Stripping Voltammetric Determination of Timolol Drug in Pharmaceuticals and Biological Fluids

doi:10.4236/ajac.2011.22020

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American Journal of Anal yt ical Chemistry, 2011, 2, 174-181

doi:10.4236/ajac.2011.22020 Published Online May 2011 (http://www.SciRP.org/journal/ajac)

Stripping Voltammetric Determination of Timolol Drug in

Pharmaceuticals and Biological Fluids

Ali F. Al-Ghamdi

Department of Chemistry, College of Science, Taibah University, Madinah, Saudi Arabia

E-mail: [email protected]

Received March 13, 2011; revised April 29, 2011; accepted May 3, 2011

Abstract

A sensitive and reliable stripping voltammetric method was developed to determine timolol drug. This me-

thod is based on the adsorptive accumulation of the drug at a hanging mercury drop electrode (HMDE) and

then a negative sweep was initiated, which yield a well defined cathodic peak at –850 mV versus (Ag/AgCl)

silver reference electrode. To achieve high sensitivity, various experimental and instrumental variables were

investigated such as supporting electrolyte, pH, accumulation time and potential, scan rate, frequency, pulse

amplitude, convection rate and working electrode area. The monitored adsorptive current was directly pro-

portional to the concentration of timolol and it shows a linear response in the range from 1 × 10–7 to 1.5 ×

10–6 mol·l–1 of this drug (correlation coefficient = 0.998) and the detection limit (S/N = 3) is 1.26 × 10–9

mol·l–1 at an accumulation time of 30 sec. The developed adsorptive stripping voltammetry (AdSV) proce-

dure shows a good reproducibility, the relative standard deviation RSD% (n = 8) at a concentration level of 1

× 10–6 mol·l–1 of timolol was 0.13%, whereas the method accuracy was indicated via the mean recovery of

110% ± 1.414. Possible interferences by several substances usually present in the pharmaceutical formula-

tions have been also evaluated. The applicability of this approach was illustrated by the determination of the

drug in pharmaceutical preparation and biological fluids such as serum and urine.

Keywords: Stripping Voltammetry, HMDE, Timolol, Urine, Serum

1. Introduction

Of the most commonly used instrumental techniques,

electroanalytical approach is the one of choice, and

stripping voltammetric method has provoked particular

interest because it is currently the most sensitive and

widely used electrochemical technique. Its possibility of

applications cover many fields ranging from environment,

pharmaceutical and clinical to food and industrial

samples. Many of the adsorptive stripping voltammetric

(adsv) approach features such as sensitivity, selectivity,

simplicity and versatility attributed to the combination of

an effective preconcentartion step based on non-electro-

lytic adsorptive accumulation process with an advanced

measurement procedures such as differential pulse (DP)

