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Influence of pH on Drug Absorption in the Gastrointestinal (GI) Tract.
Work in pairs for this practical but complete the Proforma on your own.
Aims
• To understand and identify acidic and basic functional groups in drug compounds.
• To determine the effect of pH on the solubility of drugs.
• To predict the influence of pH on drug absorption in the GI tract.
Introduction
This experiment illustrates the effect of pH on drug absorption in the human gastrointestinal (GI) tract
(Figure 1). A model using aqueous solutions of different pH in contact with ethyl acetate is used to
represent the GI tract. The stomach has an acidic pH (approx 1.5) and the small intestine has a basic
pH (approx 8); the aqueous solutions represent the aqueous contents of different sections of the
gastrointestinal tract and the ethyl acetate, which is immiscible with water, represents the lipid
component of the tissue lining.
Figure 1. Diagram of the gastrointestinal tract
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This model compares the absorption of drugs by studying how pH influences the ability of different
drugs to move from the aqueous phase into the ethyl acetate. As only unionised molecules are able
to pass through biological membranes the pH of the environment and the extent of ionisation, as
determined by pKa, has a marked influence on drug absorption. The extent of ionisation also
influences the solubility of the drug in the aqueous phase.
Hazards
Treat all of the chemicals in this experiment as toxic irritants. Wear a lab coat, safety glasses and
protective gloves.
Procedure
The drugs examined include aspirin (a weak acid), 3-aminophenol (a weak base), and paracetamol (a
neutral substance). In addition, caffeine and sodium benzoate will be examined.
Using a spatula take a small amount (approximately 5 milligrams which should be no more than 1 or
2 grains of rice) of each of the following compounds:
• aspirin
• 3-aminophenol
• paracetamol
Transfer to three separate sample vials. To each sample vial add 1mL of 0.1M aqueous hydrochloric
acid. Label the sample vials clearly. Obtain a second sample of each compound (again approximately
5mg) and to each vial add 1mL of pH 8.0 buffer. The aqueous mixtures are extracted with ethyl acetate
by adding 1mL of ethyl acetate to each of the vials, and shaking the vials gently. The organic extracts
are spotted onto a silica gel (TLC) plate and viewed under ultraviolet light.
Use a plastic pipette to measure the 1 mL aliquots.
Record and report your observations
For example:
| Strongly coloured spot visible under uv light | Nothing visible |
Complete the experiment by caffeine and sodium benzoate onto the same TLC plate. Record the
procedure of the experiment in you log book and report your observations.
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PPM1: Influence of pH on Drug Absorption in the Gastrointestinal (GI) Tract.
| Student Name: | Date: |
| Lab Partner: | |
| Question 1: Label and Name the functional groups in the aspirin and 3-aminophenol structures given | |
| below. State which groups are acidic and which groups are basic. |
Question 2: Explain in your own words what will happen to the chemical structure of aspirin when it
is in solution at pH 1.0. Compare this to what will happen to the chemical structure of aspirin at pH
8.0. Highlight your answers by drawing appropriate chemical structures for aspirin at each of the pH
values.
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Question 3: In the space below, explain the result you observed with 3-aminophenol at pH 8.0. If
appropriate include a chemical equation which shows the conjugate acid and/or base formed by 3-
aminophenol at pH 8.0. Draw and label the predominant structures of 3-aminophenol at pH 1, pH 7
and pH 10.
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Question 4: In the space below draw the shorthand chemical structures of caffeine, paracetamol and
sodium benzoate.
In addition, Complete the following table for the five compounds studied in this experiment – aspirin,
3-aminophenol, paracetamol, caffeine and sodium benzoate. You should classify the compounds as
acidic, basic or neutral.
Table 1: Classification of Compounds Analysed
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Table 2: Results and Observations
Self Reflection: In the box below write a few sentences explaining how you have used the
feedback from previous lab sessions to improve your answers to the questions for this
experiment.
