2/ 1 EXERCISE 2: VOLUMETRIC ANALYSIS: ESTIMATION OF HYDROCHLORIC ACID CONCENTRATION IN STOMACH GASTRIC JUICE

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1 2/ 1 EXERCISE 2: VOLUMETRIC ANALYSIS: ESTIMATION OF HYDROCHLORIC ACID CONCENTRATION IN STOMACH GASTRIC JUICE FOCUS QUESTION: What is the hydrochloric acid concentration in gastric juice? OBJECTIVES: 1) To develop expertise in carrying out an acid-base titration. 2) To carry out titration calculations involving acids and bases. 3) To develop an understanding of the experimental methods you use. INTRODUCTION: In volumetric analysis, a reaction is carried out by adding a solution of reactant B from a burette to a solution of A until just sufficient of B has been added to react with all of reactant A. na + mb Products (1) If the concentration C B of solution B is known and the volume V B of solution B is measured, the number of moles of A in solution A can be calculated, provided the equation for the reaction is known. This procedure is known as a titration and the point at which sufficient volume is solution B has been added to solution A to complete the reaction is called the equivalence point or end point. An indicator which changes colour at the equivalence point is often used to indicate the titration end point. The titration 1. During the titration, the solution is run from the burette into the titration flask, which is swirled to keep the contents well mixed. 2. The solution may be run in rapidly at first, but the rate of flow should be decreased as the end point is approached. (Near the end-point, the indicator may be seen to

2 2/ 2 change colour at the place where the solution from the burette, the titrant, is added. 3. Near the end-point, only one drop of solution is added at a time, with constant swirling of the flask. If smaller additions than one drop are desired near the endpoint, partly formed drops on the tip of the burette may be touched off on the inside of the neck of the titration flask and washed down with water from a wash bottle. (It is generally allowed to add any amount of distilled water to a titration flask. Such additions should not alter the end-point and the addition of some water is often desirable). Acid-base titrations In its simplest form, this involves, for, instance, titrating a known volume of a strong acid (e.g. HCl), of unknown concentration, with a standard solution of a strong base (e.g. NaOH). The reaction taking place is: HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(aq) (2) or, when eliminating the spectator ions: H + (aq) + OH - (aq) H 2 O(aq) (3) or H 3 O + (aq) + OH - (aq) 2H 2 O(aq) (4) It can be seen that this acid-base reaction involves the combination of a hydrated H + (H 3 O + ) from the acid with a hydroxide ion, OH -, from the base to form water. The end-point of such a reaction is shown by an indicator, which undergoes colour change when the reaction is complete. In reactions such as above, both the acid and the base are strong.

3 2/ 3 Acid-base indicators Acid-base indicators are organic compounds (dyes) which change colour as a solution changes from acid to alkaline, or vice versa. The indicator used in acid-base reactions may be one of the following: Indicator ph Range Suitable for titration of: Colour in acidic solution Colour in basic solution Methyl orange Strong acid - weak base Red Yellow Methyl red Strong acid- strong base Red Yellow Phenolphthalein Weak acid strong base Colourless Red When titrating a weak acid against a strong base, e.g., ethanoic (acetic) acid against sodium hydroxide, the indicator must show a colour change in the alkaline range of the ph scale; (i.e. ph 8 11), e.g., phenolphthalein. On the other hand, if a weak base like ammonia is titrated against a strong acid, like hydrochloric acid, the indicator used must show its colour change in the acid range (ph 3 5) of the ph scale; e.g., methyl red. No indicator is suitable for titrating a weak acid against a weak base. You should particularly note that the end-point shown by these indicators is not at ph 7, and that at the end-point of the titration the resulting solution is not necessarily neutral. The purpose of a titration is to determine when stoichiometric quantities of the substances involved have reacted (e.g., for this reaction when 1 mole of HCl has reacted completely with 1 mole of NaOH, as the stoichiometry of the reaction between NaOH and HCl is 1:1).

4 2/ 4 PRE-LABORATORY EXERCISES: (To be completed BEFORE coming to the laboratory and handed to your demonstrator UPON ENTERING the laboratory!) 1. If cm 3 of mol dm -3 NaOH are required to completely neutralize cm 3 of gastric juice, calculate the concentration of the hydrochloric acid (in mol.dm -3 and g dm -3 ). (3 marks) (NB: Do not use the algorithm c 1 V 1 = c 2 V 2 for this calculation). (Marks are deducted if units or significant figures are ignored) 2. What is a standard solution? 3. What is the function of an indicator in a titration? 4. Prepare your Report Sheet, including all the required tables. TOTAL = 5 marks SAFETY 1. Wear safety glasses through out the practical. 2. Hydrochloric acid causes severe burns. Handle with extreme care. If it comes to contact with your skin, rinse affected area under running water for a while. 3. Sodium hydroxide is caustic soda and can burn badly if it comes into contact with your skin. If your hands feel soapy or slippery at any point, rinse them well under running water.

