Analytical Chem Midterm

Front 1

In Lab 3 (ion-selective potentiometric determination of chloride by standard addition), why was the chloride sample dissolved in 1 F KNO3?

Back 1

The large concentration of KNO3 maintained ionic strength at a constant level, and
therefore kept activity coefficients constant. The KNO3 also acted as the electrolyte for
the cell. Another value in keeping the solution at a relatively high and constant
concentration is that junction potential will be constant.

Front 2

In Lab 2 (potentiometric titration of Fe(II) by Ce(IV)), why were the Ce(IV) solutions kept acidic?

Back 2

The acidic conditions prevented hydrolysis by the cerium (IV) and subsequent precipitation of the hydroxide. The acid also serves as the electrolyte for the cell. (Some of you noted correctly that Ce(IV) solutions are indefinitely stable in sulfuric acid, as opposed to HCl, for example.)

Front 3

In Lab 4 (acid/base titration), why didn’t we just make up a standard solution of NaOH from solid reagent?

Back 3

Solid NaOH is highly hygroscopic, and it reacts rapidly with CO2 in the air to form NaHCO3 and Na2CO3. As a result, reagent grade NaOH(s) cannot be obtained. Furthermore, the formation of carbonate represents a conversion of a strong base into a weak base.

Front 4

Why is it important to rinse and then dab off excess water from the pH electrode before putting it in the standard “buffer” solutions?

Back 4

Some of the standard pH solutions are not actually buffered (despite their being called buffers), so any contamination or dilution can change the pH.

Front 5

Briefly describe the systematic errors associated with making potentiometric determinations.

Back 5

1- Junction potential at the liquid interface between the sample solution and the reference electrode.
2- Ohmic potential due to small current flowing through the solution.
3- Activity coefficients that are unknown.
4- Complexation of the analyte, changing its electro-activity.
5- Change in temperature in the titration vessel during the experiment.
6- Presence of more than one electro-active species.
7- For ion-selective electrodes, having a Nernst factor, S, that is not equal to the ideal value (that is, a non-unity electromotive efficiency).

Front 6

What potentiometric errors does calibration eliminate?

Back 6

Junction potential at the liquid interface between the sample solution and the reference electrode.
Ohmic potential due to small current flowing through the solution.
For ion-selective electrodes, having a Nernst factor, S, that is not equal to the ideal value (that is, a non-unity electromotive efficiency).

Front 7

What potentiometric errors does titration overcome in determining concentration of an analyte?

Back 7

all of them

Front 8

In Lab 2, the observed confidence interval for the team results for the determination of the cerium sulfate unknown was usually greater than the expected error. Why was this so?

Back 8

This reflects the difference between repeatability and reproducibility. Any one person can often repeat a measurement using the same solutions and the same equipment with higher precision than can be achieved under varied conditions. In the case of Lab 2, each team member probably made certain systematic errors repeatably while the other team members made their own, different repeatable systematic errors. The difference in these errors shows up in the team precision, but not in individual precision. Thus, reproducibility is a better measure of the true uncertainty of a measurement than is repeatability.
In some cases, the difference pointed up a serious error in procedure by one or more members of the team.

Front 9

In Lab 3, your standard addition plot consisted of four points, to which you fit a straight line (of general form y = mx + b). How many degrees of freedom does this fit have?

Back 9

The two constants that must be determined from the data to define the line (m and b) each reduce the number of degrees of freedom, so two degrees of freedom are lost. Therefore, d.o.f. = 4 – 2 = 2.

Front 10

The data for some runs showed a lot of deviation from the straight line; a “better” fit (that is, one with a smaller sy) could have been obtained with a quadratic curve (y = ax2 + bx + c). Or, a “perfect” fit (sy = 0) could have been achieved for a third-order polynomial (y = ax3 + bx2 + cx + d). Would using these higher fits be a good idea? Explain.

Back 10

One degree of freedom is lost for every parameter (constant) that must be determined from the fit. Three constants are required to describe a quadratic curve, so the number of degrees of freedom that remain for this fit are 4−3 = 1, meaning a much larger t value and greater uncertainty for any values derived from the line. For the third-order fit, no degrees of freedom would be left, so the uncertainty would be infinite for this “perfect” fit.

Front 11

What are the advantages of cerium(IV) sulfate solution as an analytical oxidizing agent?

Back 11

solutions are stable indefinitley
reduces only to Ce(III).
does not oxidize chloride to any significant extent during the course of a titration.

Front 12

Because FEDS is unstable with respect to oxidation by O2 you should...

Back 12

not heat the solution above 50 C for an extended time.
dissolve it in de-gassed 1 F sulfuric acid.

Front 13

Why should you not wait too long between additions of Cl- solution to the beaker with your electrodes?

Back 13

Because Cl- drains slowly from the SCE

Front 14

Why does the oxidizing strength of ceric ion decrease in going from nitric to sulfuric to hydrochloric acid solutions (the formal reduction potential becomes less positive)?

Back 14

because ceric ion forms complexes
of increasing stability with nitrate, sulfate, and chloride.

Front 15

What was the KNO3 solution in Lab 3 used for?

Back 15

to keep ionic strength constant and, consequently, to keep the
activity coefficient of Cl⎯ constant.

Front 16

Why should you check the nerstian response of your ion selective electrode?

Back 16

because the slope may differ significantly from the ideal, and also because the slope changes as the electrode ages.