Determinación de PEG

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DESALINATION

www.elsevier.com/locate/desalDesalination 247 (2009) 423–429

Quantitative determination of polyethylene glycol withmodified Dragendorff reagent method

Zhiqian Jia*, Chunai Tian

Department of Chemistry, Beijing Normal University, Beijing 100875, PRC

Tel: +86-010-58210560; email: [email protected]

Received 5 January 2007; accepted 2 September 2008

Abstract

Quantitative determination of polyethylene glycol (PEG) is usually needed in membrane technology, water

environment and chromatography. The concentration of PEG is generally measured with the Dragendorff reagent.

But in the literature method, the hydrolysis of the Dragendorff reagent results in nonlinear calibration curve and

unsatisfied reproducibility. In this paper, a modified Dragendorff approach was provided to inhibit the hydrolysis

in the analysis. The effects of acids and its concentrations were studied. The results showed that, with the modified

method, the calibration curves for PEG 4000, 6000, 10,000 and 20,000 displayed good linearity.

Keywords: Polyethylene glycol; Dragendorff reagent; Ultrafiltration membrane; Molecular weight cut-off

1. Introduction

Quantitative determination of polyethylene

glycol (PEG) is usually needed in membrane

technology, water environment [1] and chroma-

tography. For example, in the preparation of ultrafiltration membranes, determination of

molecular weight cut-off (MWCO) is usually

indispensable [2, 3]. PEG or protein with differ-

ent molecular weight is often employed as the

standards [4, 5]. By measuring the concentrations

of the standards in the permeate and the feed, the

retention coefficient and MWCO of membranes

are obtained. As PEG is cheap and stable, it is

widely employed especially in the separation of

linear polymers because the retention coefficient

of PEG is similar to that of the linear polymers.

To determine the PEG concentration, the Dra-

gendorff reagent, widely used in themeasurement

of alkaloid [6], is often employed. The reaction of Dragendorff reagent with PEG generates an

orange-red complex, which is easily detected at

510 nm with UV-vis spectrophotometer [7, 8].

In the preparation of the calibration curve, PEG

standard solutions aremixed with theDragendorff

reagent, and then theabsorbance is measured with

UV-vis spectrophotometer. The absorbance of the

permeate and the feed solution are measured with

the same procedure and the concentrations are*Corresponding author.

0011-9164/09/$– See front matter 2009 Elsevier B.V.doi:10.1016/j.desal.2008.09.004

Published by



read from the calibration curve. Nevertheless, in

the above procedures, Dragendorff reagenthydro-

lyzes seriously, resulting in nonlinear calibration

curve, pooraccuracy and unsatisfied reproducibil-

ity. In thispaper, the effects of acids and their con-

centrations on the inhibition of the hydrolysiswere studied.

2. Experimental section [6, 7]

PEG (molecular weight of 4000, 6000,

10,000 and 20,000), Bismuth subnitrate

(BiONO3), glacial acetic acid (HAc), KI and

hydrochloric acid (HCl) were analytical pure

and provided by Beijing Reagents Factory. The

UV-vis absorbance was measured with UV-vis

spectrophotometer (Cintra 10e, Australia). The pH of the solution was determined with pH

meter (Shanghai Jingke Instrument Co. Ltd.).

2.1. Preparation of Dragendorff reagent

A 0.800 g of BiONO3 and 10 ml of glacial

HAc were added into a 50-ml volumetric flask.

Then the mixture was diluted to volume with dis-

tilled water to obtain solution A. 20 g KI powder

was added into a 50-ml brown volumetric flask

and diluted to the mark with water to obtain sol-ution B. Then 5 ml of solution A and 5 ml of

solution B were added into a 100-ml brown

volumetric flask and diluted to volume with dis-

tilled water to obtain the Dragendorff reagent.

2.2. Preparation of HAc–NaAc buffer

solution (pH 4.8)

A 590 ml of 0.2 mol L1 NaAc solution and

410 ml of 0.2 mol L1 HAc solution were added

into a 1000-ml volumetric flask and diluted tothe mark with distilled water.

2.3. Preparation of PEG standard solutions

Exactly 0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 ml of

1000 mg L1 PEG stock solution was pipetted

into 100-ml volumetric flasks, respectively and

diluted to the mark with distilled water to obtain

the PEG standard solutions with concentrations

of 0, 5, 10, 15, 20, 25 and 30 mg L1.

