ANALYTICA CHIMICA ACTA

ELSEVIER Analytica Chimica Acta 306 (1995) 225-229

Determination of trace titanium in K&PO, by differential pulse adsorption voltammetry

Xin Zhao, Wenrui Jin *, Xueying Wang

Lahorotory ofAnalyfica1 Science, Shandong University, Jinan 250100, Shandong, Chinu

Received 22 September 1994; accepted 6 November 1994

Abstract

A sensitive differential pulse adsorption voltammetric method for the determination of trace titanium(W) in KH,PO, has

been studied based on the formation of the Ti(IV) complex with diphenylguanidine and 4-[(4-diethylamino-2-hydroxy- phenyl)-azo]-5-hydroxynaphthalene-2,7-disulphonic acid (Beryllon III). The reduction peak current of the complex was proportional to the concentration of Ti(IV) in the range 7.3 X lo- lo- 1.2 X 1W’ mol/l. The detection limit of 1.7 X 1W lo mol/l can be obtained under optimum experimental conditions. The method was applied to the determination of trace Ti(IV) in KH2P0, with satisfactory results.

Keyword.\: Differential pulse voltammetry; Titanium; Trace analysis

1. Introduction

In the process of culturing KH,PO, crystals, the existence of small amounts of metal ions may influ- ence the quality of the crystal. It is therefore neces-

sary to find a simple and highly sensitive method to

determine these ions, such as Ti(IV). Atomic absorp- tion spectrometry is an unfavourable method because of the difficult atomization of titanium. The induc-

tively coupled radio frequency plasma (ICP) method is more sensitive, its detection limit being 4.18 X

lo-’ mol/l, but ICP analysis is time-consuming and

very expensive [I]. Polarographic and voltammetric studies of Ti(IV)

have been reported in the literature. In trace analysis,

* Corresponding author.

the determination of titanium by anodic stripping voltammetry is not feasible because Ti(IV) cannot be

reduced to Ti(0) [2]. The only reduction wave ob-

served in the polarogram is that of Ti(IV) to Ti(III), but this wave is also not useful for trace measure-

ment. Ti(IV) can be determined by polarography in acidic solutions based on its catalytic effect in the

presence of ligands such as oxalate [3-51, EDTA- KBrO, [6-91, N-Benzoyl-N-phenylhydroxylamine

[IO], H,C,O,-KClO,-H,PO, [ll], diphenylguani- dine (DPG)-cupferron [ 121, cupferron-KBrO, [ 131

and dihydroxyazo dye 1141. Of all these methods, the adsorption voltammetric detection of Ti(IV) based

on its complex with mandelic acid [15] and cupfer- ron (Cup) was the most sensitive. The detection limit can be 6 X lo-“’ mol/l.

In this paper, the differential pulse voltammetric (DPAV) characteristics of

adsorption the Ti(IV)

0003.2670/95/$09.50 0 lY95 Elsevier Science B.V. All rights reserved

SSDi 000.1-2670~94~00578-8

226 X. Zhao et al./Analytica Chimica Acta 306 (1995) 225-229

complex with Cup, DPG, Cup-DPG and DPG-

Beryllon III was studied and a sensitive method to determine trace Ti(IV) in KH,PO, was established.

2. Experimental

2.1. Apparatus -02 -0.4 46 -0.8 -0.2 -0.4 -0.6 -0.8

E/V VSSCE E/V vs.SCE

A PAR 384 polarographic analyzer coupled with

a Model RE 0082 digital plotter was used in connec-

tion with a cell, using potentiostatic control of the

electrode potential by means of a three-electrode

system consisting of a PAR 303 static mercury elec-

trode (HMDE mode) as the working electrode, a

platinum wire as the auxiliary electrode and an

Ag/AgCl electrode as the reference electrode. In the

preconcentration step, the solution was stirred with a PTFE-coated stirring bar, rotated by a PAR 30.5

magnetic stirrer.

-a9 -10 -1.1 E/V vs.SCE

1

-0.4 46 -0.8 E/V vsSCE

2.2. Reagents and solutions

A 1.46 X lo-* mol/l stock standard solution of

Ti(IV) was prepared by dissolving the appropriate

amount of titanium (99.99%) in 50 ml 3 M H,SO,.

Then 0.5 ml 6 M HNO, was added and gentle heating was used to obtain Ti(IV). The solution was

then diluted to 250 ml with water. 1 X lo-’ mol/l

stock solutions of diphenylguanidine, 4-[(4-diethyl- amino-2-hydroxyphenyl)-azo]-5-hydroxynaphthalene-

2,7_disulphonic acid (Beryllon III) and cupferron (all

A.R. grade) were prepared by dissolving the appro- priate amounts in water. Other reagents were of

analytical grade and all solutions were prepared with triply distilled water.

