Titration of a strong acid with a strong base with Cobra4

Transcription

1 Titration of a strong acid with a strong base with Cobra4 TEC Related topics Strong and weak acids and bases, ph value, titration curves, equivalence point, potentiometry. Principle Hydrochloric acid is a strong acid while sodium hydroxide is a strong base. Both substances dissociate in water completely and form oxonium and hydroxide ions. These ions react quantitatively with each other and form water molecules. The concentration of a strong acid can be determined using a reaction with a strong base of known concentration. Equipment 1 Cobra4 Wireless Manager Cobra4 Wireless-Link Cobra4 Sensor-Unit Chemistry Software measure Cobra4, multi-user 1 licence Retort stand, h = 750 mm ph-electrode, gel-filled Storage flask for ph electrodes,filled Holder for Cobra4 for support rod Right angle clamp Cobra4 Sensor-Unit Drop Counter Immersion probe NiCr-Ni, Teflon Magnetic stirrer, mini Magnetic stirrer bar, l = 15 mm Burette, lateral stopcock, Schellbach, 1 50 ml, graduations 0,1 ml Burette clamp, roller mount.,2pl Funnel,d =.40 mm, f.burettes Volumetric pipette, 25 ml Glass beaker, 150 ml, tall Glass beaker, 150 ml, short Glass beaker, 50 ml, tall Beaker DURAN,short form, 100 ml Pipettor Wash bottle, 500 ml Buffer solution, ph 4.62, 1000 ml Buffer solution, ph 9.00, 1000 ml Caustic soda solution, 0.1 M, 1000 ml Hydrochloric acid,0.1m, 1000 ml Water, distilled, 5 l Additional equipment PC with USB interface, Windows XP or 1 higher 1: Experimental set up. Fig. P PHYWE Systeme GmbH & Co. KG All rights reserved 1

2 TEC Titration of a strong acid with a strong base with Cobra4 Safety instructions When handling chemicals, you should wear suitable protective gloves, safety goggles, and suitable clothing. Please refer to the appendix for detailed safety instructions. Tasks 1. Measure the alteration of the ph value during a titration of an aqueous solution of hydrochloric acid with a 0.1 molar sodium chloride solution at constant temperature using the Cobra4 system. 2. From the titration curve read the equivalence point and determine the concentration of the acid. Set-up and procedure - Set up the experiment as shown in Fig Combine the Cobra4 Sensor Unit Chemistry and the Cobra4 Drop Counter with the Cobra4 Wireless-Links. - Attach them to the retort stand with the holders for Cobra4 and right angle clamps. - Connect the ph electrode to the ph input of the Cobra4 Sensor Unit Chemistry and the temperature probe to temperature input T1. - Start the PC and connect the Cobra4 Wireless Manager with a USB socket of the computer. - After the Cobra4 Wireless-Links have been switched on, the sensors are automatically recognized. Some ID numbers (01 and 02) are allocated to the sensors, which are indicated in the displays of the Cobra4 Wireless-Links. - Call up the measure programme. - Click the Unknown titration volume button and confirm with OK. - Boot the experiment Titration of a strong acid with a strong base with Cobra4 (experiment > open experiment). The measurement parameters for this experiment are loaded now. - For calibration: Pour some buffer solution with ph 4.62 and ph 9.00 in two beakers. - Immerse the well-rinsed probe into one of the solutions. - In the Cobra4 Navigator under Devices double-click the ph symbol. Now you can change some measurement parameters. - Enter the ph value for the given solution under the menu point Calibration (Step 1, see Fig. 2). - Click the Apply button. - Repeat this procedure with the other buffer solution (Step 2). - Finish the calibration with OK. - Procedure: Position the magnetic stirrer under the stand. - Pipette 25 ml of the 0.1 molar hydrochloric acid in the 150 ml beaker and slip a magnetic stirring rod in. Fig. 2: Settings for the calibration mode of the sensor. 2 PHYWE Systeme GmbH & Co. KG All rights reserved P

