Studies of preservation of water samples for the determination of non-ionic surfactants

ANALYTICA CHIMICA ACTA

Analytica Chimica Acta 3 11 (1995) 31-36 ELSEVIER

Studies of preservation of water samples for the determination of non-ionic surfactants ‘.

Andrzej Szymanski, Zbigniew Swit, Zenon Lukaszewski *

Technical Unillersity of Poznan, Institute of Chemistry, PI,-60965 Poznan, Poland

Received 4 October 1994; revised 31 March 1995; accepted 6 April 1995

Abstract

Chloroform, formaldehyde and ions of copperi and mercury(II) were tested as preservatives of water samples for determination of non-ionic surfactants (NS). River water (Warta River, Poznan) was used as hydrobiological background. Mainly the concentration of “native” NS from river water was measured, but in two series of experiments spikes of Triton X-100 and Marlipal 1618/25 were used. Indirect tensammetric measurements (ITM) were applied for control of the NS concentration. Formaldehyde is the most effective compound among the tested preservatives. A concentration of 1% is sufficient for long-term storage while 0.1% is sufficient for short-term storage (up to 6 days). Copper (II) (50 mg I-‘) or mercury(H) (2.5 mg l- ‘) may be used for short-term storage of water samples while chloroform used alone is ineffective. It

may be used together with cooling of a sample (4” C) for short-term storage, however, such a pretreatment of samples is more complicated than the use of other preservatives. Refrigeration (4” C) used alone is ineffective. The results concerning

high spikes of Triton X-100 show better preservation than the samples containing only “native” NS. Therefore, the experiments with spikes of surfactants may lead to too optimistic conclusions as for preservation of a water samples. Adsorptive stripping tensammetry was applied for observation of biodegradation of spikes of Marlipal 1618/25 both in preserved and non-preserved samples. The high potential of this technique for the examination of biodegradation has been demonstrated.

Keyw0rd.s: Tensammetry; Non-ionic surfactants; Waters; Preservation

-

1. Introduction

The present state-of-the-art for the determination of non-ionic surfactants (NS) in environmental samples is far from perfect. The BiAS [1,2] and CTAS [3] procedures recommended for the determination of

I

‘I This paper is dedicated to the memory of Professor Edmund

Kozlowski from the Technical University of Gdabsk.

* Corresponding author.

NS are cumbersome, requiring large volumes of sample (several litres) and very long sophisticated separation procedures. They are not precise and very sensitive to the choice of standard. The detection

limit of these procedures is in the order of 50-150 pg. Much more sensitive liquid chromatographic procedures supply only fragmentary information concerning mainly oxyethylated alkyl phenols [4]. Unsatisfactory results in the determination of NS may be due to the inappropriate pretreatment of a sample, i.e., sampling, storage, etc. The measuring

0003.2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved

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32 A. Szymanski et al. /Analytica Chimica Acta 311 (1995) 31-36

process may be deviated by serious errors produced

in the pretreatment stage. Storage of samples re- quires very effective preservation to stop destruction

of NS due to biodegradation. Effective preservation

of samples is very convenient for the organisation of the control of surface water. Samples of water col-

lected at different sites, if effectively preserved, can be transported even by mail to the analytical labora-

tories. Also different biodegradation studies can be

performed more easily if samples taken from the reactor can be treated with preservative to stop

biodegradation and can be analysed at the most convenient time.

Studies of preservation of real samples were lim- ited by high detection limits of procedures recom-

mended for the determination of NS. Moderately polluted river water usually contains less than 100

pug 1-l of NS [5]. If the concentration of NS drops

due to biodegradation, the level of their concentration is below the detection limits of the BiAS and CTAS procedures. The other barrier in such investigations is the necessity to use l-5 1 for a single determination. That is why the studies of preservation of water samples should be performed with

large spikes of one representative surfactant. How- ever, such spikes may affect the normal way of biodegradation and may lead to an erroneous conclusion concerning the effectiveness of preservation.