Fig. 1. Differential pulse adsorption voltammograms of the Ti(IV)

complex with different Iigands. (a) DPAV of the Ti(IV) complex

with Cup. (I) 1X10m4 mol/l Cup; (2) (1)+2~10-’ mol/I

Ti(IV). 0.02 moI/l Na,SO, + 5 X 1O-4 moI/l triaminocitrate (pH

6.0). E, = - 0.10 V, t, = 100 s. (b) DPAV of the Ti(IV) complex

with DPG. (I) 1X 10-j moI/l DPG; (2) (1)+2X10-’ moI/I

Ti(IV). 0.05 moI/I KH,PO, +0.008 mol/l K,HPO, (pH 6.41,

E, = -0.20 V, t, = 60 s. (c) DPAV of the Ti(IV)-Cup-DPG

complex. (1) 3.9X lo-’ moI/l DPG+5.5X 10m5 moI/l Cup; (2)

(1)+4.6X lo-’ mol/l Ti(IV). 5.5X lO-4 moI/l KH,PO, +4.0

X lo-” moI/l NaOH (pH 5.8), E, = -0.88 V, t, = 100 s. (d)

DPAV of the Ti(IV)-DPG-Beryllon III complex. (1) 3.9 X lo-”

mol/l DPG+~.OX lo-’ moI/l Beryllon 111; (2) (1)+5.0X lo-’

moI/l Ti(IV). 0.01 mol/l sodium acetate+O.OOl mol/l acetic

acid (pH 5.71, Ed = -0.20 V, t, = 60 s.

After a rest period of 15 s, the response curve was recorded by scanning the potential in the negative direction from -0.55 to -0.80 V. Each measure-

ment was performed with a fresh drop. All potentials

were measured against the Ag/AgCl electrode.

3. Results and discussion

2.3. Procedure 3.1. DPAV of Ti(IV) complex with different ligands

The supporting electrolyte consisted of 1 ml 0.1 M sodium acetate and 0.1 ml 0.1 M acetic acid. 0.6 ml 4.8 X 10e8 mol/l DPG, 0.01 ml 1 X lop4 mol/l Beryllon III and 9.0 ml water were added. The solution was deaerated for 4 min with pure nitrogen. The measurements were carried out after a precon- centration step in which the solution was usually stirred for a certain time, t,, of 120 s at a preconcen- tration potential, E,, of -0.55 V (VS. Ag/AgCl).

In Na,SO,-triaminocitrate buffer (pH 6.0), the

differential pulse adsorption voltammogram of the Ti(IV) complex with Cup shows a peak at -0.55 V, which lies near the reduction peak of Cup (Fig. la) and is not suitable for detection.

The Ti(IV) complex with DPG shows a differen- tial pulse adsorption voltammetric peak at -0.48 V in KZHP04-KH,PO, buffer of pH 6.4 (Fig. lb). The peak is not stable and not suitable for detection.

X. Zhao et al. /Analytica Chimica Acta 306 (1995) 225-229 227

56

PH 5.8 60

Fig. 2. Dependence of the peak current of the reduction of

Ti(IV)-DPG-Beryllon III on the pH of the acetate buffer. 1.0X

10~’ mol/l Ti(IV), 4.8X 10-s mol/l DPG, 1.0X 10mh mol/l

Beryllon III, 0.01 mol/l sodium acetate. E, = -0.50 V, t, = 60 s.

In KH2P0,-NaOH buffer (pH 5.81, the Ti(lV)- Cup-DPG complex gives a peak at - 1.0 V in the

differential pulse adsorption voltammogram (Fig. 1~).

Using this peak, a detection limit of 3 X lo- ‘” mol/l can be obtained. However, the linear range is quite

narrow, only 1.7 X 10-s-6.9 X lo-’ mol/l or 2.7 X lo-‘-6.2 X lo-’ mol/l at different ligand con-

centrations.

In order to find a sensitive method to detect

Ti(lV), the Ti(lV)-DPG-Beryllon Ill complex was studied. In acetate buffer of pH 5.7, Beryllon Ill

forms a peak, P,, at - 0.50 V (Fig. Id, curve 1) and

the Ti(lV)-DPG-Beryllon Ill complex forms a peak, P2, at -0.64 V (Fig. Id, curve 2) in DPAV. P, is

quite stable and is therefore discussed in detail.

3.2. Optimum experimental conditions

The dependence of the peak current of the reduc- tion of Ti(lV)-DPG-Beryllon Ill on the pH of the

acetate buffer is shown in Fig. 2. The peak current

values detected at different concentrations of acetic acid are summarized in Table 1. It is quite clear that a higher peak current can be obtained when the pH is in the rang of 5.65-5.80. In the subsequent experi- ments, a 0.01 mol/l sodium acetate-O.001 mol/l acetic acid buffer of pH 5.70 is used.

