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Chapter 25: Q40P (page 711)

Use Figure 25-30 for the following questions:

(a) What pH would be best for the separation of benzoic acid, 4-nitrophenol, and 3-methylbenzoic acid?

(b) What pH would be best for the separation of benzoic acid, 3-methylbenzoic acid, and 4-methylaniline?

(c) What pH would be best for separation of 4-nitrophenol, 4-methylaniline, and codeine on a typical C18-silica column?

Short Answer

Expert verified

(a).The value of pH = 2.0 .

(b).pH≈4.0

(c).pH≈7.5

Step by step solution

01

Find pH:

pH value will be best for separation of benzoic acid, 4-nitrophenol and 3 -methyl benzoic acid. To separate benzoic acid, 4 -nitrophenol and 3 -methyl benzoic acid pH2.0 is best. Because of that pH givesretention factor with in0.5≤k≤20.

So the pH is 2.0.

02

Find pH:

pH value is the best for separation of benzoic acid, 4-methylaniline and 3 -methyl benzoicacid.To separate benzoic acid, 4 -methylaniline and 3 -methyl benzoic acid pH approximately 4 is the best.

So the pH is 4.0.

03

Find pH:

pH value will be best for separation of 4-nitrophenol, 4-methylaniline and codeine on a typical C18 silica column. To separate 4-nitrophenol, 4-methylaniline and codeine on a typical C18silica column pH approximately 7.5 is best. So the pH is 7.5.

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Most popular questions from this chapter

(a) Why does mobile phase strength increase as solvent

becomes less polar in reversed-phase chromatography, whereas

mobile phase strength increases as solvent becomes more polar in

normal-phase chromatography?

(b) What kind of gradient is used in supercritical fluid chromatography?

Question: How do additives such as trienthylamine reduce tailing of certain solutes?

a) UHPLC can provide exquisite resolution when run slowly on long columns or rapid separation with reasonaple resolution if short columns are run fast. the drug acetaminophern run on a 50×2.1mmc18UHPLC column has a retention time of 0.63min and a width at half-height of 2.3s.Find the plate number and plate height. How many1.7μm diameter particles places placed dide-by are equal to one theoretical plate?

b) From figure25-3,we expect an optimum plate height of 4μm how many particles placed side-by- side are equal to one theroretical plate? Do you think the column in (a) is being run for maximum resolution or maximum speed?

(a) Make a graph showing retention times of peaks 6, 7, and 8 in Figure 25-12 as a function of %acetonitrile (%B). Predict the retention time of peak 8 at 45% B.

(b) Linear-solvent-strength model: In Figure 25-12, tm = 2.7 min. Compute k for peaks 6, 7, and 8 as a function of %B. Make a graph of log k versus Φ, where Φ= %B/100. Find the equation of a straight line through a suitable linear range for peak 8. The slope is -S and the intercept is log kw. From the line, predict tr for peak 8 at 45% B and compare your answer with (a).

(c) Gradient elution: A linear eluent gradient from 40 to 80% acetonitrile over 30 min is performed on the column in Figure 25-12. Assuming a dwell volume of 0 mL, use your data from (b) to plot the retention factor of peaks 6 and 8 during the gradient. What are the general characteristics of the plot?

(d) Why are the peaks in a gradient separation sharp?

  1. Use equation 25-1to estimate the length of a column required to achieve1.0×104plates if the stationary phase particles size is10.5,5.0,3.0,or1.5μm

  2. If the retention time was 20mins on the 10.0μm particle size column, what is the retention time on the 5.0,3.0,or1.5μmcolumns from part (a)? Assume that flow rate is constant for all columns.

  3. Use equation25-2to estimate the pressure of the column in (a) given that the pressure of the10.0μmcolumn was4.4Mpa

  4. If the flow rate was2.0mL/min , what is the baseline width for the peaks on 10.5,5.0,3.0,or1.5μmcolumns form part (a)?

  5. Which of these column configurations would require a UHPLC instrument?
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