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PPM1 pH and Drug Absorption. Each Assessment Component Counts Equally to the Overall Grade.
| All functional groups are labelled appropriately and unambiguously. pKa values are sensible and highlighted for the appropriate groups. Credible references for these values are supplied and formatted according to UH guidelines. | All functional groups are labelled appropriately and unambiguously. pKa values are sensible and highlighted for the majority of appropriate groups. Credible references for these values are supplied and formatted according to UH guidelines. | All functional groups are labelled appropriately and unambiguously. pKa values are present but with minor errors and highlighted for the majority of appropriate groups. Credible references for these values are supplied and formatted according to UH guidelines. | All functional groups are labelled appropriately and unambiguously. pKa values are present but with major errors – there may be inappropriate inclusions/omissions. Credible references for these values are supplied and formatted according to UH guidelines. | Most functional groups are labelled appropriately and unambiguously. pKa values are present but with major errors – there may be inappropriate inclusions/omissions. References for these values are supplied and largely formatted according to UH guidelines. | Some functional groups are labelled appropriately and unambiguously. pKa values are present but with major errors . References for these values are supplied but not formatted appropriately and/or are not from a credible source. | Little/No attempt to answer question. |
| Correct, succinct and reasoned account given which is supported by accurately drawn and appropriately labelled chemical structures. Clear indication that the work is written in the student’s own words. | Correct and reasoned account given which is supported by accurately drawn and appropriately labelled chemical structures. Clear indication that the work is written in the student’s own words. | Correct account but reasoning may be unclear. Supported by accurately drawn and appropriately labelled chemical structures. Clear indication that the work is written in the student’s own words. | Correct account but reasoning may be unclear. Minor errors in drawing/labelling chemical structures. Clear indication that the work is written in the student’s own words. | Correct but reasoning flawed. Errors in drawing/labelling chemical structures. Clear indication that the work is written in the student’s own words. | Incorrect account – but reasoning is presented to support statements. Clear indication that the work is written in the student’s own words. | Little/No attempt to answer question. Written work is not student’s own words. |
| Succinct and accurate explanation of observation. Clear, balanced , appropriately labelled equations. Predominant structures are correct, clearly drawn and unambiguous. | Accurate explanation of observation. Clear, balanced , appropriately labelled equations. Predominant structures are correct, clearly drawn and unambiguous. | Accurate explanation of observation. Clear, balanced , appropriately labelled equations. Predominant structures are largely correct, clearly drawn and unambiguous. | Accurate explanation of observation, but reasoning unclear. Largely appropriate equations – minor errors. Predominant structures are largely correct, clearly drawn and unambiguous. | Accurate explanation of observation, but reasoning flawed. Largely appropriate equations – minor errors. Some predominant structures are largely correct, clearly drawn and unambiguous. | Incorrect reasoning but attempts to justify any statement made. Some predominant structures are correct and drawn to a reasonable standard. | Gross errors. Little/No attempt to answer question. |
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| Proforma and Log Book completed in ink. Log Book is correctly laid out with date and title of experiment. Appropriate observations are recorded in a neat methodical manner. | Proforma and Log Book completed in ink. Log Book is correctly laid out with date and title of experiment. Appropriate observations are recorded in a largely neat and methodical manner. | Proforma and Log Book completed in ink. Log Book is mainly correctly laid out (date OR title of experiment missing). Appropriate observations are recorded in a largely neat and methodical manner. | Proforma and Log Book completed in ink. Log Book is not correctly laid out (date AND title of experiment missing). Appropriate observations are recorded in a largely neat and methodical manner. | Proforma and Log Book NOT completed in ink OR Log Book is not correctly laid out (date AND title of experiment missing). Few observations are recorded in a largely neat and methodical manner. | Proforma and Log Book NOT completed in ink AND Log Book is not correctly laid out (date AND title of experiment missing). Few/no observations are recorded in a largely neat and methodical manner. | Fails to bring log book and/or Proforma to session. |
| Student has provided a brief account which shows excellent self reflection and critical awareness. | Student has provided a brief account which shows very good self reflection and critical awareness. | Student has provided a brief account which shows good self reflection and critical awareness. | Student has provided a brief account which shows some self reflection and critical awareness. | Student has provided a brief account which shows a limited appreciation of self reflection and critical awareness. | Student has attempted to complete the section, but demonstrates a lack of appreciation for what is required in self reflection. | Student has not reflected. |
Overall Grade:
Feedback:
Degree Classification: 1ST >70% (A or B) 2:1 60-69% (C) 2:2 50-59% (D) 3RD 40-49% (E) Fail 0-39 (N/F)
PTS Skills A COM IM PS SERP
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