Tensammetric techniques offer new opportunities in investigation of surfactants. The indirect tensam-

metric measurement (ITM) of the concentration of NS is specifically useful for such a purpose [6-81. Like other tensammetric techniques, ITM exhibits much lower detection limits than BiAS and CTAS

procedures (two orders of magnitude). On the other hand, ITM shows good additivity of analytical sig-

nals of different components of mixtures, in contrast to the majority of tensammetric techniques [8]. Ex- cesses of anionic surfactants [7] can be tolerated if the starting potential is properly selected. The separation procedure required prior to the determination itself is much more simple in ITM in comparison with BiAS and CTAS procedures and needs only 100-200 ml of water sample. Therefore the ITM enables to investigate effectiveness of preservation of non-spiked samples of river water samples. Ad- sorptive stripping tensammetry (AdST) is a supple- mentary technique for the observation of specific

signals, i.e., to obtain additional information of quali- tative nature.

The aim of this paper was to study the effectiveness of different preservatives of biodegradation of

NS. Basically non-spiked samples containing only

“native” NS from river water were investigated although in two series of experiments the river water spiked with representative surfactants was used. The

river water samples contained NS in the order of 40-60 pg 1-l but during investigation their concen-

tration decreased to 5-10 pg 1-l in some samples. ITM shows excellent ability to work within this concentration range.

2. Experimental

2.1. Apparatus

A Radelkis OH-105 polarograph and EC0 Chemie

General Purpose Electroanalytical System pAUTO- LAB were used with a voltage scan rate of 400 mV min-‘. The applied amplitude of the alternating

voltage was 2 mV. Controlled-temperature HMDE equipment (Radiometer), having an additional plat- inum wire auxiliary electrode, was used. All poten-

tials cited are measured against the saturated calomel electrode. The beaker of the measuring cell was replaced by a quartz beaker, to prevent adsorptive

losses on the glass surface [9]. The ceramic frit at the

end of the salt bridge was protected by the fitting of a polyethylene tube. This ceramic frit indicates a very large adsorptive ability [9], and this protection very effectively reduces the adsorptive loss of surfactants. The measuring cell was carefully cleaned with methanol between the measurements.

2.2. Reagents

Triton X-100 (Rohm and Haas) and Marlipal 1618/25 @liils) were used without additional purifi- cation. Ethyl acetate (POCh) of gas chromatographic grade was used. Other reagents were AnalaR grade.

Purified sodium sulphate was used for the prepa- ration of the aqueous 0.5 M base electrolyte. All solutions were prepared in water triply distilled from quartz. Only freshly distilled water was used.

River water was sampled from Warta River in

A. Szymanski et al. /Analytica Chimica Acta 311 (1995) 31-36 33

Poznan, from the main stream and at 1 m depth. These samples were placed in 3-l glass bottles and the investigated preservative was added. The samples

were kept under aerobic conditions. From these bottles, samples were taken out for determination of NS

concentration. The determination of NS was per-

formed as described previously [lo]. Briefly, NS

were extracted from 50-200 ml sample previously treated with sodium chloride (100 g 1-l > and sodium

hydrogencarbonate (5 g 1-l) with two 10 or 20 ml

portions of ethyl acetate. Ethyl acetate or its aliquot was totally evaporated. The volume of sample, volume of ethyl acetate used for extraction and part of

ethyl acetate layer used for determination depended on the expected concentration of NS. The residue

after evaporation of ethyl acetate was dissolved in 1.50 ml of ethyl acetate and than this solution was dissolved in the base electrolyte (0.5 M sodium

sulphate). The tensammetric curve of ethyl acetate was recorded for ITM measurement.

