Drugs in solution PDF

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Name (SID): .................................................................Name (SID): .................................................................Name (SID): .................................................................Date: ......................Tutor: .....................................

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PHAR1812 Basic Pharmaceutical Sciences 2020

Name (SID): ................................................................. Name (SID): ................................................................. Name (SID): .................................................................

Date: ...................... Tutor:…………………………………………… Tutor:……………………………………………

Drugs In Solution (DIS) Workshop This workshop relates to the Drugs in Solution lectures and students should revise their lecture notes and bring them to the workshop. The purpose of the workshop is to assist students in meeting a number of the learning outcomes for this topic – the details are included on the following page. The completed report must be submitted to Dropbox by midnight on the day of your workshop in week 10. Revised report with marks and feedback will be provided in week 12.

The data for Questions 1,2 and 4-6 is provided on Canvas.

This workshop is designed to help you achieve the following learning outcomes: 4, 8, 9, 10, 11, 13, 14, 18, 19, 20 and 21.

Drugs in Solution workshop

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PHAR1812 Basic Pharmaceutical Sciences 2020

Exercise 1 The following calculations relate to G and G°. The biochemical reaction FDP

G-3-P + DHAP

was mentioned in lectures. The details of the compounds taking part in the reaction are not important, but G° for this reaction is 26.1 kJ/mol at body temperature (37°C). A solution is prepared at body temperature containing 10^-4 M of both G-3-P and DHAP, and you need to calculate the minimum concentration of FDP that must be present before the reaction will proceed spontaneously in the forward direction. Which equation will you use to carry out this calculation? Why? (0.5 marks)

Carry out the calculation. (1 mark)

Drugs in Solution workshop

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PHAR1812 Basic Pharmaceutical Sciences 2020

Exercise 2 Sodium cholate is a bile salt that plays an important part in the dissolution of cholesterol in the body. Sodium cholate can exist as either a monomer or a dimer in aqueous solution: 2(monomer) i.

dimer

At equilibrium at ………°C, the concentration of the monomer is ……………… M and the concentration of the dimer is ……………… M. Calculate the standard free energy change for the dimerisation process at T°C.

Which equation will you use to carry out this calculation? Why? (0.5 marks) ∆G° = ∆H° - T∆S° = -RTlnKEQ

Carry out the calculation. (1 mark)

ii.

The concentration of the monomer is held constant, and the concentration of the dimer is reduced to one-tenth of its original value. Calculate the value of G, and predict in which direction the reaction will proceed.

Which equation will you use to carry out this calculation? Why? (0.5 marks)

Carry out the calculation. (1 mark)

Drugs in Solution workshop

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PHAR1812 Basic Pharmaceutical Sciences 2020

Exercise 3 Chlorocresol is a common preservative in a number of pharmaceutical preparations. Martindale’s Extra Pharmacopoeia lists its solubilities as 1g in 260mL of water and 1g in 50mL of boiling water. It can be assumed that the temperature of “water” is 25°C, and of “boiling water” is 100°C, and the molecular weight of chlorocresol is 143. 3.87/143 i

Calculate the standard enthalpy change for the dissolution of chlorocresol.

Which equation will you use to carry out this calculation and why? (0.5 marks)

0.02689 Carry out the calculation. (1 mark)

ii

Calculate the standard free energy change for this reaction at T = 25 °C.

Which equation will you use to carry out this calculation and why? (0.5 marks)

Carry out the calculation. (1 mark)

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PHAR1812 Basic Pharmaceutical Sciences 2020

iii.

Calculate the standard entropy change for the dissolution of chlorocresol.

Which equation will you use to carry out this calculation and why? (0.5 marks)

∆G° = ∆H° - T∆S°

Carry out the calculation. (1 mark)

iv

You prepare 40 mL of an eye drop solution which contains 100 mg chlorocresol. Will you need to heat the water above room temperature (25°C) in order to dissolve the chlorocresol? Which equation will you use to carry out this calculation and why? (0.5 marks)

Carry out the calculation. (1.5 marks)

Drugs in Solution workshop

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PHAR1812 Basic Pharmaceutical Sciences 2020

Exercise 4 Some students believe that it is appropriate to use the molar solubility of an ionisable drug in the van’t Hoff isochore. This is incorrect however, as the van’t Hoff isochore requires equilibrium constants. It would be a good idea to refer to your lecture notes while doing these calculations, and to keep focusing on working out the equilibrium constant for each reaction. i.

The solubility of a nonionisable drug (MW = 250 ) in water is 2.45 mg/mL at T1 = 35C. If the enthalpy of solution is 25.5 kJ/mol, calculate its solubility in M at T2 = 50C. (1 mark)

ii.

If the solubility of AgCl is 1.5 x 10-4 g/100mL at T = 20 °C, and the enthalpy of solution is 62.7 kJ/mol, calculate the solubility of AgCl at T = 45C. (1 mark)

Drugs in Solution workshop

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PHAR1812 Basic Pharmaceutical Sciences 2020

Exercise 5 Sheets with the structures of Drugs A-E and their pKa values will be available from your tutor.

i

ii

Using the structures of the drugs and their pKa values, indicate whether each of the following drugs would be largely (ie more than 50%) in the ionised or unionised form at the pH indicated. (1 mark) a.

Drug ….. (………………) at pH …….

b.

Drug ….. (………………) at pH …….

c.

Drug ….. (………………) at pH …….

d.

Drug ….. (………………) at pH …….

Calculate the percentage ionised in each of the following cases. (2 marks) a.

Drug ….. (………………) at pH …….

b.

Drug ….. (………………) at pH …….

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PHAR1812 Basic Pharmaceutical Sciences 2020

c.

Drug ….. (………………) at pH …….

Exercise 6 Calculate the percentage of the drug which would be in the aqueous phase after the following extractions. a.

Drug ….. (………………) in 50 mL aqueous solution at pH ……. extracted with 50 mL ether, given that P = 79 for the unionised form.

What information do you have about partition coefficients? Is it the true or apparent partition coefficient? How do you know? (0.5 marks)

At pH …………., do you need to use the true or apparent partition coefficient to calculate the percentage in the aqueous phase? (0.5 marks)

Drugs in Solution workshop

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PHAR1812 Basic Pharmaceutical Sciences 2020

Carry out the relevant calculation. (1 mark)

b.

Drug …… (……………………..) in 30 mL aqueous solution at pH 12 extracted with 50 mL chloroform, given that P = 860 for the unionised form.

What information do you have about partition coefficients? Is it the true or apparent partition coefficient? How do you know? (0.5 marks)

At pH …….., do you need to use the true or apparent partition coefficient to calculate the percentage in the aqueous phase? (0.5 marks)

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PHAR1812 Basic Pharmaceutical Sciences 2020

Carry out the relevant calculation. (1 mark)

Mark (out of 20):__________________

Date:___________________

Signature:________________________

Feedback

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Drugs in Solution workshop

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