Acid-base properties p of amino acids (pp467)

Transcription

1 PH and buffer solutions(pp207) Acid-base properties p of amino acids (pp467) Mobile: (WeChat) yaobo08@zju.edu.cn aisy/ html Bo YAO Nov, 2017

2 OBJECTIVES To learn how to measure the ph of a solution To understand the operation of buffer systemsstems To study acid-base properties by titration To determine pk values for the titratable groups

3 ph PH is commonly expressed as log[h+] Pure water has [H+]=10-7 and dthus ph=7 Acids have a high [H+] and thus a low ph. (<7) Bases have a low [H+] and thus a high ph. (>7)

4 ph ph is commonly expressed as log[h + ] Pure water has [H + ]=10-7 and thus ph=7. Acids have a high [H + ] and thus a low ph. Bases have a low [H + ] and thus a high ph.

5 ph scale is logarithmic Change in just one unit of scale = tenfold change in H+ concentration.

6 Ways to measure ph Indicator dyes and test t strips Less precise Each indicator is only good for a small ph range (1-2 ph units) But may be good for field usage, or measuring small volumes. phenolphthalein ph

7 Ways to measure ph ph meter Electrode measures H + concentration Must standardize (calibrate) before usage

8 Why is ph important in biology? ph affects solubility of many substances. [A] (mol/l) Initial ph Final ph Dissolved CaCO 3 (g per liter of acid) More calcium carbonate dissolves as ph drops

9 Increased CO 2 increases the ocean s acidity Increases in H+ causes cation displacement and the dissolution of Calcium Carbonate (shell, limestone, etc.)

10 Acid/Base Definitions Brønsted Lowry acid base theory Acids proton donor Bases proton acceptor Johannes Nicolaus Brønsted and Thomas Martin Lowry, independently, formulated the idea that acids are proton (H+) donors while bases are proton acceptors. A proton is really just a hydrogen atom that t has lost it s electron!

11 ABrønsted Lowry Acid is a proton donor A Brønsted Lowry Base is a proton acceptor base acid conjugate acid conjugate base The Brønsted definition means NH 3 is a BASE in water and water is itself an ACID

12 Amphoteric Substances A substance that is amphoteric can act as either an acid or a base. In the previous slide, water acted as an acid. In the following example, water acts as a base. HCl (g) + H 2 O (l) H 3 O + (aq) + Cl - (aq) acid base conj. acid conj. base

13 Conjugate Pairs

14 Buffers Definition: a solution that resists change in ph Typically y a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid form of a chemical Can be adjusted to a particular ph value Le Chatelier Principle Equilibrium Law 化学平衡理论 Bicarbonate Buffer system is important in maintaining proper blood ph

15 PH of buffers For a weak acid, which dissociates as follows: HA H + + A -

16 AMINO ACIDS H 3 N + 3 H O C C O R Each amino acid (except for proline) has: 1. A carboxyl group (-COO - ). 2. An amino group (-NH 3+ ). 3. Side chain ("R-group") bonded to the α-carbon atom.

17 H O H + 3 N C C O H Glycine (Gly or G) H O H O H 3 N + C C O H 3 N + C C O Alanine CH Leucine CH CH(CH 3 or A) 3 2 3) 2 (Ala or A) (Leu or L)

18 TITRATION 1) Volumetric analysis Procedures in which we measure the volume of reagent needed to react with an analyte 2) Titration Increments of reagent solution (titrant) are added d to analyte until reaction is complete. - Usually using a buret Calculate quantity of analyte from the amount of titrant added. Requires large equilibrium constant Requires rapid reaction - Titrant is rapidly consumed by analyte

19 Titrations 3) Equivalence point Quantity of added titrant is the exact amount necessary for stoichiometric reaction with the analyte - Ideal theoretical result Analyte Oxalic acid (colorless) Titrant (purple) (colorless) (colorless) Equivalence point occurs when 2 moles of MnO 4- is added to 5 moles of Oxalic acid

20 Titrations 4) End point What we actually measure - Marked by a sudden change in the physical property of the solution - Change in color, ph, voltage, current, absorbance of light etc. CuCl Titration with NaOH Before any addition of NaOH After the addition of End Point Before any addition of NaOH 8 drops of NaOH

21 Titration curve of amino acids

22 Procedure ph & buffer solution Two students per group 1. Measuring ph of 0.1M HCl (0.1M acetic adic, 0.1M sodium acetate, 0.1 M carbonic acid, 0.1 M sodium bicarbonate, 0.1M ammonia, 0.1M NaOH) with ph paper 2. Measuring ph of 0.1M acetic acid (0.1M sodium acetate, 0.1M sodium carbonic, 0.1M sodium bicarbonate, 0.1M ammonia) by using ph meter. (~10mL) 3. Measuring ph of the following buffer solutions with ph meter & ph paper. a. 10mL M acetic acid + 10mL M sodium acetate b. 2mL o.1m acetic acid + 20mL 0.1M sodium acetate

23 4. Measuring ph of the following buffer solutions with ph meter &phpaper paper 10mL buffer (a) + 1mL 0.1M HCl 10mL buffer (a) + 1mL 0.1M NaOH 10mL buffer (b) + 1mL 0.1M HCl 10mL buffer (b) + 1mL 0.1M NaOH 5. Measuring ph of the following solutions with ph meter & ph paper 10mL distilled water 10mL distilled water + 1mL 0.1M HCl 10mL distilled water + 1mL 0.1M NaOH

24 Procedure Acid-base properties of Amino acids Two students per group 1. Pipet 20mL of 0.1M Glycine solution (ph1.5) into a 100mL beaker 2. Fill a 25mL buret with 0.25M NaOH solution 3. Add NaOH solution from the buret in 1.0mL increments to the beaker. After each increment, stir the contents and then read the ph 4. Continue the titration until reaching ph12 5. If you have enough time, you can try Alanine and Leucine (optional)

25 Notes 1. Using a pipet filler Before inserting the pipet end into the pipet filler, lubricate the glass by rubbing the opening with a drop of water or glycerin

26 Notes 2. Using a buret for titration 1. Clean 2. Rinse 3. Fill 4. Remove bubblesbbl 5. Drain to zero 6. Take initial reading 7. titrate Do not use your mouth when using the pipet. Use the pipet filler youku com/v show/id XMjU3NTQ4MzIw ml?x&sharefrom=android&sharekey=87b41150db e92e d75444

27 Notes 3. Using a ph meter 1. A bulb made from a specific glass 2. Internal electrode 3. Internal solution, usually a ph=7 buffered solution of 0.1 mol/l KCl 4. AgCl 5. Reference electrode, usually the same type as 2 6. Reference internal solution, usually 0.1 mol/l KCl 7. Junction with studied solution 8. Body of electrode htt // k / h /id XNjY ODI OT l#paction

28 Lab report 1. Your and your partner s names and the date 2. Title of the experiment 3. Objectives-what the purpose of the lab is (ph, buffer, titration, etc.) 4. Principle of the experiment 5. Phenomenon (what have you found during the operation) and data record 6. result, data analysis and discussion

29 Q&A