Separating a Weakly Acidic Analyte from a Weakly Basic Interferent

Imagine  you have an aqueous sample containing a weak acid analyte, HA, and a weak base interferent, HI.  The following information about HA, HI, the method of analysis, and the sample are available to you:

Ka,HA = 1.00 x 10-3 Kb,HI = 1.00 x 10-7 KHA,HI = 0.500   [HI]o/[HA]o = 10

(a) What is the expected percent relative error for the quantitative analysis of HA if no attempt is made to separate the analyte and interferent?

(b) One way to separate the analyte and interferent is to use a liquid-liquid extraction.  As a starting point, evaluate the efficiency of a single extraction using 50.0 mL of sample buffered to a pH of 5.0, and 50.0 mL of an organic solvent for which KD,HA is 50.0 and KD,HI is 100.0.  Be sure to calculate the extraction efficiency for both the analyte and the interferent.

(c) In which phase is the analyte preferentially found?  This is the phase in which we will isolate and analyze the analyte.

(d) Calculate the recoveries for the analyte and interferent in the phase in which the analyte is enriched.  What is the percent relative error for the quantitative analysis after this extraction?

(e) To improve the separation you might consider the following modifications: make the pH of the aqueous phase more acidic; make the pH of the aqueous phase more basic; use a larger portion of organic solvent; use a smaller portion of organic solvent; use more sample; use less sample; perform more extractions.  Which of these do you think will improve the separation.  Be sure that you have good reasons for your decisions.

(f) Based on your answer to the previous question, design a modified separation that will produce a percent relative error of less than 5%.

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