3. Results and discussion

3.1. The influence of Triton X-100 spikes on the

stability of the water samples preserved with chloro-

f orm

The aim of this series of experiments was to check whether spikes of model surfactant may affect the stability of the preserved water sample. Triton

X-100 was selected as the model surfactant. It is frequently used for this purpose. Except for non-

spiked samples, three spikes of Triton X-100 were

applied: 50, 200 and 1000 pg 1-l. The moderately effective preservative chloroform was used at a con-

centration of 4 ml l-‘, recommended for preservation of water samples prior to the determination of NS [ 111. The non-spiked sample preserved in the

same way was also investigated just like non-spiked non-preserved water sample. The total concentration of non-ionic surfactants (i.e., spike plus “native” surfactants from the water sample) was measured during 20 days. The results are shown in Fig. 1 To show the curves for very different concentrations in the same figure, the concentrations of non-ionic surfactants have been given in the relative form.

A rapid drop of the concentration of non-ionic

1.0

0.5

0

0 0 a D

I

5 10 15 zbdays

Fig. 1. Changes of relative total concentration of non-ionic surfac-

tants in samples of river water (Warta River, Poznan) spiked with

Triton X-100 and preserved with chloroform. Spike of Triton

X-100 ( pg I-’ 1: (a and b) 0, (c) 50, (d) 200 and (e) 1000. Total

concentration of “native” non-ionic surfactants in sample, 37 pg

1-l. Concentration of chloroform (ml I-’ ): (a) 0. (b, c, d and e) 4.

surfactants in the non-preserved water sample is

apparent (see curve a); it shows the importance of immediate preservation of water samples. Even a

few hours of delay is unacceptable from the point of view of effective preservation. The drop of concentration of non-ionic surfactants preserved with chloroform is also apparent, although it is much slower.

It depends on the magnitude of the spike of Triton X-100. Generally, the samples are insufficiently preserved, however, looking at the curve e concerning

the sample with 1000 pg 1-l spike, it may lead to the wrong conclusion that the water sample is effectively protected during approximately 10 days. Tak-

ing into account these results the majority of further

investigations were performed only with non-spiked water samples, i.e., those containing only “native” non-ionic surfactants. These results truly reflect conditions of a sample-to-be-preserved in contrast to spiked samples.

3.2. Effectiveness of different preservatives of water

samples

Chloroform, formaldehyde and ions of copper(B) and mercury(B) were investigated as agents for the preservation of water samples. Only one preservative concentration, selected on the basis of literature search, was normally used in the experiments, except


lb 20 t/days

Fig. 2. Changes of total concentration of “native” non-ionic

surfactants in refrigerated and not refrigerated samples of river

water (Warta River, Poznan) preserved with chloroform. Concen-

tration of chloroform (ml 1 -I): (a and a’) 0, (b and b’) 4.

Temperature of samples (” C): (a and b) 20, (a’ and b’) 4.

for formaldehyde, which was found to be the most

promising one during the investigation. The effect of refrigeration of stored water samples was also exam-

ined. The changes in concentration of “native” non-

ionic surfactants in two water samples stored at 20 and 4” C were measured during 20 days. The results

are shown in Fig. 2 (curves a and a’). It is obvious that refrigeration of the sample is ineffective against the biodegradation loss although the process runs

much more slowly at 4” C than at 20” C. The low effectiveness of chloroform has already

been shown in Fig. 1. In this series of experiments it was investigated whether the sample of “native”

non-ionic surfactants preserved with 4 ml 1-l of chloroform and simultaneously refrigerated to 4” C is effectively preserved. The results are shown in Fig. 2 (curves b for 20” C and curve b’ for 4” C). It is apparent from the figure that refrigeration of a sam-

ple very effectively improves the stability of samples preserved with chloroform (in terms of concentration of NS). The drop in NS concentration is very slow and such a preservation of water samples can be recommended for short term storage.