3 Titration of a strong acid with a strong base with Cobra4 TEC - Fill the burette up to the 50 ml mark with 0.1 molar sodium hydroxide solution. - Position the beaker containing the hydrochloric acid on the magnetic stirrer so that the ph measuring electrode dips into the solution (Note: The ph electrode must dip at least so deep in the solution that the diaphragm is completely immersed in the solution. Add more water if necessary). - Position the tap of the burette so that sodium hydroxide solution can drop into the beaker. Also ensure that individual drops will be recorded by the drop counter. - Adjust the stirrer to a medium stirring speed (Note: Do not allow the magnetic stirring bar to hit against the measuring electrode.) - Start measurement with a click on in the icon strip. - Add caustic soda solution drop-wise to the hydrochloric acid from the burette. (Note: Take care that the addition of the drops is not so rapid that the light barrier cannot register individual drops.) - When about 20 ml of caustic soda solution has been so added, close the tap of the burette and click on the icon to terminate measurement. - Send all data to measure (see Fig. 3). Save the measurement (File > Save measurement as ). - Fig. 4 shows the graph as presented by the programme. To Fig. 3: Window which appears after measurement. Fig. 4: Measurement curve of a titration of 25 ml of 0.1 molar hydrochloric acid with caustic soda (c = 0.1 mol/l). have the equivalence point displayed use. Theory - Hydrochloric acid is a strong acid and caustic soda is a strong base. Both substances dissociate in water completely: HCl + H 2 O H 3 O + + Cl - NaOH + H 2 O OH - + Na + - The formed oxonium and hydroxide ions react quantitatively with each other forming water molecules. H 3 O + + OH - 2 H 2 O P PHYWE Systeme GmbH & Co. KG All rights reserved 3

4 TEC Titration of a strong acid with a strong base with Cobra4 - The ph value and the poh value are measures for the concentrations of oxonium and hydroxide ions in aqueous solution. ph = -log c(h 3 O + ) (1) poh = -log c(oh - ) (2) pk w = ph + poh = 14 (3) - In a neutral solution ph and poh are equal. ph = poh = 7 (4) - One can determine the unknown concentration of a strong acid by observing the change of ph while adding a strong base with a known concentration. To do this one have to determine the volume of the strong base at the point of neutralisation. c = n/v (5) n = c. V (6) - At the point of neutralisation the number of moles of oxonium ions and hydroxide ions are equal: n(h 3 O + ) = n(oh - ) (7) - Because of the complete dissociation of strong acids and bases the following equations are valid: - From these equations follows: c(h 3 O + ) = c(acid) (8) c(oh - ) = c(base) (9) c(acid) = [c(base). V(base)]/V(acid) (10) - The ph measurement is 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: 4 PHYWE Systeme GmbH & Co. KG All rights reserved P

5 Titration of a strong acid with a strong base with Cobra4 TEC - 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. Results - Fig. 4 shows the typical curve of a titration of a strong acid against a strong base. - At the beginning the ph value is very low because hydrochloric acid as a strong acid is completely dissociated. In theory the ph value should be 1 (c(hcl) = 0.1 mol/l). But the real value is approximately 1.7. This is because the 0.1 M solution has been diluted by adding some water before the measurement started. - During the performance of the titration the oxonium ions are neutralised because of the reaction with the added hydroxide ions. So the ph value should increase. - Because of the logarithmical presentation of the concentration (see equation (1)) the rising of the curve is at first very low. Near the point of equivalence the ph value shows an abrupt rise. - In the end of the titration the rise of the ph value is low again. - Under the given conditions, the equivalent point is equal to the point of neutralisation and should be reached on the addition of 25 ml of caustic soda solution. Notes - Any other strong acid can be used in this procedure in place of hydrochloric acid. - The ph electrode should be stored in a 3 molar potassium chloride solution when not in use. Disposal The diluted and neutralised solutions of the used acid and base can be disposed by rinsing into the drain. Appendix Hazard symbol, signal word Caustic soda solution, 0.1 N Hazard statements Precautionary statements Attention H290: May be corrosive to metals. P234: Keep only in original container P PHYWE Systeme GmbH & Co. KG All rights reserved 5

6 TEC Titration of a strong acid with a strong base with Cobra4 Hydrochloric acid, 0.1 N H290: May be corrosive to metals. P234: Keep only in original container Attention 6 PHYWE Systeme GmbH & Co. KG All rights reserved P