Solvent Extraction Using a Polymer as Solvent with an Amperometric

16

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 24, 2017 | http://pubs.acs.org Publication Date: December 15, 1986 | doi: 10.1021/ba-1987-0214.ch016

Solvent Extraction Using a Polymer as Solvent with an Amperometric Flow-Injection Detector You-Wei Feng and CalvinO.Huber Department of Chemistry and Center for Great Lakes Studies, University of Wisconsin—Milwaukee, Milwaukee,WI53201 Solvent extraction offers unique advantages among separation techniques. A system based on extraction into a polymer [poly(vinyl chloride)] as solvent was examined here because of possible advantages in speed, simplicity, sample size, solvent handling, etc., especially when coupled with flow injection and an amperometric detector. Solutes examined included salicylic acid and 8-hydroxyquinoline. The apparatus typically consisted of 0.8-mm i.d. X 170-cm coiled tubing that could be connected directly to the injection loop of a flow-injection amperometric detector system containing a nickel oxide electrode.

- L l I Q U I D E X T R A C T I O N FOR T H E SEPARATION and enrichment of organic

compounds in aqueous samples has been used successfully. Automated solvent extraction with flow-injection analysis has been reported (J). The handling and disposal problems associated with the use of liquid solvent extractors have resulted in increased attention to the separation and preconcentration of organic compounds in water by collection in synthetic polymers followed by elution with an organic solvent (2). For example, selective collection and concentration of organic bases on methylacrylic ester resin from dilute water samples have been reported (3). Such collection techniques are especially wellsuited to flow-injection measurement techniques. In this study, ionizable organic analytes such as salicylic acid and 8-hydroxyquinoline (oxine) were extracted into a polymer and then back extracted by an aqueous solution. Amperometric measurement using a flow-injection technique was employed to monitor the process. 0065-2393/87/0214/0349$06.00/0 © 1987 American Chemical Society

Suffet and Malaiyandi; Organic Pollutants in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1986.

350

ORGANIC POLLUTANTS IN WATER

The detection step involves electrochemical oxidation at a nickel electrode. This electrode has been applied to measurements of glucose (4) ethanol (5), amines, and amino acids (6, 7). The reaction mechanism involves a catalytic higher oxide of nickel. The electrolyte solution consists of 0.1 M sodium hydroxide containing ΙΟ" M nickel as suspended nickel hydroxide to ensure stability of the electrode process. The flow-injection technique offers the advantages of convenience and speed in solution handling and ready maintenance of the active electrode surface. y


4

Experimental The polymer extractor consisted of 0.8-mm i.d. poly(vinyl chloride) (PVC) tubing (Norton Company) typically 170 cm in length and coiled about a 1- or 2-cm diameter rod. The outflow of the extractor tube was transferred to the detector by direct connection or by collection and transfer (see Figure 1). The continuous-streamflow-injectionsystem (Figure 2) consisted of a gravity-feed electrolyte reservoir, a sample injection valve (Rheodyne, Model 50) fitted with a 30 μί^πιρίβ loop, and a flow-through electrochemical detector cell. The channel diameter of the Teflon tubing for the stream was 0.8 mm. The tubing length from injector to detector was 10 cm. The detector cell was a three-electrode system consisting of a flow-through nickel working electrode, a saturated calomel reference electrode (SCE), and a stainless steel outlet tubing counter electrode. The tubular-type electrode cell housing was constructed of molded Teflon, which was machined to provide the channels and to accommodate the fittings. The working electrode area was

BACK

POLYMER EXTRACTOR

WASTE

TO S A M P L E INJECTOR

Figure 1. Extraction apparatus.


F E N G A N D HUBER

16.

351

Extraction Using a Polymer as Solvent ELECTROLYTE (0.1 Ν NaOH)

A M P E R O M E T R I C


DETECTOR

S E P A R A T O R —

S A M P L E

LOOP

Figure 2. Flow-injection detector apparatus. about 1 mm . The SCE salt bridge was located downstream from the working electrode. The working electrode applied potential was +0.56 V versus SCE. The circuitry consisted of a two op-amp based potentiostat and current-to-vol tage converter with offset. It was powered by a ±15-V dc power supply. A potentiometric recorder was used for readout. 2

