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Chapter 28: Q24P (page 792)

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

Expert verified

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

01

Concept used

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

02

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\({\rm{pH}}8:{\rm{CrO}}_4^{2 - }\)

03

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.

04

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.

05

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.

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Most popular questions from this chapter

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.")

Barium titanate, a ceramic used in electronics, was analyzed by the following procedure: Into a Pt crucible was placed \(1.2\;{\rm{g}}\)of \({\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3}\) and \(0.8\;{\rm{g}}\)of \({\rm{N}}{{\rm{a}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7}\)plus \(0.3146\;{\rm{g}}\)of unknown. After fusion at \({1000^\circ }{\rm{C}}\)in a furnace for\(30\;{\rm{min}}\), the cooled solid was extracted with \(50\;{\rm{mL}}\)of\(6{\rm{MHCl}}\), transferred to a \(100 - {\rm{mL}}\) volumetric flask, and diluted to the mark. A \(25.00 - {\rm{mL}}\)aliquot was treated with \(5\;{\rm{mL}}\)of \(15\% \)tartaric acid (which complexes \({\rm{T}}{{\rm{i}}^{4 + }}\)and keeps it in aqueous solution) and \(25\;{\rm{mL}}\)of ammonia buffer,\({\rm{pH}}9.5\). The solution was treated with organic reagents that complex\({\rm{B}}{{\rm{a}}^{2 + }}\), and the \({\rm{Ba}}\)complex was extracted into \({\rm{CC}}{{\rm{l}}_4}.\)After acidification (to release the \({\rm{B}}{{\rm{a}}^{2 + }}\) from its organic complex), the \({\rm{B}}{{\rm{a}}^{2 + }}\)was backextracted into\(0.1{\rm{MHCl}}\). The final aqueous sample was treated with ammonia buffer and methylthymol blue (a metal ion indicator) and titrated with \(32.49\;{\rm{mL}}\) of \(0.01144{\rm{M}}\)EDTA. Find the weight per cent of Ba in the ceramic.

Many metals in seawater can be preconcentrated for analysis by coprecipitation with\({\rm{Ga}}{({\rm{OH}})_3}\). A \(200 - \mu {\rm{L}}\)HCl solution containing \(50\mu {\rm{g}}\)of \({\rm{G}}{{\rm{a}}^{3 + }}\)is added to \(10.00\;{\rm{mL}}\) of the seawater. When the \({\rm{pH}}\)is brought to \(9.1\)with\({\rm{NaOH}}\), a jellylike precipitate forms. After centrifugation to pack the precipitate, the water is removed and the gel is washed with water. Then the gel is dissolved in \(50\mu {\rm{L}}\) of \({\rm{HN}}{{\rm{O}}_3}\)and aspirated into an inductively coupled plasma for atomic emission analysis. The preconcentration factor is\(10\;{\rm{mL}}/50\mu {\rm{L}} = 200\). The figure shows elemental concentrations in filtered and unfiltered seawater as a function of depth near hydrothermal vents.

(a) What is the atomic ratio (Ga added): (Ni in seawater) for the sample with the highest concentration of\({\rm{Ni}}\)?

(b) The results given by gray lines were obtained with seawater samples that were not filtered prior to coprecipitation. Colored lines are from filtered samples. Results for Ni do not vary between the two procedures, but results for Fe vary. Explain what this means.

If 105 particles are taken, what is the relative standard deviation of each measurement?

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?
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