In the Laboratory
Secondary School Chemistry
Potentiometric Determination of CO2 Concentration in the Gas Phase: Applications in Different Laboratory Activities
W
Eduardo Cortón, Santiago Kocmur, Liliana Haim, and Lydia Galagosky* Centro de Formación en la Enseñanaza de las Ciencias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Cuidad Universitaria – Pabellón II, 1428 Buenos Aires, Argentina; *
[email protected] The aim of this work is to provide high school experiments through which students may appreciate how concepts from different scientific disciplines can be shared, resulting in an understanding of science. The principles of gaseous equilibrium and electrochemical potentials are applied in a set of laboratory activities using a CO2 potentiometric detector. The device is calibrated during the first lab session. The CO2 and CO2-free air samples required for calibration are produced in the lab by an inexpensive and simple apparatus. In the second lab session, the CO2 potentiometric device is used to measure CO2 uptake and release during two metabolic processes. Also, the variation of CO2 concentration is estimated while changing the air/fuel proportion in a Bunsen burner. It is very important to emphasize practical applications and interdisciplinary relationships when teaching difficult physical chemistry concepts. Also, students should be stimulated to acquire a more integrated view of scientific investigation. These simple experiments help to accomplish both goals. First Laboratory Session The gas-sensor electrode is built by adding a gaspermeable membrane to a pH electrode (1). A commercial CO 2 electrode could be used as well by following manufacturer’s instructions. A thin-film solution of electrolyte is intercalated between the membrane and the electrode. When the gas diffuses across the membrane, it dissolves in the electrolyte solution, producing a pH change. The glass-membrane electrode registers the pH variation. The study of the pH changes can be used for the determination of carbon dioxide concentration in a gas atmosphere. This is shown below. From Bates (2), EH = ᎑R′T pH ᎑1
where R′ = (R ln 10)/F. When R = 8.3146 J K mol , F =
To potentiometric detector Bunghole Beaker covered with Parafilm Wash flask with NaOH solution Magnetic stirrer Figure 1. Calibration chamber.
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∆E = ᎑R′T ∆(pH)
(1)
If we consider the carbon dioxide constants in aqueous solution K1 and K2 (1), defined as H2O + CO2 HCO3᎑ + H+ ᎑ 2᎑ HCO3 CO3 + H+
(K1) (K2)
K1 is the reaction constant resulting from the following equilibria: CO2(g) CO2(aq) (K0) CO2(aq) + H2O H2CO3 (K0 ′) H2CO3 HCO3᎑ + H+ (K0 ′′) ——————————————————— CO2 + H2O HCO3᎑ + H+ (K1) Thus, K1 (= K0 × K0′ × K0 ′′) relates gaseous and aqueous CO2 species with the carbonic acid dissociation equilibria and therefore with carbonate and bicarbonate species. By using eq 1, eq 2 is obtained: 2
H+ + Na+ H+ – K w CO2 = 2K K1 1 + +2 H
(2)
If the sodium hydrogen carbonate concentration range is 0.01–0.001 M, eq 2 approaches to Severinghaus (3) :
CO2 =
Na+ H+ K1
Rearrangement and replacement in eq 1 gives ∆E = ᎑R ′T ∆(᎑log[CO2]) = ᎑R ′T ∆(pCO2)
᎑1
Air inlet
96,487 C/equiv and T is in kelvins. In terms of pH change,
(3)
therefore, ∆E shows a linear response to ᎑ log[CO2] changes owing to CO2 diffusion from the atmosphere through the membrane into the inner electrolyte solution.
Hazards Safety goggles must be worn. Concentrated NaOH(aq) is extremely caustic and should not be allowed to contact skin or eyes. Calibration Procedure with Gas Mixtures 1. The calibration chamber consists of a 100-mL beaker covered with Parafilm and containing a magnetic bar, placed on a magnetic stirrer (Fig. 1). The stirring creates a gaseous vortex that rapidly mixes the gases in the calibration chamber.
Journal of Chemical Education • Vol. 77 No. 9 September 2000 • JChemEd.chem.wisc.edu
In the Laboratory
2. The electrode is placed in the calibration chamber by piercing the Parafilm coat. 3. Carbon dioxide is produced by gently dropping 10 N sulfuric acid into 0.5 L of a saturated sodium hydrogen carbonate solution, contained in a 1-L Erlenmeyer flask that is covered with Parafilm having a small bunghole. To obtain relatively pure CO2, use an amount of sulfuric acid that releases 2–3 L of carbon dioxide. The gas will be vented through the bunghole. 4. Carbon dioxide–free air is obtained by bubbling air in a wash flask containing concentrated sodium hydroxide solution (as shown in Fig. 1). A delivery tube conducts the CO2-free air into the calibration chamber. 5. After the calibration chamber has been equilibrated with the atmosphere, the value 300 ppm is assigned to the potentiometric reading, in millivolts, that is obtained. 6. The potentiometer value corresponding to 0 ppm is obtained by allowing the electrode to become stabilized in the CO2-free air current. 7. The injection of different volumes of CO2 into the chamber by syringe allows the calibration curve to be built, as shown in Table 1 and Figure 2. To determine the accuracy of the calibration method, the following experiment was carried out. The necessary amount of sodium bicarbonate to produce a 1% v/v CO2 concentration in the air was placed in an Erlenmeyer flask containing an excess of sulfuric acid. The results indicate an error
