Potentiometric measurement of ph

Transcription

1 Potentiometric measurement of ph Determination of ph can be done by simple colorimetric methods using acid-base indicators (ph test strips). Nevertheless, the precision of such methods is mostly insufficient. For exact ph measurement, laboratories are equipped with ph meters with usual resolution of 0.01 ph units, high-end instruments with resolution of ph units. There is a ph meter with combined electrode (consisting of both the glass and the reference electrode) at your working place. Electrode must be kept moist using the storage solution. Remove the protective plastic cover with storage solution before using the electrode. Keep the storage solution inside the plastic cover, do not pour it out. At the end of your work, you have to place the electrode back into the cover with storage solution. Never touch the glass membrane of the electrode with your fingers! Electrodes should be rinsed between samples with distilled water. After rinsing, gently blot the electrode with cotton paper piece to remove excess water. Procedure: Into a clean small beaker, transfer the sample which ph you want to determine, by pouring from the small plastic bottle. Estimate the ph using universal indicator strip. Dip the electrode of the ph meter into the solution in the beaker, gently shake the content by slowly moving the beaker to make the membrane of the electrod be in contact with the solution. (Electrode is "dirty" with distilled water used for rinsing between samples.) After stabilization of the value, read the ph on a display. Put the sample back into the small plastic bottle. Sample ph estimated by indicator strip ph measured by ph meter 1

2 Demonstration of buffer functioning Buffers are solutions that maintain a relatively stable ph and resist changes in ph, upon addition of small amounts of acid or base. At your working place, there are two bottles with solutions to compare how they are resistant to changes in ph. One bottle contains unbuffered solution - deionized (ultrapure) water (in theory, ph=7), the other one contains phosphate buffer at ph close to 7. Unbuffered solution - water Using a graduated cylinder, measure 50 ml of deionized water, pour it into a clean beaker and determine the ph by ph meter. Record the value (measurement 1). Pour out the water from the beaker. Using the same graduated cylinder, measure again 50 ml of deionized water, pour it into the same beaker and determine the ph by ph meter (measurement 2). After the second measurement, do not remove the combined electrode from the beaker, let it dipped there. By pipetting add 100 μl of HCl solution (c=0.1 mol/l) into the solution in the beaker and gently mix the content. After stabilization of the value record the ph. ph of deionized water measurement 1 measurement 2 ph after addition of HCl Describe how stable is the ph of an unbuffered solution: Buffer Using a graduated cylinder, measure 50 ml of phosphate buffer, pour it into a clean beaker and determine the ph by ph meter. Record the value (measurement 1). Pour out the buffer from the beaker. Using the same graduated cylinder, measure again 50 ml of phosphate buffer, pour it into the same beaker and determine the ph by ph meter (measurement 2). After the second measurement, do not remove the combined electrode from the beaker, let it dipped there. By pipetting add 100 μl of HCl solution (c=0.1 mol/l) into the solution in the beaker and gently mix the content. After stabilization of the value record the ph. ph of the buffer measurement 1 measurement 2 ph after addition of HCl Describe how stable is the ph of a buffer solution: Describe the differences between the behaviour of the buffer solution, and the pure water, upon the addition of a small amount of a strong acid: 2