Three concentrations of formaldehyde used for preservation were investigated: 0.1, 1.0 and 5.0%. Non-preserved water sample was also investigated. The results are shown in Fig. 3. Formaldehyde appears to be very effective. Even 0.1% of this compound keeps the concentration of non-ionic surfactants stable during 6 days. The sample preserved

with 1 or 5% of formaldehyde were stable concerning the concentration of NS during the whole period

of experiments (20 days). Experiments with river

water samples spiked with 1000 pug 1-l of Marlipal

1618/25 preserved with 1% of formaldehyde were also conducted for additional checking of the effec-

tiveness of this way of preservation. Marlipal

1618/25 is one of the most representative non-ionic surfactants. The results have been added to Fig. 3. They support the conclusion concerning the high

effectiveness of 1% formaldehyde as a preservative. Non-preserved samples of river water containing a

spike of 1000 pg 1-l of Marlipal 1618/25 undergo fast biodegradation, as it may be concluded from the tensammetric curves a, b and c in Fig. 5. After 10

days the tensammetric curves show no Marlipal 1618/25 to be present.

The samples of non-spiked river water for investigation of effectiveness of copper(B) and mercury(B)

ions as preservatives were acidified with 5 ml of concentrated sulphuric acid. 50 mg 1-l of copper(B) or25mgl-’ of mercury(B) were added. The results are shown in Fig. 4. The curve for non-preserved

water sample has also been added (curve a>. Both copper(B) and mercury(B) cause short-term stability

of the preserved river water samples. Mercury(B) is slightly more effective than copper(B).

r/&f

1500

1000

500

rys


surfactants in samples of river water (Warta River, Poznan) (a, b,

c and d) preserved with formaldehyde (b, c and d). Changes of

total concentration of non-ionic surfactants in sample of river

water (Warta River, Poznan) spiked with 1000 pg I-’ Marlipal

1618/25 and preserved with formaldehyde (e). Concentration of

formaldehyde (o/o): (a) 0, (b) 0.1, (c and e) 1 and (d) 5.

A. Szymanski et al./Analytica Chimica Acta 311 (1995) 31-36 35


surfactants in samples of river water (Warta River, Poznan) (a, h

and c) preserved with ions of copper(R) (b) or mercury(R) (c).

Samples corresponding to curves (b) and (c) were acidified with 5

ml I - ’ of concentrated sulphuric acid. Concentration of metal ions

(mg I- ’ ): (a) 0, (b) 50 and (c) 25.

Undoubtedly, formaldehyde is the most effective compound among the preservatives tested. A concen-

tration of 1% is sufficient for long-term storage while 0.1% will do for short-term storage (up to 6 days). Copper (II) (50 mg 1-l) or mercuryfI1) (25 mg 1 - ’ ) may be used for short-term storage of water samples while chloroform used alone is not effective. It may be used together with refrigeration of a

sample (4” C) for short-term storage, however, such a

sample pretreatment is more complicated than the use of other preservatives. Refrigeration (4” C) used alone is ineffective. Application of 1% formalin is

recommended for the preservation of water samples for the determination of oxyethylated alkyl phenols

[4]. The same authors recommended refrigeration as an effective measure for preservation of water samples (used alone) during 4 weeks. Our results are different but recommendations of Kubeck and Nay- lor [4] concern oxyethylated alkylphenols while our results concern “native” surfactants from Warta River in Poznan of which the biodegradation ability

may be different. A certain weak point in the conclusions drawn on the basis of the results of this paper is that only one hydrobiological background (Warta River) was used in the investigation. Other water samples may differ in terms of quality and quantity of species of microfauna and microflora being re- sponsible for biodegradation.

3.3. Adsorptir>e stripping tensammetric obseroation

of water samples spiked with Ma&pal 1618 / 25 and

preserved with chloroform and formaldehyde

To obtain more information concerning the differ-

ence between formaldehyde and chloroform as

preservatives. adsorptive stripping tensammetry (AdST) was applied. AdST gives a more specific

analytical signal (or signals) in comparison with the non-specific indirect tensammetric methods. The

main disadvantage of AdST is the very complicated behaviour of mixtures of different surfactants. How- ever, it is a very useful technique if a single surfac-

tant is present in the system. Therefore the spike of

the tested surfactant should be so high in comparison

with “native“ surfactants from the water sample

that their influence would be negligible. Spikes of

1000 pg 1-l of Marlipal 1618/25 were applied. The biodegradation process as observed by AdST appears more complicated than that observed on the basis of

total concentration measured by ITM. This is apparent from the tensammetric curves shown in Fig. 5.