or square wave (SW) [1-5]. Unlike conventional strip-

ping approaches (anodic and cathodic stripping voltam-

metry), which are based on an electrolytic nature of

preconcentration step, adsorptive stripping voltammetric

approach in contrast is based on adsorptive accumulation

of the analyte on the electrode at open circuit with no

charge transferred. Consequently, for a wide range of

surface-active organic and inorganic species, which can-

not be preconcentrated electroanalytically, the adsorption

approach allows these analytes to be interfacially accu-

mulated on the electrode and hence analysed. There have

been many reviews devoted to emphasize and illustrate

the wide spectrum and scope of adsorptive stripping vol--

tammetric applications and potentialities in the analysis of

metal ions [6,7] organic analytes [8,9] and pharma--

ceutical drugs and biomedical compounds, such as, the

anti-Inflammatory drug Lornoxicam, the antidepressant

drug sulpiride and Josamycin, a Macrolide Antibiotic

[10-14]. Timolol was the first beta (β) blocker to be used

as an anti-glaucoma agent and to date remains as the

standard because none of the newer β blockers were

found to be more effective. Timolol maleate is the generic

name for (Ocumol 0.5, Timolo 0.5%, 5 mg/ml- Riyadh,

Pharma-SA), an eye drop (ophthalmic solution) indicated

for the treatment of elevated intraocular pressure for

those patients with primary open-angle glaucoma and

ocular hypertension. Timolol maleate should be used

A. F. Al.-GHAMDI175

with care. It is important not to contaminate the solution,

because this can cause infection and/or inflammation in

the eye. If you are using more than one eye-drop me-

dication, administration of the drugs should be spaced at

least ten minutes apart to provide adequate time for

absorption. Timolol is soluble in distilled water, its mole-

cular weight is 316.421 g/mol and systematic IUPAC

name is (S)-1-(tert-butylamino)-3-[(4-morpholin-4-yl-1,

2,5-thiadiazol-3-yl)oxy]propan-2-ol[15,16]. The chemical

structure of this drug and the mechanism for the elec-

trochemical reduction process of its compound are

shown in Scheme 1 (The mechanism proposed of elec-

trochemical reduction for timolol drug). Timolol drug

has been analysed in pharmaceutical formulations and

biological samples by various analytical methods such as

spectrophotometric [17-19], chromatographic [20,21],

high performance liquid chromatography (HPLC) [22]

and differential pulse voltammetric [23]. However, to the

best of our knowledge, the square wave voltammetric

behavior of timolol and thus its square-wave adsorptive

stripping voltammetry (SW-adsv) have not been per-

formed and reported so far. Consequently, the aim of this

work was to develop more sensitive, reliable and simple

SW-adsv procedure for the determination of timolol drug

in biological media and pharmaceutical formulations.

2. Experimental

2.1. Apparatus

All adsorptive stripping measurements were carried out

with 797 VA computrace (Metrohm, Switzerland) in

connection with Dell computer and controlled by VA

computrace 2.0 control software. Stripping voltammo-

grams were obtained via a hp color laserjet CP1215

printer. A conventional three electrode system was used

in the hanging mercury drop electrode (HMDE) mode.

pH values were measured with Hanna instruments pH

211 (Romania made) pH meter. The labofuge 200 instru-

ment , Heraeus sepatech (Germany) was used for centri-

fuging of biological fluids to suite for stripping analysis.

Scheme 1. The mechanism proposed of electrochemical re-

duction for timolol drug.

2.2. Reagents

All chemicals used were of analytical reagent grade and

were used without further purification. Timolol stock

solution of 1 × 10–2 mol·l–1 were prepared by dissolving

the appropriate amount of this drug in distilled water in

10 ml volumetric flask. This stock solution was stored in

the dark and under refrigeration in order to minimize

decomposition. Standard solutions of this drug with lower

concentrations were prepared daily by diluting the stock

solution with distilled water. Britton-Robinson support-

ing buffer (pH ≈ 2, 0.04 M in each constituent) was pre-

pared by dissolving 2.47 g of boric acid in 500 ml dis-

tilled water containing 2.3 ml of glacial acetic acid and

then adding 2.7 ml of ortho-Phosphoric acid and diluting

to 1 L with distilled water. In addition, phosphate sup-

porting buffer (0.1 M NaH2PO4 and 0.1 M H3PO4) was

prepared by dissolving 12 g of NaH2PO4 and 6.78 g of

H3PO4 in 1000 ml distilled water. Acetate supporting

buffer (0.02 M in each constituent) was prepared by dis-

solving 1.68 g of sodium acetate in 500 ml distilled water

containing 1.12 ml of acetic acid and diluting to 1 L with

distilled water. Finally, carbonate supporting buffer (0.1

M in each constituent) was prepared by dissolving 10.6 g

of sodium carbonate and 8.4 g of sodium hydrogen car-

bonate in 1000 ml distilled water.

