Electrochemistry LEC Potentiometric ph titration (phosphoric acid in a soft drink) What you need: What you can learn about

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1 Electrochemistry LEC 06 What you can learn about Galvanic cell Types of electrodes Nernst equation Potentiometry Principle and tasks The cell voltage and the Galvani voltage of the electrodes of an galvanic cell are dependent upon the concentration of the ions involved in the potential forming process. Measuring the change of the cell voltage in the titrations of phosphoric acid and a beverage containing phosphoric acid (E 338) conclusions can be made about the beverage s acid content. What you need: Set automatic titration with Cobra3 Chem-Unit Glass beaker, 50 ml, tall Volumetric flask, 100 ml Volumetric pipette, 5 ml Volumetric pipette, 50 ml Graduated pipette, 1 ml Pipette dish Pipettor Wash bottle, 500 ml Ortho-phosphoric acid, 85%, 250 ml Caustic soda solution, 0.1 M, 1000 ml Buffer solution, ph 4.62, 1000 ml Buffer solution, ph 7.01, 1000 ml Buffer solution, ph 9.00, 1000 ml Water, distilled, 5 l PC, Windows XP or higher Titration curve of a beverage containing phosphoric acid. with Cobra3 Chem-Unit 88 Laboratory Experiments Chemistry PHYWE Systeme GmbH & Co. KG D Göttingen

2 LEC Related concepts Galvanic cell, types of electrodes, Galvani voltage, cell voltage, Nernst equation, potentiometry, volumetry. Principle The cell voltage and the Galvani voltage of the electrodes of an galvanic cell are dependent upon the concentration of the ions involved in the potential forming process. Thus, conclusions can be made about the concentration of the ions to be investigated from the measured cell voltage at a constant potential of a suitable reference electrode (potentiometric titration). Tasks Using the Cobra3-System, measure the change in the cell voltage in the titration of a) diluted phosphoric acid with 0.1 molar sodium hydroxide solution b) a sample of a carbonated beverage (Cola) containing phosphoric acid (E 338) with 0.1 molar sodium hydroxide solution and calculate the beverage s phosphoric acid content from the consumption of the standard solution. Equipment Set automatic titration with Cobra3 Chem-Unit Glass beaker, 50 ml, tall Volumetric flask, 100 ml Volumetric pipette, 5 ml Volumetric pipette, 50 ml Graduated pipette, 1 ml Pipette dish Pipettor Wash bottle, 500 ml Ortho-phosphoric acid, 85 %, 250 ml Caustic soda solution, 0.1 M, 1000 ml Buffer solution, ph 4.62, 1000 ml Buffer solution, ph 7.01, 1000 ml Buffer solution, ph 9.00, 1000 ml Water, distilled, 5 l PC, Windows 95 or higher Fig. 1. Experimental set-up. PHYWE series of publications Laboratory Experiments Chemistry PHYWE SYSTEME GMBH & Co. KG D Göttingen 1

3 LEC Set-up and procedure Set up the experiment as shown in Fig. 1. Prepare the solutions required for the experiment as follows: 0.08 molar H 3 solution: Pipette 0.55 ml of 85% phosphoric acid into a 100 ml volumetric flask and make up to the mark with distilled water. Soft drink containing phosphoric acid: Pour approximately 150 ml of the soft drink into a 250 ml beaker. Place the beaker on the magnetic stirrer and heat it carefully to remove dissolved carbon dioxide. Allow to cool to room temperature. Connect the control input of the motor piston burette to the TTLoutput of the Cobra3 Chem-Unit with the cable supplied for this particular purpose. Connect the ph electrode to the ph-input, and the temperature probe to temperature input T1. Call up the Measure programme in Windows and enter <Chem-Unit> as measuring instrument. Set the measurement parameters as shown in Fig. 2. In <Preferences> under <ph/potential> select <Potential> as mode and <mv> as unit. In menu prompt <Displays> set the display range to mv. Set Digital display 1 to <Potential>, and select, under Diagrams, <Line diagram> for Diagram 1, <Potential> for Diagram 1a, and for the display range <0-10 ml> and <no auto range>. Confirm your entries with <OK>. Now calibrate your ph electrode using two buffers. To do this, enter the appropriate ph value in <ph / Potential> under the menu point <Calibrate>, dip the electrode into the buffer and save with <Calibrate>. To calibrate the temperature sensor, either balance it against a temperature measured with a thermometer, or with the level of a temperature probe connected to T2 or T3. After having made all settings, press <Continue> to reach the field for the recording of measured values. Arrange the displays as you want them. Measure the potentials of the two buffers used above and a further one, in each case immersing the electrode in the buffer and recording the value that is shown in the display. Fill 100 to 150 ml of water into a 250 ml glass beaker, add a magnetic stirrer bar and place the beaker on the magnetic stirrer. Pipette 5 ml of 0.08 molar phosphoric acid into the beaker. Cut the rubber stopper with central hole lengthwise from one side to the hole and fit the temperature probe into it. Fix the dispensing tip of the motor piston burette, together with the ph electrode and the temperature probe, in the electrode holder. The sensing diaphragm of the ph electrode must be covered by the solution. Adjust the magnetic stirrer to a medium stirring speed and start the measurement with <Start measurement>. The first measurement (for V = 0.0) is immediately recorded. Following this, the motor piston burette dispenses the first portion of sodium hydroxide solution. The titration now proceeds according to the parameters that have been set, and is automatically stopped after the addition of 10 ml of sodium hydroxide solution. On completion of the series of measurements, save the data with <File><Save measurement as...>. Now titrate the soft drink containing phosphoric acid. To do this, pipette 60 ml of the prepared soft drink into the 250 ml glass beaker, add approximately 100 ml of distilled water and a stirrer bar, place the beaker on the magnetic stirrer, fix the burette tip and ph electrode in position and carry out the titration and saving of data exactly as above. Figs. 3 and 4 show the graphs as they can be presented when the measurement is stopped. Fig. 3: Cell voltage U and ph value as a function of the volume of added standard solution for the titration of an approximately 0.08 molar phosphoric acid with a 0.1 molar sodium hydroxide solution. Follow the operating instructions supplied with the motor piston burette to fill it with 0.1 molar sodium hydroxide solution. Set the motor piston burette to <Dosing>, <nom.vol. = 0.2 ml> and <Speed = 99.9 ml/min>. Fig. 2: Measurement parameters Fig. 4: Titration diagram of the neutralisation of a beverage containing phosphoric acid (V = 50 ml) with a 0.1 molar sodium hydroxide solution. 2 PHYWE series of publications Laboratory Experiments Chemistry PHYWE SYSTEME GMBH & Co. KG D Göttingen

