HCl and Na 2 CO 3. One 250 ml volumetric flask One 50 ml buret ph 7.00 and ph standard buffer solution (shared by groups)

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1 Potentiometric Titration of Collect HCl and Na 2 CO 3 One 250 ml volumetric flask One 50 ml buret ph 7.00 and ph standard buffer solution (shared by groups) Two 125 ml Erlenmeyer flasks (check if broken) One pipet filler (check for gas leak) One magnetic stir bar (from GTA) (2016/03/12 revised) 1

2 I. Objective: Objective & Skills To prepare and standardize 0.1 M HCl solution To determine the equivalence point of HCl/Na 2 CO 3 titration by using the electric potential method To determine the dissociation constants, K a, of carbonic acid II. Skills: To use analytic balance for weighing chemicals To prepare solutions by volumetric flask To operate graduated pipet and titration To calibrate and operate ph-meter To determine the equivalence point by using titration curves 2

3 Standardization of Hydrochloric Acid Primary standard: larger molar mass and high purity Common primary - standard acid, potassium hydrogen phthalate (KHP) - standard base, sodium carbonate (Na 2 CO 3 ) Standardize HCl(aq) with Na 2 CO 3 HCl (aq) + CO 3 2- (aq) HCO 3 - (aq) + NaCl (aq) HCl (aq) + HCO 3 - (aq) H 2 CO 3(aq) + NaCl (aq) mole of Na 2 CO 3 VHCl MHCl

4 ph Determining the Equivalence Point The ph value of the reacting solution changes significantly near the equivalence point Base on the color change of the acid-base indicator or monitoring the change in ph values to determine the equivalence point Equivalence point Phenolphthalein Bromothymol blue Methyl orange NaOH 滴定體積 Weak acid / strong base titration curve Indicator Methyl orange Bromothymol blue Phenolphthalein Acid form ph range Basic form Red 3~4 Orange Yellow 6~7 Blue Colorless 8~10 Pink red 4

5 Choosing a Suitable Acid-Base Indicator In a Titration Acid-base indicator is a weak organic acid or base Weak acid (HIn) and its conjugate base (In - ) with different colors HIn + H 2 O H 3 O + + In - Acidic Color HIn Color Change Range Basic Color In - pk a - 1 pk a + 1 ph increases Predict the ph range of the equivalence point Strong acid/weak base titration: ph < 7 Weak acid/strong base titration: ph > 7 Strong acid/strong base titration: ph = 7 Choose the appropriate indicator to match the end-point with the equivalence point 5

6 ph/ V ph Equivalence Point Equivalence point 1. Acid-base titration curve The equivalence point is the point on the curve with the maximum slope 2. First derivative of titration curve The maximum point is the equivalence point 3. Second derivative of the titration curve 0 crossing is the equivalence point 2 ph/ V NaOH (aq) V (ml) V 1 (ml) 150 Equivalence 100 point A Equivalence point V 2 (ml) B 6

7 ph K a Acid Dissociation Constant of a At Half-Equivalence point [HA] = [A - ] [H 3 O + ] = K a A H O 3 HA Weak Acid Weak acid / strong base titration curve Therefore, ph = pk a For example Equivalence volume = ml Half-equivalence volume = ml V = 18.00, ph = 4.60 V = 19.10, ph = 4.65 ph of the half-equivalence volume = 4.63 pk a = ph = 4.63 K a = ph = pk a Half-equivalence volume V (NaOH, ml) 7

8 Components of ph Meter ph meter consists of three parts: ph electrode Reference electrode (usually made of silver and silver chloride), the potential is a fixed value Indicator electrode (usually made of glass), the potential changes when the concentration of H + varies Thermoprobe: used to measure the temperature of soln Voltmeter: used to measure the potential difference between the two electrodes 8

