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Chapter 28: Q15P (page 791)

To pre-concentrate cocaine and benzoylecgonine from river water described at the opening of this chapter, solid-phase extraction was carried out at \({\rm{pH}}\,\,2\) using the mixed-mode cation-exchange resin in Figure 28-19. After passing \(500\;{\rm{mL}}\)of river water through \(60{\rm{mg}}\)of resin, the retained analytes were eluted first with \(2\;{\rm{mL}}\)of \({\rm{C}}{{\rm{H}}_3}{\rm{OH}}\)and then with \(2\;\,\,{\rm{mL }}of\,\,\,2\% \) ammonia solution in\({\rm{C}}{{\rm{H}}_3}{\rm{OH}}\). Explain the purpose of using \({\rm{pH}}2\) for retention and dilute ammonia for elution.

Short Answer

Expert verified

At pH 2: amine is protonated and carboxylic acid is neutral, so analytesare retained by cation exchanger.At high \({\rm{pH}}\) the amine would be neutral and carboxylic acid (carboxylate) would be negative,the anion isn't retained by the cation exchange column and would be eluted with methanol

Step by step solution

01

Definition of\({\rm{pH}}\).

  • \(pH\) is a scale used in chemistry to quantify the acidity or stipulation of awaterless result. It's preliminarily denoted as" eventuality of hydrogen" (or" power of hydrogen").
  • Acidic results (those containing a larger attention of \(H\) ions) have a lower \(pH\) than introductory or alkaline results.
  • The \(pH\) scale is logarithmic, indicating the attention of hydrogen ions in a result in inverse order.
02

Determine the purpose of using \({\rm{pH}}2\) for retention and dilute ammonia for elution.

  • In this task we will explain the purpose of using\({\rm{pH}} = 2\) for retention and also dilute ammonia for elution.
  • Here solid- phase extraction was carried out at\({\rm{pH}} = 2\)via mixed-mode cation-exchange resin.
  • Considering that cocaine is an amine base it would be a cation at low\({\rm{pH}}\)such as\({\rm{pH}} = 2\)in our case and also it would be neutral in ammonia.
  • The cocaine cation at \({\rm{pH}} = 2\)would be retained by cation-exchange resin and the neutral cocaine would be easily eluted by methanol
  • Benzoylecgonine (metabolite of cocaine) contains both amine and carboxylate functionality which is seen on the picture below

  • At \({\rm{pH}} = 2\) the amine would be protonated and carboxylic acid would be neutral - the compound would be retained by the cation exchange column
  • Whereas, at high \({\rm{pH}}\) the amine would be neutral and carboxylic acid(carboxylate) would be negative - the anion isn't retained by the cation exchange column and would be eluted with methanol.

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Most popular questions from this chapter

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.)

When you flip a coin, the probability of its landing on each side is \(p = q = \frac{1}{2}\)in Equations 28-2 and 28-3. If you flip it \(n\)times, the expected number of heads equals the expected number of tails \( = np = nq = \frac{1}{2}n.\)The expected standard deviation for \(n\)flips is\({\sigma _n} = \sqrt {npq} \). From Table 4-1, we expect that \(68.3\% \)of the results will lie within \( \pm 1{\sigma _n}\) and \(95.5\% \)of the results will lie within\( \pm 2{\sigma _n}\).

(a) Find the expected standard deviation for the number of heads in \({\bf{1000}}\) coin flips.

(b) By interpolation in Table 4-1, find the value of \(z\)that includes \(90\% \)of the area of the Gaussian curve. We expect that \(90\% \)of the results will lie within this number of standard deviations from the mean.

(c) If you repeat the\({\bf{1000}}\)coin flips many times, what is the expected range for the number of heads that includes\(90\% \) of the results? (For example, your answer might be, "The range \({\bf{490}}\) to \({\bf{510}}\) will be observed \(90\% \)of the time.")

Why is it advantageous to use large particles \(\left( {{\bf{50}}{\rm{ }}\mu {\bf{m}}} \right)\) for solid phase extraction, but small particles \(\left( {{\bf{5}}{\rm{ }}\mu {\bf{m}}} \right)\) for chromatography?

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 ?

Barbital can be isolated from urine by solid-phase extraction with\({{\bf{C}}_{18}} - \)silica. The barbital is then eluted with\({\bf{1}}:{\bf{1}}\) vol/volacetone: chloroform. Explain how this procedure works.
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