4 LEC Theory and evaluation The titration curves recorded show the cell voltage U as a function of the volume of sodium hydroxide solution added. To obtain the relationship between voltage and ph, plot the potentials measured for the three buffers against their ph values. ph of the buffer Voltage measured mv mv mv With the help of the best straight line through the three measured points and the equation for a straight line f(x) = ax + b (with a = ph / U and b = ph at U = 0 mv), you can have the diagram for the corresponding ph values calculated with <Channel modification> under menu prompt Analysis. For this, set the parameters as given in Fig. 5. The ph measurement is therefore a potentiometric measurement. It can only be carried out, when the potential of the indicator electrode is measured against that of a reference electrode. Both electrodes are contained in single-rod ph measuring systems. A glass electrode is used as the indicator electrode. When this is immersed in an aqueous solution, a swelling layer (gel) is formed at the ph sensitive glass membrane. This also occurs at the inner side of the membrane, which is in contact with a defined buffer solution, commonly of ph 7. According to the ph of the test solution, hydronium ions diffuse either out of, or into, the outer swelling layer, whereby the potential of this layer changes. The ph, and so also the potential, of the inner side of the glass membrane remains constant while this occurs. With regard to the pick-up electrode, the same system is chosen as for the reference electrode (AgCl), so that the potential difference between the two electrodes is equal to zero. The voltage measured with the single-rod measuring system therefore results from the potential difference between the inside and outside swelling layers of the glass electrode. When the system is dipped into a test solution of ph 7, then a potential of 0 mv should be given, as the inner potential is equal to the outer potential. Theoretically, when the ph changes by a single unit, the voltage should change by mv. This can be calculated using the Nernst equation, which is as follows for hydrogen: Two equivalence points can be clearly seen in the titration curve for pure phosphoric acid. They correspond to the following dissociation equilibria: H 3 + Na + + OH - S H Na + + H 2 O H Na + + OH - S H 2- + Na + + H 2 O The second equivalence point is only weakly shown in the titration curve for the soft drink. This is because, in addition to phosphoric acid, further acids are present here which also take part in the titration with sodium hydroxide solution and exert a buffering effect in the region of the second equivalence point. A knowledge of the first equivalence point suffices for the calculation of the quantity of phosphoric acid contained in the soft drink. The concentration c 2, and so the content m 2 of acid-forming substance in the sample of volume V 2, can be calculated from the corresponding consumption V 1 of standard solution of known concentration c 1. where M c 1 V 1 = c 2 V 2 = m 2 / M 2 Molar mass of phosphoric acid (= 98 g/mol) Data and results The first point of inflection of the titration curve gives V 1 =3.34 ml NaOH, from which a phosphoric acid concentration c 2 = can be calculated, and from this a content of m = mg H 3 in the volume of the sample tested (V 2 =50ml). A 1.5 l bottle of the soft drink tested therefore contains mg of phosphoric acid. Fig. 5: Parameters of the channel modification E = E o RT/F log a (H + ) where E 0 = 0 log a (H + ) log c(h + ) = - ph 2.3 RT/F = mv (at T = K) From this it follows that: E = mv ph T R F Absolute temperature Universal gas constant Faraday constant This rise in the ph-characteristic line is called the slope of the single-rod ph measuring system. The slope of real ph measuring systems deviates like the zero point from the theoretical value given by the Nernst equation. You can have the equivalence point calculated with <Detect point of equivalence> under menu prompt <Analysis>. PHYWE series of publications Laboratory Experiments Chemistry PHYWE SYSTEME GMBH & Co. KG D Göttingen 3

5 LEC 4 PHYWE series of publications Laboratory Experiments Chemistry PHYWE SYSTEME GMBH & Co. KG D Göttingen