9 Cell Potential and ph Value E m = K 2.3RT(pH)/nF E m = mt(ph) + K Second standard solution First standard solution The Relationship Between Measured Cell Potential and ph value E m : measured cell potential K: constant, determined by the type of electrode used R: gas constant T: absolute temperature of the solution ph: ph value of solution n: number of moles of electrons transferred through the electrodes during a reaction F: Faraday constant 9

10 Outline of Procedures (I) Prepare HCl(aq) (II) Standardization of HCl with Na 2 CO 3 (III) Calibrate the phmeter and titration of unknown 10

11 Procedure I: Prepare 0.1 M HCl (1) Take 4.17 ml of 6 M HCl (2) Place in 250 ml volumetric flask 6 M HCl (3) Add water till mark to dilute (1) Invert the flask several times (2) Mix thoroughly (3) Pour into beaker (1) Use approx. 5 ml of 0.1 M HCl (2) Rinse buret twice and fill with solution Read initial volume of buret (V i ) to 0.01 ml 11

12 Procedure II: Standardize HCl with Na 2 CO 3 Measure ca. 0.1~0.2 g Na 2 CO 3 with analytical balance Place in a 125 ml Erlenmeyer flask Record accurate weight (1) Dissolve with 30 ml distilled water (2) Add 3 d. of bromocresol green (3) Titrate with 0.1 M HCl to appear green Boil the soln for 2-3 min. till soln turn to blue color (Use stir bar to avoid bumping) After cooling, continue titrating to soln appear yellow 藍色 綠色 黃色 Record V i and V f Carry out a duplicate test Calculate average concentration of HCl(aq) 12

13 Procedure III: Calibrate ph-meter NT$ 4500!! Push the POWER button, warm up for 10 minutes Remove the electrode cap by rotating it Place thermoprobe into solution Use washing bottle to clean the electrode Blot dry with a tissue Setup of ph meter HOLD Press HOLD when cleaning the electrodes and the screen will freeze 13

14 Procedure III: Calibrate ph-meter (1) Collect standard buffer solution (2) Start calibrating ph meter Calib button ph 10.0 Slope button ph 7.0 Immerse thermoprobe and electrodes into ph 7.00 buffer solution Adjust Calib button until meter says 7.00 ph 10.0 Clean thermoprobe and electrodes Immerse in ph 10.0 buffer solution Adjust Slope button until meter says

15 Manipulate ph Meter Do not take the electrode off of the holder The end of the electrode should be fully immersed in the test solution and not touching the walls of the container Both thermoprobe and the electrode should be placed in solution 100 ml beaker is used for testing in this experiment Position the electrode in the soln so that the stir bar will not strike the electrode Turn the magnetic stir on during titration Every time the testing solution is changed, the electrode should be rinsed with distilled water and blot dry with tissues When the electrode is not in use, it needs to be immersed in clean distilled water When the electrode is not in use for long periods of time, it should be immersed in 3 M KCl solution 15

16 Procedure III: Titration of Unknown Take 10.0 ml unknown concentration of Na 2 CO 3 soln into a 100 ml beaker Add 30 ml distill water Add 3 d. of universal indicator (1) Place stir bar, electrode, and thermoprobe in soln (2) Setup apparatus NT$ 1000!! Titrate with standardized 0.1 M HCl Add ~1 ml aliquots of HCl and record V i, V f, and ph value after each addition During ph 9-7 and 5-3, add titrant in 0.2 ml increments When ph is lower than 2, stop titration Observe and record the change in color of solution during titration 16

17 After Experiment Clean and check ph electrodes Place electrode in plastic-cap that containing 3 M KCl Turn the ph meter off Hand in magnetic stir bar to GTA Wash buret and place it back Waste liquids (salts) can be discarded in sin after neutralization 17

18 Data Analysis V 1 ( ) ( ) 9.50 V First derivative Second derivative V HCl ph V 1 ph/ V V 2 ( ph/ V)/ V

19 Plots and Results M HCl titrate unknown Na 2 CO 3 solution Make three plots Determine equivalence points Calculate the concentration of unknown soln According to the halfequivalence points determine K a1 and K a Titration curve First derivative Second derivative

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