Table 1

Peak current of reduction of the Ti(IV)-DPG-Beryllon III com-

plex detected at different concentrations of acetic acid (c)

PH i,, (nA)

4.70 6.00 11.60

6.70 5.85 17.20

8.70 5.75 IX.65

10.0 5.70 1X.80

12.7 5.58 15.65

14.7 5.52 13.10

16.7 5.45 7.80

Conditions as in Fig. 2

The dependence of the peak current of the reduc- tion of Ti(lV)-DPG-Beryllon Ill on the concentra-

tion of DPG is shown in Fig. 3. A higher peak

current can be obtained when the DPG concentration

is 2.8 X lo-” mol/l. This DPG concentration is

therefore selected for subsequent detections. The dependence of the peak current of the reduc-

tion of the Ti(lV)-DPG-Beryllon Ill complex on the concentration of Beryllon Ill is shown in Fig. 4. The

peak current increases with increasing Beryllon Ill

concentration. When the concentration of Beryllon Ill is too high, the reduction peak of Beryllon Ill will

influence the detection of the reduction peak of the

2 0 10 2.0 3.0 L.0

105r,,,/mol L’

Fig. 3. Dependence of the peak current of the reduction of

Ti(lVbDPG-Beryllon III complex on the concentration of DPG.

0.01 mol/l sodium acetate +O.OOl mot/l acetic acid buffer (pH 5.7), 1.0X 10mh mol/l Beryllon III, 2.9X IO-* mol/l Ti(IV),

E,=-0.50V, f,=6Os.

228 X. Zhao et al. /Analytica Chimica Acta 306 (1995) 225-229

ip/nA

Y 0 2 k 6 8

106c/mol L-'

Fig. 4. Dependence of the reduction peak current of the Ti(IV)-

DPG-Beryllon III complex on the concentration of BeryIIon III.

2.9 X 10e5 mol/l DPG, other conditions as in Fig. 3.

complex. So in the experiments, 1 X 10m6 mol/l

Beryllon III is used for the detection of 10-8-10~7

mol/l Ti(IV) and 1 X lo-’ mol/l Beryllon III is used for the detection of 10-‘“-10-9 mol/l Ti(IV).

Fig. 5 shows the relationship between the reduc-

tion peak current of the complex and the preconcen-

tration potential, E,. The peak current is higher at

more negative potentials. When E, is more negative

than -0.55 V, however, the whole reduction peak cannot be obtained. Therefore, -0.50 V is used as

preconcentration potential in the determination.

14 ip/nA

L fu 0.50 0.52 O.SL

E/V vs.SCE

Fig. 5. Relationship between the reduction peak current of

Ti(IV)-DPG-BeryIIon III and the preconcentration potential. 2.9

x lo-’ mol/I DPG, other conditions as in Fig. 3.

Table 2

The reduction peak current of the Ti(IV)-DPG-Beryllon III

complex detected at different preconcentration times

I, (s) i, (PA)

20 0.90

40 2.78 60 4.72

80 6.95

100 9.00

E, = - 0.55 V, other conditions as in Fig. 4.

The reduction peak current of the Ti(IV)-DPG-

Beryllon III complex detected at different preconcen-

tration times, t,, is summarized in Table 2. The longer the 1, the higher the peak current. t, = 120 s

is used for subsequent determinations.

3.3. Analytical application

Under optimum conditions, when the concentra-

tion of Beryllon III is 1.0 X 10d7 mol/l, a linear relationship between the reduction peak current of

the complex and the concentration of Ti(IV) can be obtained in the range of 7.28 X lo-‘O-9.85 X 10m9

mol/l, and when the concentration of Beryllon III is

1.0 X 10e6 mol/l, the linear range is 1.0 X lo-‘-

1.2 X lo-’ mol/l. When the concentration of Beryl- lon III is 1.0 X lo-’ mol/l, E, = -0.55 V and t, = 120 s, the limit of detection is 1.7 X lo-*’

mol/l. Experimental results showed that a 2000-fold ex-

cess of K+, Na+, Cl-, SOi- and PO:-, a lOOO-fold

excess of Sn(IV), Mn(II), Cd(I1) and V(V), a lOO-fold excess of Ca(I1) and Mg(II), a lo-fold excess of Zn(I1) and Cu(I1) and a 5-fold excess of Fe(II1) do

not interfere during the determination of Ti(IV). Samples of KH,PO, were first dissolved in water

to form a solution of 1 mol/l. 0.01 ml of sample solution was added to the electrolytic cell containing

0.01 mol/l sodium acetate-O.001 mol/l acetic acid, 4.8 x 10m5 mol/l DPG and 1.0 X lo-’ mol/l Beryllon III. The standard addition method was ap-

Table 3

Results of the determination of Ti(IV)

Sample DPAV(pgg-‘1 UV(cLgg_‘1

~2PO4 0.913 0.895

NaCl 0.876 0.866

X. Zhao et al. /Analytica Chimica Acta 306 (1995) 225-229 229

plied to determine the concentration of TKIV) in

KH,PO,. The Ti(IV) concentration found in

KH,PO, was 0.914 pg/g. The recovery was 99%. This method can also be used in the determination of

Ti(IV) in NaCl. The concentration of Ti(IV) found in NaCl was 0.876 pg/g. The recovery was 97%. The

results obtained by the DPAV method were in agree-

ment with those obtained by UV spectrometry (Table

3).

Acknowledgements

This project was supported by the State Key

Laboratory of Crystal Materials, Shandong Univer-

sity.

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