3 Determination of concentration by potentiometric titration Potentiometric titration is a volumetric method in which the potential between two electrodes is measured as a function of the added reagent volume. In potentiometric titrations, there is no need for indicator. The titration is not stopped at the equivalence point, the whole titration curve is constructed instead. Volume of the standard reagent consumed to reach the equivalence point is found by analysis of the titration curve. A titration curve has a characteristic sigmoid shape. The part of the curve that has the maximum change marks the equivalence point of the titration. You will be performing two potentiometric titrations to determine the unknown concentrations of two acids. In order to perform these titrations you will utilize a ph meter. 1) Strong acid: hydrochloric acid (HCl) 2) Weak acid: acetic acid (CH 3 COOH) 1) Titration of a strong acid, graphical analysis of the titration curve Procedure: Into a clean small beaker, using a glass pipette measure exactly 10.0 ml of the sample (strong acid HCl) which concentration you want to determine. Add a small stirring bar (little white corpuscle similar to Tic Tac mints) into the beaker. Close the burette tap, and fill the burette with the standard solution of NaOH (c=0.100 mol/l) until the meniscus is about 1-2 cm above the zero mark. Remove the funnel. Open the tap and allow the solution to drain (into the waste bottle) until the meniscus falls to zero mark. Fill the burette with the standard solution. The burette is ready for the titration. Set up the apparatus so that you can perform the potentiomeric titration. Place the beaker with the sample on a magnetic stirrer. Dip the electrode of the ph meter into the solution in the beaker. If the electrode is not sufficiently submerged, add a little of distilled water. Place the burette so that the orifice is slightly above the beaker and is possible to use it for adding standard solution to the sample. Switch on the stirring (not heating!) and adjust the speed of it to optimal (not extremely fast to prevent damage of the electrode). Before adding any NaOH from the burette, record the initial ph on the display of the ph meter. Put the data into the table further in this instruction sheet. You are now ready to begin the titration. You will be adding NaOH solution in increments 0.5 ml until 10.0 ml. After every addition of NAOH, record the ph. Be sure that the ph has stabilized! There is a delay between the addition of NaOH and a stable ph reading. There is a computer in the students' laboratory with a MS Excel file prepared to put the collected data and plot the graph (titration curve). Print the filled MS Excel form. Use a graphical method to find out the equivalence point. Two parallel lines are drawn tangent to the flat portions of the curve. A line perpendicular to both the upper and the lower tangent is drawn to help you find the middle between them so that you can plot the third parallel line exactly in the middle. The point where this middle parallel crosses the titration curve is the equivalence point. 3

4 Potentiometric titration of a strong acid (HCl) data sheet Added NaOH volume (ml) ph HCl Consumption of NaOH read from titration curve: Calculation: (M = 36.5 g/mol) Molar (substance) concentration c = mmol/l Mass concentration = g/l 4

5 1) Titration of a weak acid, graphical + methematical analysis of the titration curve Procedure: (the same as for strong acid) Into a clean small beaker, using a glass pipette measure exactly 10.0 ml of the sample (weak acid CH 3 COOH) which concentration you want to determine. Add a small stirring bar (little white corpuscle similar to Tic Tac mints) into the beaker. Close the burette tap, and fill the burette with the standard solution of NaOH (c=0.100 mol/l) until the meniscus is about 1-2 cm above the zero mark. Remove the funnel. Open the tap and allow the solution to drain (into the waste bottle) until the meniscus falls to zero mark. Fill the burette with the standard solution. The burette is ready for the titration. Set up the apparatus so that you can perform the potentiomeric titration. Place the beaker with the sample on a magnetic stirrer. Dip the electrode of the ph meter into the solution in the beaker. If the electrode is not sufficiently submerged, add a little of distilled water. Place the burette so that the orifice is slightly above the beaker and is possible to use it for adding standard solution to the sample. Switch on the stirring (not heating!) and adjust the speed of it to optimal (not extremely fast to prevent damage of the electrode). Before adding any NaOH from the burette, record the initial ph on the display of the ph meter. Put the data into the table further in this instruction sheet. You are now ready to begin the titration. You will be adding NaOH solution in increments 0.5 ml until 10.0 ml. After every addition of NAOH, record the ph. Be sure that the ph has stabilized! There is a delay between the addition of NaOH and a stable ph reading. There is a computer in the students' laboratory with a MS Excel file prepared to put the collected data and plot the graph (titration curve). Print the filled MS Excel form. Use both, a graphical method and mathematical analysis to find out the equivalence point. Mathematical analysis of the titration curve Consumption of the standard reagent in equivalence point (V) can be calculated using the formula: 2 Δ ph V V 2 2 Δ ph Δ ph ΔV V + "added NaOH volume" at last positive 2 ph 2 ph + the last positive 2 ph 2 ph - V absolute value of the first negative 2 ph the difference in "added NaOH volume" between the last positive and the first negative 2 ph (In our experiment, it must always be 0.5 ml.) 5

6 Potentiometric titration of a weak acid (CH 3 COOH) data sheet Added NaOH volume (ml) 0.0 ph ph 2 ph ph = ph x+1 - ph x 2 ph = ph x+1 - ph x 6

7 CH 3 COOH Consumption of NaOH read by graphical method from a titration curve: Consumption of NaOH calculated: Calculation: (M = 60.0 g/mol) Molar (substance) concentration c = mmol/l Mass concentration = g/l 7

8 Determination of the isoelectric point of the amino acid Added NaOH volume (ml) ph Added NaOH volume (ml) ph Calculation of pi: 8