+( 1 .0)-U022+(0), Cd2+( I .0)-Ag2+(0.20)

Cd2+( 1 .0)-Pb2+(0), Cd2+( 1 .0)-TnO), Cd2+( I .O)-Bi3+(0)

MOHAMMAD & IRAQI: RPTLC OF METAL IONS 225

Table 2-Separalion of some metal ions achieved under different experimental conditions by reversed-phase TLC

Stationary Phase

Ion-exchange gel +silica gel G (I: I ) impregnation by 1 M TBP in acetone

Ion-exchanger gel + Silica gel G (3: I) impregnation by 1 M TBP in acetone

I on-exchange gel +Silica gel G (I :3) impregnation byl M TBP in acetone

Results and Discussion

Mobile Phase

Distilled water

I M Formic Acid 1 0M Formic Acid Pure Formic Acid Water + HCI + Acetone (8 + 2 + 90) Wuter + HCI + Acetone (0 + 10 + 90) Water + HCI + Acetone (2 + 8 + 90) WateHFormic Acid+Acetone (2 + 8 + 90) Water+Formic Acid+Acctone ( 1 0 + 0 + 90) Water+Formic Acid+Acetone (8 + 2 + 90) Water+Formic Acid+Acetone (0 + 1 0 + 90) Water + HCI + Acetone (90 + 1 0 + 0) I M Sodium Formate- I M Formic Acid (1: 1 ) I M Sodium Formate-l M KI (20:80)

I M Sodium Formate-I M KI (80 : 20) I M Sodium Formate-I M KI (60: 40) I M Sodium Formate-I M KI (50:50) I M Sod. Formate-I M K I (80: 20)

Zirconium tungstophosphate gel mixed with silica gel G was used as the layer material. The organophosphorous compound, tri-n- butyl phosphate (TBP) was used in the concentration range 0.1- 2 M in acetone both as an eluent (i.e. mobile phase) and as an impregnant. TBP when used as impregnant in reversed-phase TLC (RPTLC) , yields better separations of metal ions compared to its use as mobile phase in normal-phase TLC. The optimum impregnation concentration level was I M of TBP. The effect of formic acid concentration (I-10M) on the mobility of metal ions on S" S2, S4 and S, stationary phases was examined. Better results in terms of clear detection and compactness of spots of metal ions were obtained with 1.0 M formic acid

Separations (RF)

Ni2+(0.76)-Pb2+/ Bi3+/ TI+ / UO/+/ Fe'+ (-0. 1 0)

Cu2+ / U022+ (0.80)-Pb2+ / TI+ (0. 1 0) Co2+CO.80) / Cd2+(0.75)-Pb2+(0) / Ag+(0. 1 5), TI+CO.65)-Pb2+(0) Cu2+/UO/+/Cd2+/Co2+ (-0.90)-Ph2+fn+/ Ag+(·-O. I 0)

eluent and Ss as stationary phase. Comparative studies of the mobility of metal ions in one of the selected mobile phases (mobile phase Mg) revealed that the RF values of almost all metal ions except Pb

2+ and Ag+

on S, stationary phase were 1.0 while on S2 stationary phase the RF values of several metal ions were in the range 0.50-0.80. However, on stationary phases S4 and S5, the variation in RF values of metal ions results in some important binary separations which are listed in Table 2.

Crown ethers were also tried as impregnants and as mobile phases with S,-S6 stationary phases in order to explore their utility in TLC separation of metal ions. 0.1 % Benzo-15-crown-6 in methanol and 0.1 % dibenzo 18 crown-6 in a mixture of 1,4- dioxane and formic acid in I: 1 ratio were used. The crown ether

226 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2000

impregnated layers prepared from the mixture of ion­exchanger gel and silica gel in I: I ratio by weight were developed with 1 .0 M sodium formate + 1 .0 M KI (3:2) and the unimpregnated layers were developed with M2 and M.,. Generally, Ni2+, Co2+, Cd2+ and Zn2+ produce tailed spots with both crown ethers, irrespective of their use as impregnant or as mobile phase. However, less tailed spots were observed when benzo-I5-crown-5 was used as impregnant compared to its use as mobile phase, whereas less tailed spots were noticed when dibenzo ]8 crown-6 was used as eluent compared to its usc as impregnant . Besides these aqueous-organic mixed solvent systems (water-HCI-acetone, water-formic acid-acetone) and sodium formate-KI (M7) were also used as mobile phases mainly with S" S2 and Ss stationary phases. The separations achieved in these systems are given in Tables I and 2.

Acknowledgement The authors gratefully acknowledge the financial

assistance provided by the Council of Science and Technology, Lucknow, India.

References 1 Winchester J W, Rep US Atomic Energy Commission, CF

58-12-43, 1 958.

2 Brinkman U A Th, Vries G De & Kuroda R, J Chromatogr, 85 ( 1 973) 1 87.

3 Sherma J & Fried B, (Eds), Handbook of Thin-Layer Chromatography (Marcel Dekker, New York, USA), 1 996.

tt Korkisch J, Navratil J D & Schulz W W, Science alld Techllol Triblltylphosphate 1987,2 (PE-B), 1 987.

5 Panesar K S, Singh 0 V & Tandon S N, Anal Lt'lt, 23 ( 1 990) 1 25.

6 Kuroda R & Volynets M P, in CRC Handbook of Chromatography: III organics, edited by Qu .. eshi M, Vol I (CRC Press, Boca Raton, Fla), 1 987, 87.

7 Chemistry Division Quarterl y Report for Period Ending June 30, 1 949, US Atomic Energy COII/lllission, Report ORNL-286, Oak Ridge National Lahoratory, Oak Ridge, Tennessee, 1 949.

8 Brinkman U A Th & Vcltkanag H, J Chroll/etogi", 24 ( 1 966) 489.

9 Muchova A & loki V, ChellJ Zvesti, 30 ( 1 976) 629.

1 0 Pierce TB & Flint RF, J ChrclIlatogr, 24 (1966) 14 1 .

II Mohammad A, Najar M P A & Iraqi E, Indian J Chelll Technol, 6 ( 1 999) 38.