Results and Discussion Detector Characteristics. The applied potential that produced the largest analytical signal was 0.56 V versus S C E . The decrease in analyti cal signal at potentials more positive than 0.56 V suggested a decrease in the active surface area of the electrode due to competitive solvent oxidation at the active sites. The analytical signal increased with flow rate up to 1.4 m L / m i n , and no further increases at higher flow rates occurred. A linear relationship between peak current and salicylate concentra tion i n 0.1 M N a O H carrier electrolyte was found for the range 8 X 10" M to 1 Χ Ι Ο M with a sensitivity of 2.5 μΑ/mM. The linear range for salicylate in acetate buffer solution (0.025 M N a A c / H A c , p H 4.8) was found to be 2.0 Χ 10" to 5.0 X 10 M with a sensitivity of 2.2 μΑ/mM. β

2

5

3

Polymer Selection. T h e polymer was selected on the basis of observations using salicylic acid-salicylate as analyte. T h e following organic polymers were examined: polystyrene, methyl methacrylate-ethyl acrylate, Teflon, silicone rubber, P V C , and polyester. T e n millimolar salicylic acid in 0.01 M HC1 was first extracted for 30 s and then back extracted with 0.1 M N a O H . Peak currents for back extractants (nA) were as follows: P V C , 1780; methyl methacrylate-ethyl


352



acrylate, 380; Teflon, 90; polyester, 410; polystyrene, 290; and silicone rubber, 1420. P V C tubing afforded the most efficient extraction and was used for the remainder of the study. The selection of an optimum polymer depends upon analyte species. Separation Dynamics. The mass transport dynamics of the system were examined with respect to extraction, back extraction, and time interval between extraction and back extraction. Concentration gradients limited b y diffusion rate must be considered; that is, equilibrium with homogenous concentrations of analyte does not describe this system. Extraction time effects are summarized in Figure 3. F o r extraction times greater than about 2 m i n , the extraction rate is apparently limited b y concentration gradient establishment in one of the phases. When the concentration decrease of the extraction sample rather than the back extracted amount was observed, the extent of extraction also leveled within 2 m i n , as shown b y the following data: F o r 0.1 m M salicylic acid, the peak currents (μΑ) were 0.180, 0.116, 0.043, and 0.045 after extraction times of 0, 1.0, 2.0, and 4.0 m i n , respectively; for 1.0 m M salicylic acid, the peak currents (μΑ) were 1.79, 1.14, 0.90, and 0.91

2

4

6

8

10

12

T i m e (minutes)

Figure 3. Extraction and back extraction time effects: (Φ), extaction times followed by 0.50-min back extractions; ( X ) , back extraction times after 1.0-min extractions. Extraction was from 10 mM salicylic acid in 0.01 m HCl, and back extraction was into 0.1 M NaOH.



16.

F E N G A N D HUBER

Extraction Using a Polymer as Solvent

353

after extraction times of 0, 1.0, 2.0, and 4.0 m i n , respectively. Extraction leveling data were obtained over a salicylic concentration range of 0.1-5 m M with an extraction efficiency of 30-50$. These results indicate that within 2 m i n a steady-state concentration profile is reached. When the extraction step is done with continuous f l o w , the amount of sample extracted per minute increases as the f l o w rate increases but levels at higher f l o w rates. This result indicates eventual mass transfer control of extraction efficiency. F o r back extraction rate studies, an extraction time of 1.0 m i n was used, and the back extraction time was varied up to 12 m i n , as shown in Figure 3. The time dependence in the back extraction process indicates that a diffusion rate to or f r o m the interface is regulating the process. The gradient in the aqueous solution is very steep because of ionization of the analyte; thus, diffusion of analyte molecules in the polymer to the interface limits the back extraction rate. This rate limitation supports the v i e w of a solvent extraction rather than a surface adsorption mechanism. The longer time dependence for back extraction (~12 min) versus that for extraction (~2 min) can be attributed to the collapse of the concentration gradient in the polymer on proceeding to back extraction. Back extraction for analytical applications was b y continuous f l o w of 0.1 M N a O H with detector injection of the initial portion of the back extract. This concentration gradient broadens further during the period between extraction and back extraction while the extraction tubing contains air. Figure 4 shows results of experiments in w h i c h this time interval was varied. As predicted, the rate of back extraction is decreased for larger time intervals. The rapid drop during the first 2 or 3 m i n in Figure 4 suggests that diffusion coefficients in the polymer are comparable to those in liquids. The analytical procedure accordingly requires both short and constant intervals between extraction and back extraction. Between samples, the residual solute in the polymer is removed b y continued exposure to back extractant. This process requires about 2 min for 1 m M sample solutions. As expected f r o m diffusion laws, this time period for residual removal varies with the square of the sample concentrations. Higher sampling rates require keeping analyte concentrations below about 1 m M . Plasticizer. The polymer tubing extractor contains the plasticizer l,2-bis(ethylhexyl) phthalate. Longer exposures to 0.1 M N a O H indicated that electroactive hydrolysis products of plasticizer, probably ethyl and hexyl alcohols, were produced, as suggested b y the following peak currents given in nanoamperes (the times of the peak currents are given in parentheses): 30 (2 min), 7 (1 min), 5 (30 s), and 2 (15 s). The analytical procedure presented here uses back extraction times of less