5 2/ 5 PROCEDURE: 1. Collect about 100mL of gastric juice and NaOH solutions in clean separate beakers (mark the beakers so that you know which is which). 2. Rinse the burette following the steps outlined in Appendix 1A.2.1, pg Fill your burette with NaOH solution following the steps outlined in Appendix 1A.2.2, pg 24, steps Read (2 decimal places) and record the initial volume of NaOH solution in the burette. (See figure 1A.2, diagram B, Appendix 1, pg 27). 5. Clean your pipette following the steps outlined in Appendix 1A.1.1, pg Transfer ± 20 cm 3 of gastric juice you collected into a clean dry beaker. Use 5 cm 3 portions of this solution to rinse the pipette. Allow the solution to run into the sink and repeat the operation twice more. 7. Pipette cm 3 of gastric juice into a clean 250 cm 3 Erlenmeyer flask following the technique described in figure 1A.1, pg Add 2-3 drops of methyl red indicator to the flask. Place a piece of white paper or white tile under the flask and then titrate gastric juice with NaOH solution. Do not forget to swirl the conical flask to keep the contents well mixed. 9. Carry out a rough titration until the colour changes from red to pale yellow (see figure 1A2, diagram A, Appendix 1, on pg 27 on the handling of the burette). 10. Rinse down residual titrant on flask and cut fractions of a drop using a wash bottle, before the endpoint. 11. Read (to 2 decimal places) and record the volume of NaOH solution titrated. DO NOT START WITH A VOLUME OF 0.00 CM 3. START BETWEEN 0.00 AND 2.00 CM 3. WHY? 12. Empty and rinse your titration flask and carefully repeat the titration. The titration should be repeated until at least 3 successive readings agree to within 0.1 cm 3 of each other.

6 2/ Do not let the solution get below the 50 cm 3 mark. Refill before carrying out the third titration. 14. From the 3 titration volumes, determine an average titration volume. 15. From the average titration volume, calculate the concentration in mol. dm -3 of gastric juice. 16. Now rinse out the burette 3 or 4 times with distilled water and fill it with NaOH. Titrate the gastric juice with NaOH. Do one titration only. Explain your titration volume in relation to those obtained in the previously. REPORT SHEET Read the information on page 7 of this manual to determine what information should appear on the top of your report sheet. Copy this table below into your report sheet. Volume of gastric juice pipetted: cm 3 Concentration of standard NaOH (given): mol dm -3 Table1: Volume of standard NaOH solution used in titration (Burette readings to two decimal places) Titration Rough titration Titration 1 Titration 2 Titration 3 Initial / cm 3 Final / cm 3 Difference / cm 3 (3 marks) Average titration volume =.. (Marks are deducted if units are missing or if decimal places are incorrect)

7 2/ 7 Concentration of HCl in gastric juice (Marks are deducted if units or significant figures are ignored) 1. Calculate the concentration of gastric juice in mol. dm -3. (3 marks) (NB: Do not use the algorithm c 1 V 1 = c 2 V 2 for this calculation). 2. Comment on the precision of your titres (i.e. titration volumes). 3. Obtain the value for the exact concentration of gastric juice from your demonstrator. Comment on the accuracy of your calculated gastric juice concentration by comparing it with the exact concentration obtained from your demonstrator. 4. Mention one source of error that may have occurred resulting in a concentration value different from your demonstrator. 5. Convert the concentration of HCl calculated in 1 above to g. dm (a) Explain how the titration volume in step 16 differs with the other 3 obtained previously. (2 marks) (b) How does this in turn affect the concentration of gastric juice calculated? 7. Why is it important to wipe the outside of the burette after filling it with solution? 8. Why is it not advisable to start with a volume of 0.00 cm 3 when titrating? (2 marks) 9. Always place a piece of white paper under the flask when titrating. Why? 10. Record the burette readings in figure 1A3, from diagram E to H in Appendix 1, pg 28. (2 marks) (Marks are deducted if units and decimal places are ignored) Pre-lab exercise = 5 marks Lab report = 20 marks Total = 25 marks