2.3. Procedure

The general procedure adopted for obtaining adsorptive

stripping voltammograms was as follows: A 10 ml ali-

quot of B-R supporting buffer (unless otherwise stated)

at the desired pH (e.g. 3.5) was pipetted into a clean and

dry voltammetric cell and the required standard solution

of timolol was added. The test solution was purged with

nitrogen for 5 minutes initially, while the solution was

stirred. The accumulation potential of –0.8 V vs. Ag/AgCl

was applied to a new mercury drop while the solution

was stirred for 30 seconds. Following the preconcentra-

tion period, the stripping was stopped and after 20 sec-

onds had elapsed, cathodic scans were carried out over

the range 0.0 to –1.2 V. All measurements were made at

room temperature.

3. Preliminary Observations

When the differential pulse polarographic behavior was

investigated for timolol drug in acetate buffer at pH 3.5,

a broad polarographic wave at Ep = –0.850 V was ob-

served and this obtained polarographic wave, as shown

in Figure 1 (Differential Pulse Polarographic of 5 × 10–5

mol·l–1 Timolol in pH 3.5 Acetate buffer, scan rate 25

mV·s–1), is probably due to the electrochemical reduction

A. F. Al.-GHAMDI

176

Figure 1. Differential Pulse Polarographic of 5 × 10–5 mol·l–1

Timolol in pH 3.5 Acetate buffer, scan rate 25 mV·s–1.

of the double bond (–N=C) as shown in the previous

scheme1, which including a proposed mechanism for the

electrochemical reduction of this drug. This mechanism

suggesting that the electrochemical reaction is an irre-

versible process, an assumption which was confirmed by

cyclic voltammetric measurement at a scan rate of 50

mV–1 of timolol in acetate buffer (pH 3.5) as shown in

Figure 2 (Cyclic voltammogram of 5 × 10–5 mol·l–1 Ti-

molol in pH 3.5 Acetate buffer, scan rate 50 mV·s–1).

In order to obtain a voltammetric peak with better de-

finition and higher sensitivity, a HMDE was used to

study the adsorptive prosperities of timolol compound.

The AdSV behavior of the drug was investigated in var-

ious supporting electrolytes at different pH values. This

Drug compound yielded a well-developed and defined

AdSV peak corresponding to the electroactive –N=C at

peak potential of –0.85 V. A typical adsorptive stripping

voltammogram for 5 × 10–7 mol·l–1 timolol in acetate buf-

fer is shown in Figure 3 (SW-AdSV voltammogram

Figure 2. Cyclic voltammogram of 5 × 10–5 mol·l–1 Timolol

in pH 3.5 Acetate buffer, scan rate 50 mV·s–1.

Figure 3. SW-AdSV voltammogram of 5 × 10–7 mol·l–1 Ti-

molol in pH 3.5 Acetate buffer. Accumulation time 30 sec,

accumulation potential –0.8 V and scan rate 250 mV·s–1.

of 5 × 10–7 mol·l–1 Timolol in pH 3.5 Acetate buffer. Ac-

cumulation time 30 sec, accumulation potential –0.8 V

and scan rate 250 mV·s–1), which illustrates a well ob-

served electrochemical peak indicating a strong and rea-

dily adsorption process at the surface of the working

electrode.

3.1. Parameters Affecting the Adsorptive

Stripping Response

3.1.1. Effect of Supporting Electrolyte and pH

The nature and acidity of the supporting buffer are some

of the most important factors which strongly influence

the stability of the analyte and its cathodic reduction and

adsorption processes. Among the various investigated

buffers (B-R, acetate, carbonate and phosphate) the best

voltammetric signal in terms of sensitivity (peak height)

and resolution (peak shape) have been secured using

acetate buffer. In addition, when the AdSV peak current

was measured as a function of pH over 2-6 range, the

stripping voltammetric signal increased steadily over the

acidic region and the peak current reached its maximum

value at pH 3.5 which was selected as optimal value for

subsequent studies. It is noteworthy that when more than

3.5 acetate supporting electrolyte was used, timolol was

barely detectable and nearly no stripping voltammetric

signal was observed. The variation of AdSV peak current

with pH, obtained for 1 × 10–6 mol·l–1 Timolol drug con-

centration accumulated for 30 sec is exhibited in Figure

4 (Effect of pH on AdSV peak current of 1 × 10–6 M Ti-

molol at Acetate buffer).

