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Chapter 28: Q20P (page 791)

(a) Explain how dispersive liquid-liquid microextraction reduces the use of solvent in comparison with liquid-liquid extraction.

(b) What is the purpose of the disperser solvent, which is used in much greater volume than the extraction solvent?

Short Answer

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(a)How dispersive liquid-liquid microextraction reduces the use of solvent in comparison with liquid-liquid extraction is explained.

(b)The purpose of the disperser solvent is explained.

Step by step solution

01

Derivation of liquid-liquid extraction and dispersive liquid-liquid microextraction.

  • The extraction technique can be used to purify compounds or separate compound mixtures, such as isolating a product from a reaction mixture (known as an extractive work-up). It can also be used to isolate natural products, such as caffeine from tea leaves.
  • Dispersive liquid-liquid microextraction (DLLME) is a quick and easy method for extracting and purifying organic compounds found in trace amounts in aqueous samples.
02

Working of liquid-liquid extraction and dispersive liquid-liquid microextraction.

a) Liquid-liquid extraction:

Liquid-liquid extraction would use of organic solvent to extract aqueous phase via continuous distillation.

Dispersive liquid-liquid microextraction:

Whereas, dispersive liquid-liquid microextraction would use of immiscible organic solvent and also of dispersant solvent in order to make cloudy emulsion for extraction

How the dispersive liquid-liquid microextraction reduces the use of solvent in comparison with liquid-liquid extraction is explained.

b) Disperser solvent can be mixed with both aqueous and organic phases, thereby lowering interfacial energy and also it would permit the formation of high-surface-area emulsion * rapid mass transfer

Hence the purpose of the disperser solvent, which is used in much greater volume than the extraction solvent is explained.

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Most popular questions from this chapter

Acid-base equilibria of Cr(III) were summarized in Problem 10-36. Cr(VI) in aqueous solution above pH 6 exists as the yellow tetrahedral chromate ion, \({\rm{CrO}}_4^{2 - }.\)Between\({\rm{pH}}2\)and \(6,{\rm{Cr}}\)(VI) exists as an equilibrium mixture of\({\rm{HCrO}}_4^ - \) and orange-red dichromate,\({\rm{C}}{{\rm{r}}_2}{\rm{O}}_7^{2 - }.{\rm{Cr}}({\rm{VI}})\) is a carcinogen, but \({\rm{Cr }}(III)\)is not considered to be as harmful. The following procedure was used to measure\({\rm{Cr }}({\rm{VI}})\) in airborne particulate matter in workplaces.

1. Particles were collected by drawing a known volume of air through a polyvinyl chloride filter with \(5 - \mu {\rm{M}}\)pore size.

2. The filter was placed in a centrifuge tube and \(10\;{\rm{mL}}\)of \(0.05{\rm{M}}{\left( {{\rm{N}}{{\rm{H}}_4}} \right)_2}{\rm{S}}{{\rm{O}}_4}/0.05{\rm{MN}}{{\rm{H}}_3}buffer,{\rm{pH}}8,\) were added. The immersed filter was agitated by ultrasonic vibration for\(30\;{\rm{min}}\)at \({35^\circ }{\rm{C}}\)to extract all \({\rm{Cr }}(III)and{\rm{Cr}}\)(VI) into solution.

3. A measured volume of extract was passed through a "strongly basic" anion exchanger (Table 26-1) in the \({\rm{C}}{{\rm{l}}^ - }\)form. Then the resin was washed with distilled water. Liquid containing \({\rm{Cr}}\)(III) from the extract and the wash was discarded.

4. Cr(VI) was then eluted from the column with\(0.5{\rm{M}}{\left( {{\rm{N}}{{\rm{H}}_4}} \right)_2}{\rm{S}}{{\rm{O}}_4}/0.05{\rm{MN}}{{\rm{H}}_3}\) buffer, \({\rm{pH}}8,\)and collected in a vial.

5. The eluted \({\rm{Cr}}\)(VI) solution was acidified with \({\rm{HCl}}\)and treated with a solution of 1,5 -diphenylcarbazide, a reagent that forms a colored complex with Cr(VI). The concentration of the complex was measured by its visible absorbance.

(a) What are the dominant species of \({\rm{Cr}}\)(VI) and \({\rm{Cr}}\)(III) at\({\rm{pH}}8\)?

(b) What is the purpose of the anion exchanger in step 3 ?

(c) Why is a "strongly basic" anion exchanger used instead of a "weakly basic" exchanger?

(d) Why is Cr(VI) eluted in step 4 but not step 3 ?

EXAMPLE- Particles designated \(50/00\)mesh pass through a 50 mesh sieve bou are retained by a lo0 mesh sieve. Their size is in the range 0.150-0.300 mm.

does not pass is retained for your sample. This procedure gives particles whose diameters are in the range \(0.85 - 1.18\;{\rm{mm}}.\) We refer to the size range as \(16/20{\rm{mesh}}.\)

Suppose that much finer particles of \(80/120\)mesh size (average diameter \( = 152\mu {\rm{m}},\) average volume\( = 1.84\;{\rm{nL}}\)) were used instead. Now the mass containing \({10^4}\) particles is reduced from \(11.0to0.0388\;{\rm{g}}.\) We could analyze a larger sample to reduce the sampling uncertainty for chloride.

The following wet-ashing procedure was used to measure arsenic in organic soil samples by atomic absorption spectroscopy: A 0.1- to \({\bf{0}}.{\bf{5}} - \)g sample was heated in a \({\bf{150}} - {\bf{mL}}\) Teflon bomb in a microwave oven for \(2.5\;{\rm{min}}\) with \(3.5\;{\rm{mL}}\)of\(70\% \,\,\,{\rm{HN}}{{\rm{O}}_3}\). After the sample cooled, a mixture containing \(3.5\;{\rm{mL}}\)of \(70\% \,\,\,{\rm{HN}}{{\rm{O}}_3},1.5\;{\rm{mL}}\) of\(70\% \,\,{\rm{HCl}}{{\rm{O}}_4}\), and \(1.0\;{\rm{mL}}\) of \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)was added and the sample was reheated for three \({\bf{2}}.{\bf{5}} - {\bf{min}}\) intervals with 2 -min unheated periods in between. The final solution was diluted with \(0.2{\rm{M}}\,\,\,{\rm{HCl}}\)for analysis. Why was \({\rm{HCl}}{{\rm{O}}_4}\) not introduced until the second heating?

How does solid-supported liquid-liquid extraction differ from solid-phase extraction?

From their standard reduction potentials, which of the following metals would you expect to dissolve in \({\rm{HCl}}\)by the reaction\({\rm{M}} + n{{\rm{H}}^ + } \to {{\rm{M}}^{n + }} + \frac{n}{2}{{\rm{H}}_2}:{\rm{Zn}},{\rm{Fe}},{\rm{Co}},{\rm{Al}},{\rm{Hg}},{\rm{Cu}},{\rm{Pt}}\),\({\bf{Au}}\)?

(When the potential predicts that the element will not dissolve, it probably will not. If it is expected to dissolve, it may dissolve if some other process does not interfere. Predictions based on standard reduction potentials at \({\bf{2}}{{\bf{5}}^{^{\bf{o}}}}C\) are only tentative, because the potentials and activities in hot, concentrated solutions vary widely from those in the table of standard potentials.)
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