Question

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 ?

Short answer

a.To find the dominating species,

\( \to {\rm{Cr }}(III)\)dominant species at \(pH8:{\rm{Cr}}({\rm{OH}})_2^ + \) and \({\rm{Cr}}{({\rm{OH}})_3}\)

\( \to {\rm{Cr}}({\rm{VI}})\)dominant species at\({\rm{pH}}8:{\rm{CrO}}_4^{2 - }\)

b.The resin was then rinsed with distilled water. The extract and wash liquids containing\({\rm{Cr}}\) (III) were discarded."

c."highly basic" exchanger - contains an unprotonated amine \({ - ^ + }{\rm{N}}{{\rm{R}}_3}\) that would be a stable cation in basic solution.

d. When \(\left( {{\rm{SO}}_4^{2 - }} \right.)\), would be eluted from the exchanger. grows from \(0.05{\rm{M}}(step2)\) to \(0.5{\rm{M}}\) in the buffer

Step-by-step solution

Concept used

In this exercise, we'll utilise a technique to measure \({\rm{Cr }}({\rm{VI}})\)in workplace airborne particulate matter.

Step 2:Define the dominant species of  \({\rm{Cr}}\) (VI) and  \({\rm{Cr}}\) (III) at \({\rm{pH}}8\)

a)

Determine which species of \({\rm{Cr }}({\rm{VI}})\) is the most prevalent.\({\rm{Cr }}({\rm{III}})\)at\({\rm{pH}}8\)

To find the dominating species, we shall employ acid dissociation constants.

\( \to {\rm{Cr }}(III)\)dominant species at \(pH8:{\rm{Cr}}({\rm{OH}})_2^ + \)and \({\rm{Cr}}{({\rm{OH}})_3}\)

\( \to {\rm{Cr}}({\rm{VI}})\)dominant species at\({\rm{pH}}8:{\rm{CrO}}_4^{2 - }\)

Step 3:What is the purpose of the anion exchanger

b)

explain what the anion exchanger is for. "In the Cl2 form, a determined amount of extract was passed via a "highly basic" anion exchanger (Table 26-1). The resin was then rinsed with distilled water. The extract and wash liquids containing \({\rm{Cr}}\)(III) were discarded."

the anion's retention\(\left( {{\rm{CrO}}_4^{2 - }} \right)\)

allowing cation to flow through\(\left( {{\rm{Cr}}({\rm{OH}})_2^ + } \right)andneutral({\rm{Cr}}({\rm{OH}})/{\rm{x}} - {\rm{tex }} > - \{ 3\} 3)\)

\({\rm{Cr }}(III)\)and\({\rm{Cr }}({\rm{VI}})\) are separated.

Step 4:Describe why "strongly basic" anion exchangers are used instead of "weakly basic" anion exchangers:

c)

"weakly basic" exchanger - possesses a protonated amine.If treated to basic solution, \({\rm{:}}{ - ^ + }{\rm{NH}}{{\rm{R}}_2}\)may lose its positive change - it would be deprotonated.

- "highly basic" exchanger - contains an unprotonated amine \({ - ^ + }{\rm{N}}{{\rm{R}}_3}\)that would be a stable cation in basic solution.

Step 5:Explain why \({\rm{Cr }}({\rm{VI}})\)was created

(d)

The anion \(\left( {{\rm{CrO}}_4^{2 - }} \right)\)was eluted in step 4 instead of step 2 - When \(\left( {{\rm{SO}}_4^{2 - }} \right.)\), would be eluted from the exchanger.grows from \(0.05{\rm{M}}(step2)\) to \(0.5{\rm{M}}\)in the buffer.