2.4. Preparation of calibration curve

A 5.0 ml of PEG standard solutions with con-

centrations of 0, 5, 10, 15, 20, 25 and 30 mg L1

were added into 10-ml volumetric flasks. Then

1.0 ml of HAc–NaAc buffer solution and 1.0 ml

of the Dragendorff reagent were added, respec-

tively. Lastly, the mixed solution was diluted to

volume with distilled water. After 15 min, the

absorbance was determined with UV-vis spectro-

photometer to obtain the calibration curves.

3. Results and discussions

According to the literature [6, 7], in the

preparation of Dragendorff reagent, 5.0 ml of

solution A (i.e., BiONO3 solution) was mixed

with 5.0 ml of solution B (i.e., KI solution),

and then diluted to 100 ml with water. Figure 1

shows that solution A has no absorption band

from 300 nm to 800 nm, and solution B only

shows absorbance below 340 nm. When solution

A was mixed with B, I reacts with Bi3+ and forms BiI3 precipitates. As KI is in excess

in the mixture (the molar ratio of I to Bi3+

is 43.3), BiI3 reacts with I and forms [BiI4]

complex:

3I+ Bi3+ ? BiI3

BiI3 + I? BiI4

The stepwise formation constants of [BiI4]

are K 1 = 3.63 and K 4 = 14.95 [9]. The maximumabsorbance of [BiI4] is displayed at 465 nm

(Fig. 1) [10, 11].

When the mixed solution was diluted to

100 ml with water to obtain the Dragendorff

reagent, the mixture became turbid and golden-

yellow precipitates settled down after a certain

time. These phenomena were attributed to the

Z. Jia, C. Tian / Desalination 247 (2009) 423–429424



hydrolysis of BiI4 and the formation of black

BiI3 and orange BiOI precipitates [12].

BiI4 ! BiI3 # þI

BiI3 þ H2O ! BiOI # þ2HI

In the preparation of calibration curve, 5.0 ml

of standard PEG 10,000 solutions with concen-trations of 0, 5, 10, 15, 20, 25 and 30 mg L1

were added into 10-ml volumetric flasks. Then

1.0 ml of HAc–NaAc buffer solution, 1.0 ml of

Dragendorff reagent and water were added,

respectively. After 15 min, the absorbance was

determined with UV-vis spectrophotometer. In

these procedures, large amount of precipitates

were formed in the mixture. Figure 2a shows

that the absorption spectra of the solutions are

plat and there is no characteristic band of PEG–BiI4 complex at 510 nm. The scattering

phenomena on the long wavelength areattributed

to the suspended precipitates. The relation

between the absorbance at 510 nm and the

concentrations of PEG isnot linear (Fig. 2b),

indicating that the reproducibility and accuracy

of the method arevery poor.

From the above results, it can be seen that, in

the preparation of Dragendorff reagent and

calibration curve, the hydrolysis of BiI4 should

be effectively inhibited. For the lack of solubility

data of BiI3 and BiOI, the proper pH value had to

be estimated empirically. In solution A, the HAc

concentration, C , was 3.494 mol L

1

. The disso-ciation constant of HAc, K a,HAC, is 104.76. Then

pH =[lg H+] =lg ( K a,HACC )1/2 = 2.11. When

solution A was mixed with solution B, the HAc

concentration decreased to 1.747 mol L1 and

the pH increased to 2.26. In this case no precipi-

tates were formed. When the mixture was diluted

with water to obtain the Dragendorff reagent, the

300 400 500 6000

1

A b s o r b a n c e

Wavelength (nm)

Solution A

A+B+HAc

Solution B

Fig. 1. Absorption spectra of solution A, solution B and the mixture.

300 400 500 600 700 800 900

0.2

0.4

0.6

0.8

1.0

1.2

1.4 a)

A b s o r b

a n c e

Wavelength (nm)

30 mg L1

25 mg L1

20 mg L1

15 mg L1

0 mg L

1

10 mg L1

5 mg L1

0 5 10 15 20 25 300.45

0.50

0.55

0.60

0.65

0.70

0.75

b)

A b s o r b a n c e

Concentration (mg L1

)

Fig. 2. Results obtained according to the procedures in

the literature. (a) Absorption spectra of PEG 10,000 sol-utions with different concentrations. (b) The calibrationcurve of PEG 10,000.