3.1.2. Effect of Accumulation Time and Potential

Preconcentration of the analyzed drug on the surface of

the working electrode (HMDE) is one of the essential

A. F. Al.-GHAMDI177

Figure 4. Effect of pH on AdSV peak current of 1 × 10–6 M

Timolol at Acetate buffer.

conditions for highly sensitive determinations. Variation

of the accumulation time over 0 - 150 sec period for 1 ×

10–6 mol·l–1 timolol drug solution at a preconcentration

potential of 0.0 V, showed a gradual enhancement for the

monitored peak current over the range 0 - 30 sec. The de-

pendence of peak current on accumulation time is pre-

sented in Figure 5 (Effect of accumulation time on AdSV

peak current of 1 × 10–6 M Timolol in pH 3.5 Acetate buf-

fer). The proportional relationship was nearly ob- served

up to 30 sec and then it became virtually curved and lev-

eled off owing to the saturation of the hanging mercury

drop by the analyte. For further experiments an accumula-

tion time of 30 sec was selected as optimal because it pro-

vided relatively high peak current with adequate practical

time. The variation of accumulation time did not produce

significant shifts in peak potential value.

In addition, as can be seen from Figure 6 (Effect of

accumulation potential on AdSV peak current of 1 × 10–6

M Timolol in pH 3.5 Acetate buffer and accumulation

time 30 sec), when the influence of accumulation poten-

tial on the monitored electrochemical response was ex-

amined over the –0.8 to +0.8 V range at 30 sec precon-

centration time, the peak current was directly reached its

maximum value at Ep = –0.8 V then it decreased sharply

after potential –0.8V. Thus, Eacc= 0.8 V will be adopted

as optimum operational value for the following works as

it ensured the highest AdSV signal.

Figure 5. Effect of accumulation time on AdSV peak cur-

rent of 1 × 10–6 M Timolol in pH 3.5 Acetate buffer.

Figure 6. Effect of accumulation potential on AdSV peak

current of 1 × 10–6 M Timolol in pH 3.5 Acetate buffer and

accumulation time 30 sec.

3.1.3. Effect of Scan Rat

The cathodic peak current of timolol drug was found to

be forthwith proportional to the scan rate, particularly at

low scan rate values, a phenomenon characterized for

adsorbed materials [24]. When the stripping voltammet-

ric peak current of 1 × 10–6 mol·l–1 timolol drug in pH 3.5

acetate buffer was measured over the range 10 - 300

mV/s, it was found that peak height was observed in the

scan rate 250 mV/s as it was shown in Figure 7 (Effect

of scan rate on AdSV peak current of 1 × 10–6 M Timolol

in pH 3.5 Acetate buffer, accumulation time 30 sec and

accumulation potential –0.8 V). However, after this

maximum value the peak current started to decrease di-

rectly with faster scan rate. Accordingly, 250 mV/s scan

rate value was adopted as optimum condition for further

investigations.

3.1.4. Effec t of Pulse Amplitude and Freq uency

In addition, the impact of varying the excitation wave

pulse amplitude on the voltammetric current intensity

was also evaluated. The effect of this operating variable

was studied over the rang 10 - 120 mV (see Figure 8:

Effect of pulse amplitude on the peak current of 1 × 10–6

M Timolol in pH 3.5 Acetate buffer, accumulation time

30 sec, accumulation potential –0.8 V and 250 mV/s scan

Figure 7. Effect of scan rate on AdSV peak current of 1 ×

10–6 M Timolol in pH 3.5 Acetate buffer, accumulation time

30 sec and accumulation potential -0.8 V.