The initial signal of Marlipal 1618/25 (see curve a) consists of two almost overlapping tensammetric

?oornv

I 0.2~1A

a

/

b I C / d i’ e

Fig. 5. Tensammetric curves of Marlipal 3618/25 extracted from

non-preserved (a. b and c) and preserved with 1%’ of formalde-

hyde (d) or 4 ml I-’ of chloroform (e) river water samples (Warta

River, Poznan). Sampling after (days of experiment): (a) 0 (initial

sample), (bf 1, (c) 5 (d and e) 6.


peaks. It is in agreement with more extensive investigations concerning the behaviour of oxyethylated

alcohols with AdST [12,13]. Biodegradation of non-

preserved samples concerning Marlipal 1618/25 cause surprising changes of the tensammetric curves.

On the second day, the tensammetric curve of Marli-

pal 1618/25 shows a decrease in height of the left peak and a simultaneous shift in the less negative direction. This peak becomes distinctly shaped and it

does not overlap with the other peak (see curve b). Most surprising is the substantial rise of the second

more negatively located peak. Such a behaviour is

characteristic of mixtures in AdST [14]. The ob-

served behaviour seems to show that a part of the components of a polydispersal mixture (which in fact is Marlipal 1618/25) was biodegraded while the

other components were not. The further biodegradation causes a reduction of both peaks (see curve c corresponding to five days of biodegradation). The

tensammetric curve of Marlipal 1618/25, corresponding to the sample of river water preserved with 1% of formaldehyde after six days of experiments, is the same as curve a showing the initial signal. On the other hand curve e corresponding to a similar sample

preserved with 4 ml l- ’ of chloroform after six days of experiments shows similar effects as the non-pre-

served sample on the second day of the experiments but less pronounced (compare curves e and a>. The left peak becomes more distinct and the right peak increases. This experiment shows that biodegradation

of sample spiked with Marlipal 1618/25 and preserved with chloroform proceeds similar to a non- preserved sample, but more slowly. These curves show also how useful AdST can be in the investigation of biodegradation of non-ionic surfactants.

Acknowledgements

This work was supported by the Committee of

Scientific Research (grant No. 4 4008 92 03).

References

[II El

[31

[41

[51

[61

[71

[Sl

[91

[lOI

illI

ml

[131

1141

R. Wickbold, Tenside Deterg., 9 (1972) 173.

J. Waters, J.T. Garrigan and A.M. Paulson, Water Res., 20

(1986) 247.

T.M. Schmitt, M.C. Allen, D.K. Brain, K.F. Guin, D.E.

Lemmel and Q.W. Osburn, J. Am. Oil Chem. Sot., 67

(1990) 103.

E. Kubeck and C.G. Naylor, J. Am. Oil Chem. Sot., 67

(1990) 400.

W.K. Fischer, Tenside Deterg., 17 (1980) 250.

A. Szymanski and Z. Lukaszewski, Anal. Chim. Acta, 260

(1992) 25.


(1993) 313.


(1994) 77.


(1990) 77.

A. Szymanski, B. Wyrwas and Z. Lukaszewski, Anal. Chim.

Acta, 305 (1995) 256.

Polish Standard PN-88-C-04550/11. Determination of non-

ionic surfactants in waters by spectrophotometric method

using Dragendorff reagent, Polish Committee for Standard-

ization, Measures and Quality Control, 1988.

A. Szymanski and Z. Lukaszewski, Electroanalysis, 3 (1991)

17.

A. Szymanski and Z. Lukaszewski, Electroanalysis, 3 (1991)

963.

M.K. Pawlak and Z. Lukaszewski, Anal. Chim. Acta, 202

(1987) 97.

Documents

Studies of preservation of water samples for the determination of non-ionic surfactants