Z. Jia, C. Tian / Desalination 247 (2009) 423–429 425



HAc concentration declined to 0.174,7 mol L1

and the pH was 2.76. At this pH, large amounts

of precipitates were formed. So the pH value

should be lower than 2.76. In the modified proce-

dures, the mixture of 5.0 ml of solution A and

5.0 ml of solution B was diluted with 3.85mol L1 HAc solution to 100 ml. In the modified

Dragendorff reagent, the HAc concentration was

3.669molL1 and the pH was 2.10.The modified

reagent appeared as orange-yellow transparent

solution with an absorption band at 465 nm, and

kept stable for several months.

In the preparation of the calibration curve, to

inhibit the hydrolysis, the mixture of PEG and

Dragendorff reagent should also be diluted

with HAc solution. To elucidate the effects of HAc concentration, 5.0-ml aliquots of standard

20 mg L1 PEG 10,000 solution was pipetted

into 10-ml volumetric flasks. Then 3.0 ml of

HAc solutions with concentrations of 0, 2.16,

4.33, 6.06, 8.66, 12.98 and 16.44 mol L1

were added, respectively. Lastly, 1.0 ml of

modified Dragendorff reagent was added, and

the mixture was diluted to volume with the cor-

responding HAc solution. The pH values of the

mixtures were 2.60, 2.40, 2.22, 2.16, 2.09, 2.01

and 1.95, respectively. With the increasing con-centration of HAc solutions, the transparency of

the mixture increased, and the absorbance of

BiI4 at 465 nm and that of PEG–BiI4

at

510 nm increased (Fig. 3). The reason was that

high acidity favors the inhibition of hydrolysis

of Bi3+ and the formation of BiI4 and PEG–

BiI4 complex.

HOðCH2CH2OÞnHþ Hþ

! HOðCH2CH2 O

Hþ

ÞnH

HOðCH2CH2 O

Hþ

ÞnHþ Bil4

! HOðCH2CH2 O

Hþ

ÞnHBil4

In the following experiments, in the prepara-tion of the calibration curves, 3.0 ml of 8.66

mol L1 HAc solution was used in the analysis.

That was, 5.0 ml of PEG standard solutions

(0, 5, 10, 15, 20, 25 and 30 mg L1) were pipetted

into 10-ml volumetric flasks, respectively. Then

3.0 ml of 8.66 mol L1 HAc solution and 1.0

ml of modified Dragendorff reagent were added.

Lastly the mixture was diluted to volume with

8.66 mol L1 HAc solution. The final pH value

was 2.09. It was found that there were not appa-

rent precipitation phenomena occurring in allthe samples. The samples with PEG concentra-

tions of 0 mg L1 and 5 mg L1 displayed yellow

solutions, whereas those with PEG concentrations

from 10 mg L1 to 30 mg L1 were orange.

Figure 4a and b shows that the absorbance at

510 nm increases with the concentrations of

PEG 10,000 and the absorbance–concentration

relation is linear. Figure 4c shows that, for PEG

4000, 6000 and 20,000, the calibration curves

also display good linearity.Instead of HAc, in the preparation of the Dra-

gendorff reagent, H2SO4 and HCl were also tried

in dissolving BiONO3 powder. It was found that

BiONO3 was almost insoluble in dilute H2SO4,

but soluble in HCl solutions with concentrations

equal to or larger than 1.0 mol L1. The stepwise

formation constants of Bi3+ –Cl complexes are

400 500 6000

1

2

A b s o r b

a n c e

Wavelength (nm)

16.44 mol L1

12.98 mol L1

8.66 mol L1

6.06 mol L1

4.33 mol L1

2.16 mol L1

0 mol L1

Fig. 3. Effects of HAc concentration on the absorbancespectra.




K 1 = 2.44, K 2 = 4.2, K 3 = 5.0 and K 4 = 5.6. It

seemed that the coordination of anions (e.g.,

Cl, Ac) with Bi3+ may be important in

dissolving BiONO3. In the preparation of

Dragendorff reagent, 0.800 g of BiONO3 was

added into a 50-ml volumetric flask, then 1.0

mol L1 HCl was added to the mark to obtain

solution A. 20 g of KI powder was added into

a 50-ml brown volumetric flask and dissolved

with water to obtain solution B. Lastly, 5.0 ml

of solution A and 5.0 ml of solution B were

added into a 100-ml brown volumetric flask,and diluted to volume with distilled water to

obtain Dragendorff reagent (pH 1.3).