A. F. Al.-GHAMDI

178

Figure 8. Effect of pulse amplitude on the peak current of 1

× 10–6 M Timolol in pH 3.5 Acetate buffer, accumulation

time 30 sec, accumulation potential –0.8 V and 250 mV/s

scan rate.

rate) and it was concluded that in order to assure maxi-

mum peak current, 100 mV pulse amplitude was the ideal

choice for this operational parameter. Moreover, varying

the value of square wave frequency also plays an impor-

tant role for the measured signal of square wave- adsorp-

tive stripping voltammetric (SW-AdSV) approach. When

the voltammetric peak current of 1 × 10–6 mol·l–1 timolol

drug in acetate buffer pH 3.5 was measured over the

range 10 - 50 Hz, as in Figure 9 (Effect of frequency on

the peak current of 1 × 10–6 M Timolol in pH 3.5 Acetate

buffer, accumulation time 30 sec, accumulation potential

–0.8 V, Scan rate 250 mV/s and pulse amplitude 0.10 V),

it was found that peak height was observed at a low fre-

quency 10 - 15Hz only, after this value (15Hz) the peak

current started to decrease with increasing frequency. Ac-

cordingly, for farther work 15 Hz square wave frequency

value was adopted.

3.1.5. Effect of Instrumental Parameters

The monitored adsorptive stripping voltammetry (AdSV)

peak height could be further maximized by optimizing

other experimental factors that can affect the adsorption

Figure 9. Effect of frequency on the peak current of 1 × 10–6

M Timolol in pH 3.5 Acetate buffer, accumulation time 30

sec, accumulation potential –0.8 V, Scan rate 250 mV/s and

pulse amplitude 0.10 V.

process of the formed drug. The influence of both the

surface size of the mercury drop working electrode and

electrode convection rate was also evaluated. An increase

in the surface of the working electrode (over 0.15 - 0.60

mm2) yielded, as expected, a linear enhancement in the

analytical signal and did not affect the value of the strip-

ping voltammetric potential. In addition, an increase in

the stirring rate (raising it from 0.0 to 3000 rpm) yielded,

a linear enhancement in the analytical signal from 0.0 to

2000 rpm, after that it is decreased and did not affect the

value of the stripping voltammetric potential. Thus, for

optimal sensitivity, 0.60 mm2 drop size and 2000 rpm

stirring speed were chosen for subsequent practical

works (see Figures 10: Effect of electrode area on the

peak current of 1 × 10–6 M Timolol in pH 3.5 Acetate

buffer, accumulation time 30 sec, accumulation potential

–0.8 V, Scan rate 250 mV/s, pulse amplitude 0.10 V and

frequency 15 Hz, Figure 11: Effect of convection rate on

the peak current of 1 × 10–6 M Timolol in pH 3.5 Acetate

buffer, accumulation time 30 sec, accumulation potential

–0.8 V, Scan rate 250 mV/s, pulse amplitude 0.10 V,

frequency 15 Hz and drop size 0.6 mm2).

3.2. Analytical Performance (Method Validation)

Once the most ideal and suitable chemical conditions and

instrumental parameters for the adsorptive determination

were established, a calibration plot for the analyzed drug

was recorded to estimate the analytical characteristics of

the developed method.

3.2.1. Calib ra t i on Graph

Under the optimum conditions a very good linear corre-

lation was obtained between the monitored voltammetric

peak current and timolol concentration in the range 1 ×

10–7 - 1.5 × 10–6 mol·l–1, is constant in all measurements,

(see Figure 12: SW-AdSV voltammogram for Timolol

in acetate buffer, pH = 3.5, Tacc = 30 sec, Eacc = –0.80 V.

Figure 10. Effect of electrode area on the peak current of 1

× 10–6 M Timolol in pH 3.5 Acetate buffer, accumulation

time 30 sec, accumulation potential –0.8 V, Scan rate 250

mV/s, pulse amplitude 0.10V and freque nc y 15 Hz.

A. F. Al.-GHAMDI179

Figure 11. Effect of convection rate on the peak current of 1

× 10–6 M Timolol in pH 3.5 Acetate buffer, accumulation

time 30 sec, accumulation potential –0.8 V, Scan rate 250

mV/s, pulse amplitude 0.10V, fre quency 15Hz and drop size

0.6 mm2.