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350

nA

300

250 1

10

20

30

Time (minutes) Figure 4. Effect of time interval between extraction and back extraction. Extraction was from 1 mM salicylic acid in 0.01 M HCl, and back extraction was into 0.01 M NaOH. than 30 s so that the plasticizer contribution to analytical signals is negligible. Blank determinations further supported the conclusion that, for the prescribed procedure, plasticizer contribution is insignificant. To determine whether the plasticizer was functioning as solvent, attempts were made to extract the analyte (salicylic acid) with liquid diethyl phthalate. The distribution ratio for this extraction was negligibly small. This finding indicated that the P V C polymer rather than the phthalate ester plasticizer served as the solvent. Evaluation of Distribution Parameters. The fraction of salicylic acid extracted (%E) was found to be proportional to the equilibrium fraction (a ) of the un-ionized neutral form present in the p H range 1.65-3.01, as shown in Table I. To analytically describe the partition Q


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Extraction Using a Polymer as Solvent

Table I. Relation of the Fraction of Salicylic Acid Extracted (%E) to the Equilibrium Fraction (a ) of the Un-ionized Neutral Form in the pH Range 1.65-3.01


Q

pH

%E

1.β5 1.88 2.23 2.49 2.93 3.01

44.8 38.9 34.8 33.1 28.8 26.3

E/oo 0.95 0.92 0.85 0.75 0.53 0.48

0.47 0.42 0.41 0.44 0.55 0.54

process, it was assumed that mass transport into the polymer was limited b y diffusion of solute molecules in the bulk aqueous solution toward the interface. T h e mass exchange near the interface reached steady state in about 2 m i n . Assuming that the effective volumes of the thin layers of water and polymer phases were equal, one can estimate the apparent distribution coefficient (K ) in terms of the observed capacity ratio (fc')r D

K

D

= (n /V )/(n /V ) 0

0

w

= n /n

w

0

w

= k'

(1)

where η is the number of molecules of un-ionized salicylic acid; V is the volume; and subscripts ο and w designate polymer and water phases, respectively. In the water phase, the fraction of un-ionized salicylic acid can be stated as (2)

a = n /n 0

w

s

where n is the sum of salicylic acid and salicylate species in the water phase. The total solute species (n ) in both phases is s

f

(3)

n = n + n t

0

s

E q 1-3 can be combined to yield a linear relationship i n reciprocal parameters: l/n

0

= (1/n,) + (l/n k'a ) t

0

(4)

The peak current (t ) of the analytical signal after back extraction is proportional to the amount extracted: p

ip = kn

Q


(5)

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ORGANIC POLLUTANTS I N WATER

C o m b i n i n g e q 4 and 5 gives


l/i

p

= (l/kn ) + (l/k'kn a ) t

t

(6)

0

B y using experimentally obtained data for 1 m M salicylic acid, a plot of reciprocal analytical signal versus reciprocal oto yielded a linear relationship for the p H range 1.65-3.01. This result supported the solvent extraction model. T h e corresponding estimate of capacity ratio and distribution coefficient using this treatment was 8.5. Selectivity. Several additional analytes were examined. Extraction efficiencies f r o m 1 m M solutions for a 2-min extraction time were as follows: glucose, 2$; salicylic acid, 34?; oxine, 75$; phenol, 22$; and p-cresol, 79$. 8-Hydroxyquinoline a n d p-cresol are significantly more extractable than salicylic acid. 8-Hydroxyquinoline can be back ex tracted either b y basic solution ( p H >12) or acidic solution ( p H

Solvent Extraction Using a Polymer as Solvent with an Amperometric

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