In the preparation of the calibration curve,

HCl solution was also added to inhibit hydro-

lysis. To elucidate the effects of HCl concen-

tration, 5.0 ml of 20 mg L1 PEG 10,000

standard solution was pipetted into 10-ml

400 500 6000.0

0.5

1.0a)

b)

c)

A b s o r b

a n c e

Wavelength (nm)

30 mg L1

25 mg L1

20 mg L1

15 mg L1

10 mg L1

5 mg L1

0 mg L1

0 5 10 15 20 25 300.0

0.2

0.4

A b s o r b a n c

e


)

0 5 10 15 20 25 30 350.0

0.2

0.4

0.6 PEG 4000

PEG 6000 PEG 20000

A b s o r b a n c e

Concentration (mg L1)

Fig. 4. Results obtained when HAc was employed in themodified method. (a) Absorption spectra for PEG10,000 with different concentrations. (b) Calibrationcurve for PEG 10,000. (c) Calibration curve for PEG4000, 6000 and 20,000.

500 6000.0

0.2

0.4

0.6a)

b)

30 mg L1

25 mg L20 mg L

1

10 mg L1

5 mg L1

A b s o r b a n c e

Wavelength (nm)

0 5 10 15 20 25 300.0

0.2

0.4

A b s o r b a n c e


)

1

Fig. 5. Results obtained when HCl was employed in themodified method. (a) Absorption spectra for PEG

10,000. (b) Calibration curve for PEG 10,000.




volumetric flasks. Then 3.0 ml of HCl solutions

with concentrations of 0, 0.013, 0.025, 0.038,

0.050, and 0.075 mol L1 were added. After

mixing, 1.0 ml of Dragendorff reagent was

added. Lastly the mixture was diluted to volume

with the corresponding HCl solution. The pHvalues of the mixture were 2.3, 2.0, 1.82, 1.70,

1.60 and 1.46, respectively. It was found that

for 0 and 0.013 mol L1 HCl solutions, large

amount of precipitates were formed. For 0.050,

and 0.075 mol L1 HCl solutions, no precipi-

tates were formed. In the following experiments,

0.050 mol L1 HCl solution was used in the

analysis. Figure 5 shows that the calibration

curves are in good linearity. Similar linear cali-

bration curves were also obtained for PEG4000, 6000 and 20,000. It seemed that HAc

and HCl were suitable for the inhibition of

hydrolysis, whereas H2SO4 and HNO3 were

not suitable because of the unsatisfied dissolu-

tion ability or oxidizing property.

4. Conclusions

The Dragendorff reagent method for the

measurement of PEG concentration was

modified to inhibit the hydrolysis of Bi3+. Itseemed that HAc and HCl were suitable in the

acidification. When HAc was employed, in the

preparation of Dragendorff reagent, 5.0 ml of

solution A and 5.0 ml of solution B were diluted

to 100 ml with 3.85 mol L1 HAc solution. In the

preparation of the calibration curves, 3.0 ml of

8.66 mol L1 HAc solution was added to the

mixture of PEG and Dragendorff reagent.

When HCl was employed, in the preparation

of Dragendorff reagent, 0.800 g of BiONO3 wasdissolved with 1.0 mol L1 HCl. Then, 5.0 ml of

solution A and 5.0 ml of solution B were diluted

to 100 ml with distilled water. In preparing the

calibration curves, 3.0 ml of 0.050 mol L1

HCl solution was added to the mixture of PEG

and Dragendorff reagent. The results showed

that, with the modified methods, the calibration

curves of PEG 4000, 6000, 10,000 and 20,000

were in good linearity. As the Dragendorff

reagent is widely used in the determination of

PEG concentration, the modified methods

seem to be helpful and interesting.

Acknowledgments

The authors gratefully acknowledge the

supports from the National Natural Science

Foundation of China (No. 20676016), the Youth

Foundation of Beijing Normal University (BNU,

No. 107189) and the Analytical Foundation of

BNU.

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