Figure 12. SW-AdSV voltammogram for Timolol in acetate

buffer, pH = 3.5, Tacc =30 sec, Eacc = –0.80 V. Drug Conc.:-

(A = 1 × 10–7 M, B = 3 × 10–7 M, C = 5 × 10–7 M, D = 7 × 10–7

M, E = 1 × 10–6 M, F = 1.5 × 10–6 M). Drug Conc.:- (A = 1 ×

10–7 M, B = 3 × 10–7 M, C = 5 × 10–7 M, D = 7 × 10–7 M, E =

1 × 10–6 M, F = 1.5 × 10–6 M).

Least-square treatment of the calibration graph yielded

the following regression equation:

ip (nA) = 53.1 + 3.1 × 108 C (mol l–1) r = 0.998, n = 6.

where ip is the adsorptive stripping peak current, C is the

analysed drug concentration and r is the correlation coef-

ficient.

3.2.2. Detect i o n Li mi t

The lowest detectable concentration of this drug was

1.26 × 10–9 mol·l–1(0.4 ppb), which was estimated based

on the signal-to-noise ratio (S/N = 3) and this obtained

analytical sensitivity is very promising since it was

achieved after employing very short accumulation time

(30 s) comparing to that reported for the previous volt-

ammetric determination of timolol using differential pulse

voltammetry (DPV) technique which required 2.5 ppb

detection limit [23].

3.2.3. Reproducibility

The high sensitivity of adsorptive voltammetry is ac-

companied by very good reproducibility. This analytical

performance was evaluated from eight repeated meas-

urements of electrochemical signal of 1 × 10–6 mol·l–1 ti-

molol drug solution. The precision of the electrochemical

developed method in terms of the relative standard de-

viation (RSD%) was 0.13%.

3.2.4. Accuracy

The accuracy of the proposed method was checked by

calculating the recovery of known amount of timolol (5 ×

10–7 mol·l–1) solution added to acetate buffer solution and

analysed via the optimized stripping voltammetric pro-

cedure. The value of the recovery obtained by the stan-

dard addition method was 110% ± 1.414.

3.2.5. Stability

Under the optimum conditions, the stability of 1 × 10–6

mol·l–1 timolol solution was evaluated by monitoring the

changes in the height of adsorptive stripping voltammet-

ric (AdSV) peak over a period of 90 min. The electro-

analytical signal was gradually constant with time. The

acidic media (pH 3.5) of the acetate electrolyte solution

probably initiated a slow degradation process for the

drug.

3.3. Interference Studies

In order to evaluate the selectivity of the developed

AdSV procedure, the influence of various interferences

was examined. Considerable interference can be caused

by co-existing surface-active compounds capable of com-

peting with the analyte of interest for the adsorption site

on the electrode surface, resulting in decreased or in-

creased peak height. The competitive co-adsorption in-

terference was evaluated in the presence of various sub-

stance usually occur in the pharmaceutical eye-drops and

formulations. For these investigations, the interfering

species were added at different concentrations (one,

5-fold and 50-fold) higher than the concentration of ti-

molol (1 × 10–6 mol·l–1). The addition of starch at these

concentration levels caused the adsorptive stripping vol-

tammetric (AdSV) peak current to decrease by about 3%,

4% and 6%, respectively, of its original peak current.

Also the addition of 50-fold of sucrose in the test drug

solution (1 × 10–6 mol·l–1), caused the stripping voltam-

metric peak current to decrease by about 8%. Apparently,

A. F. Al.-GHAMDI

180

these inhibition effects were caused by the working elec-

trode surface blockage due to adsorption of interferences.

In contrast, the addition of 50-fold of lactose in the drug

solution, caused the square wave adsorptive stripping

voltammetry (SW-AdSV) response of the drug to in-

crease by about 16%.

