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Pharmaceutical Analytical Chemistry

Pharmaceutical Analytical Chemistry. 213 PHC Dr. Mona AlShehri 4 th Lecture. Acid-Base Titrations (1). (1) Gary D. Christian, Analytical Chemistry, 6 th edition. By the end of the lecture you should be able to:. Define buffer and buffer ratio.

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Pharmaceutical Analytical Chemistry

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  1. Pharmaceutical Analytical Chemistry 213 PHC Dr. Mona AlShehri 4th Lecture

  2. Acid-Base Titrations (1) (1) Gary D. Christian, Analytical Chemistry, 6th edition.

  3. By the end of the lecture you should be able to: • Define buffer and buffer ratio. • Calculate the pH of acidic and basic buffers. • Describe the indicator theory. • Select a suitable indicator for a particular reaction.

  4. Buffers

  5. Definition • A solution that resists change in pH when a small amount of an acid or base is added • A mixture of a weak acid and its salt or a weak base and its salt at predetermined concentrations or ratios

  6. Why do we use buffer? • Maintaining the pH for a reaction at an optimum value • Keeping the pH constant

  7. Buffer of weak acid HA H+ + A- (Weak acid) (Salt) Apply Henderson-Hasselbalchequation pH = pKa + log [A-] / [HA] pH = pKa + log [salt] / [weak acid]

  8. Problem 1: • Calculate the pH of a buffer solution containing 0.1M acetic acid and 0.1M sodium acetate (Ka = 1.75 x 10-5)

  9. Buffer of weak base B + H+ BH+ Weak base salt Applying Henderson-Hasselbalch equation pOH = pKb + log [BH+]/ [B] or pH = pKa + log [B] / [BH+]

  10. Problem 2: • Calculate the pH of a solution containing 0.07M NH3 and 0.08M NH4Cl (pKb = 4.76)

  11. Buffer Ratio The ratio of salt / acid [salt] / [acid] = 1 pH = pKa By changing the buffer ratio, buffers of different pH values can be prepared

  12. HomeworkWhat is the pH of the following substances?

  13. Indicator

  14. How to detect the end point of a reaction? The point at which the reaction is observed to be complete is called the end point The end point can be detected by adding an indicator to the solution and visually detect a color change

  15. Indicators • It is a weak acid or weak base that is highly colored • The color of the ionized form is markedly different from that of the unionized form

  16. Weak acid indicator HIn H+ + In-   unionized ionized form form pH = pKIn + log [In-] / [HIn] (Henderson-Hasselbalch equation)

  17. [In-] / [HIn] = 1/10 (only color of unionized form is seen) pH = pKa + log (1/10) = pKa – 1 [In-] / [HIn] = 10/1 (only color of ionized form is seen) pH = pKa + log (10/1) = pKa + 1

  18. The pH in going from one color to the other has changed from pKa- 1 to pKa+ 1 (a pH change of 2 units) Most indicators require a transition range of about 2 pH units

  19. At midway of transition range, the concentration of the two forms are equal, and the pH = pKa The pKa of the indicator should be close to the pH of the equivalence point

  20. Weak base indicator At midway of transition range, the pOH = pKb A weak base indicator should be selected such that pOH = pKb

  21. A good indicator must have the following properties: • The color change must be easily detected. • The color change must be rapid. • The indicator molecule must not react with the substance being titrated. • To minimize errors, the indicator should have a pKin that is within one pH unit of the expected pH at the equivalence point of the titration.

  22. pH ranges and colors of some common indicators

  23. Natural IndicatorRed cabbage juice contains a mixture of substances whose color depends on the pH value. Each test tube contains a solution of red cabbage juice in water, but the pH of the solutions varies from pH = 2 - 11

  24. Homework:Make your own indicator • You can make a very effective pH indicator in a few steps with hot water and red cabbage leaves. Follow the steps below: Record your reu results

  25. Summary • Buffer Definition. • buffer ratio. • pH Calculation of weak acid and weak base buffers. • The indicator theory. • Weak acid and weak base indicators.

  26. Thank you

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