3.4. Practical Applications

The reliability of the proposed adsorptive stripping vol-

tammetry (AdSV) method for the determination of ti-

molol drug was investigated by assaying this drug in

some real samples. Following the developed electroana-

lytical procedure described above, timolol drug was ana-

lysed in pharmaceutical formulation. The timolol content

of commercially available eye drop (Ocumol 0.5, Ti-

molol 0.5%, 5 mg/ml) was determination directly by the

square wave adsorptive stripping voltammetry (SW-

AdSV) method after the required dissolving steps. Four

aliquots of the dissolved sample were diluted to the re-

quired concentration level and measured via the standard

additions approach. For these studies, results obtained

gave a recovery mean 80% with standard deviation of ±

2.94%. As can be seen from Table 1 (Analysis of Ti-

molol drug in its commercial eye drop).

In addition, the applicability of the stripping voltam-

metric procedure for the analysis of timolol drug in bio-

logical samples was also evaluated by estimating its re-

coveries from spiked human urine and serum samples. A

simple and fast pretreatment (clean-up) procedure, which

is in fact a slight modification of the sample preparation

method develop for the determination of some antagonist

drugs [25] was used. By adding a small amount of 5%

ZnSO4·7H2O solution, NaOH and methanol to the serum

sample and centrifuging the mixture, most of the inter-

fering substances (mainly proteins) were simply removed

and eliminated by precipitation. As can be extracted from

Table 2 (Analytical results for timolol drug recoveries

from urine and serum samples), this adsorptive stripping

voltammetric (AdSV) method (after appropriate dilution)

allowed the determination of timolol drug in urine and

Table 1. Analysis of Timolol drug in its commercial eye drop.

Found (mg) Recovery %

Labeled 3.85 77

Content 3.90 78

5 mg/ml 4.10 82

Timolol 4.15 83

Mean 80

Standard Deviation ±2.94

Table 2. Analytical results for timolol drug recoveries from

urine and serum samples.

Recovery%: Spiked Urine Spiked Serum

86 108

88 107

Added Timolol 85 109

5.0 × 10–7 mol·l–1 89 106

Means 87 107.5

Standard Deviations±1.83 ±1.3

serum samples with mean recoveries 87% ± 1.83 and

107.5% ± kmi 1.3, respectively.

4. Conclusions

Voltammetric methods have proved to be very sensitive

for the determination of organic molecules, including

drugs and related molecules in pharmaceutical dosage

forms. These techniques have several advantages, in-

cluding that they are quick and reproducible, present low

limits of detection and quantification and have relatively

low cost compared with the more traditional techniques.

Moreover, they provide a better discrimination against

background currents.

In this study, a new, simple, selective, accurate and

precise SW-AdSV method developed for the determina-

tion of Timolol in pharmaceutical formulation. In this

study, all experimental and instrumental parameters were

optimized, as their values strongly affect the sensitivity

of the voltammetry. The peak current was investigated

using differential pulse polarography and cyclic voltam-

metry. The electrochemical reduction of Timolol is an

irreversible process controlled by adsorption under the

conditions described in this work. The proposed method

with the optimized parameters demonstrated a good lin-

ear relationship between the peak current and the Ti-

molol concentration for a wide range of concentration.

The applicability of the proposed procedure was tested

using a commercial pharmaceutical formulation of Ti-

molol. This drug was quantified in the pharmaceutical

preparation and biological fluids such as serum and urine,

and no pretreatment or time - consuming extraction was

required prior to the analysis. The results are in good

agreement with the labeled values. Accuracy and selec-

tivity of the developed method were demonstrated by

recovery studies. Reproducibility, stability, and interfer-

ences studies of this proposed method suggest that this

method could be used in quality control analysis, clinical

laboratories, and pharmacokinetic studies.

A. F. Al.-GHAMDI

181

5. Acknowledgements

I would like to thank Prof. Mohammed Hefnawy and

Prof. Mohammed Al-Omar at King Saud University for

their assistances.

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