edited by
Experiments with "Calo-pH Meter" Michel R. Paris and Daniel J. Aymes Laboratoire de Chimie de Coordination, Universite de Bourgogne, B.P. 138, 21004 Dijon Cedex. France In the article, "Development of a New Design for Multipurpose Meter: 'CalwpH Meter"' we descrihed a new instrument design called a "CalwpH meter" that can he used as a high-impedance millivoltmeter (redox or acid-base titrations) or as a calorimeter (enthalpimetric titrations). In this paper some experiments are descrihed that use this instn~rnent. I t is unnecessary to detail here the redox and acid-base experiments that can be performed with the "Calo-pH meter". Adequate information on adaptable experiments can he ohtained in this Journal or in svecialized hooks. We shall, however, describe enthalpimetricmeasurements that are es: pecialiy novel. ~~
Enthalplmeirb Measurements Principles Chemical reactions such as acid-base neutralizations. when carried out in the calorimeter descrihed in the ahovementioned article. result in an imbalance in the hridee. due a change in thd temperature in the calorimeter. ~ e y o n d the eouivalence voint. the thermal effect is due onlv t o the heat i f dilution of the'excess hase. This is generallykegligihle comoared t o the heat evolved during- the neutralization itself. A plot of E (mV) versus the volume of hase added may have three possible shapes (Fig. 1). There is always a break point a t the endpoint corresponding to V,. The case of Figure l a is ohtained when the titrating solution is a t the same temperature as the titrated solution. Figure l b is the case where the titrating solution is warmer than the titrated solution. Figure l c corresponds to the opposite case. More complete studies are developed in specialized hook^.^,^ Two types of determinations can he made from these enthalnoerams. ~he;mimetric Titrations. These measurements do not need anv calihration. I t is sufficient to olot the voltaee versus the volume of titrant. The determidation of V., ismade from the break ooint (cf. Fie. 1).This is an elementaw titration that c& he applied & aeid-base, precipitation, or complexation reactions. Examples of such reactions are fouid in footnote 1. Measurements of Heat of Reaction. These measurements require first the determination of W (water equivalent mass of the calorimeter). This calihration probably will need to he carried out by the instructor or laboratory assistant and not hy the students because this aspect of the measurement techniaue reouires a raoid mixine of two volumes of water a t two dikerenttemperaiures (see'below). If the calorimeters and their accessories are all identical,. onlv - one of the calorimeters need he calibrated. The students must calibrate their aooaratus in order to know the amount of heat that correspdnds to the measured potential change. This calihration is made by the titration of 510
Journal of Chemical Education
Plot of E(mV) versus the volume of base added at different temperature mmblnation. la) Titratino solution and titrated solutlon are both the same temperatdre (b) Tllranng solut on is warmer than lhe t lrated $01~1on tcl T m t m g solut on is coo er than tne tltrated salut on
-
a strong acid with a strong hase, corresponding to the reaction:
+
H+(aq) OH-(aq) 6 H,O(liq)
(1)
for which the AH is k n o ~ n .Once ~ , ~ this calihration is done, the students can determine any other heat of reaction, such as the heat of dissociation of a weak acid, a heat of precipitation, or a heat of complexation. Some values for heats of reactions may he found in footnotes 2 and 5. orperimnts (1) Determination of W (water equivalent mass of the calorime-
ter). ~~~.
The simplest method to determine W consists of rapidly mixing Mgofwater te.g.,50g)in thecalorimeterat the temperature T with the same mass Mof water at temperature TI and in determining the new temperature T after mixing in the calorimeter. The heat quantities involved are respectively: or = MC(T - TI) = heat of cwline- of M e of hot water from ted;perature TI to temperature T. q, = MC(T - To) = heat of heating of M g of cold water from temperature To to temperature T. q = WC(T - To)= heat of heating of the calorimeter.
'See previous artlcle In this issue. Paris. M. R.: Aymes. D. J.: Poupon. R.: Gavasso, R. l990,67,507-509. Bark, L. S.: Bark S. M. Thermometric TiRimeiry:Pergamon: 1969. "ordan, J. "Thermometric Enthalpy Titrations" in Treatise on AnalyticalChemistry; Kolthoff,I. M.; Elving. P. J.. Eds.: Inter-Science: 1968: Vol. 8. Part 1. p 5175. Vanderzee, C. E.; Swanson, J. A. J. Chem. Phys. 1963, 87, 285. Meites, L. Handbook of Analflical Chemistry; McGraw-Hill: 1963 pp 8-5.
If the calorimeter is well insulated,
C = 4.18 J/g
(heat capacity of water)
Several determinations gave an average of 10 g for this water equivalent mass. (2) Calibration of the Instrument by the Students. This calibration is made by titrating 100 mL of 0.05 mol.dm-3 HCL solution with 1 mobdm-3 NaOH solution. The heat of the neutralization is taken as A H 0 = -57.3 KJ/ mol. For n mol of HCL, the thermal effect is q = n. A H o corresponding t o a temperature change AT and a potential change E,. These quantities are related by q = n.AHo = (M
+ WGAT
(2)
with M = mass of dilute HCl solution (the density is taken as 1). W = water equivalent mass of the calorimeter (determined as indicated in paragraph 1). Equation 2 gives:
As W is about 10 g (see above), the change of temperature during the titration is then 0.6 OC. Dividing the E,, value (obtained by extrapolation on the neutralization plot) by AT gives the sensitivity of the calorimeter in millivolts per Celsius degrees:
'Jordan. J. Chlmla 1963, 67, 285.
The value of this coefficient is about 200 mVPC; it is essentially a function of the voltage that powers the Wheatstone bridge in the electronic circuitry. (3) Determination of the Dissociation Heat of Boric Acid. This experiment is particularly interesting because the usual acid-base titration is not possible by classical pHmetry6; only the thermometric titration enables one to obtain a well-defined equivalence point. For the experiment, 100 mL of boric acid of unknown concentration is titrated with 1 mobdrn-WaOH solution. The concentration of HB02 is first determined from the extrapolation of E,, on the enthalpogramE = f(V). Then the number x of neutralized HsO+ ions, the temperature change AT (eq 4), the heat quantity q and the molar heat AH1 of neutralization of HBOz (eq 2), are determined. The molar heat dissociation AH2 is then calculated by the relation: AH2=A H - A H o
(5)
In another experiment, i t is possible in the same way to study the two neutralizations of glycine chlorhydrate; the quantities of glycine and the procedure are the same as above for HBO1. We have found that the following range of concentrations of boric acid and of glycine chlorhydrate are ~ appropriate for a student "unknown": 0.045 m ~ l . d m -and 0.055 m ~ l . d m - ~ . (4) Further Experiments The same procedure may he used for determining a heat of precipitation or a heat of complexation (Cu2+by EDTA). In these experiments, the solutions and the distilled water used for rinsing out the reactor must be a t the same temperature, i.e., they should stay a t least 24 h in the lab prior to the experiments. During the manipulation, any sudden temperature change should be avoided. One should also be careful to add increments of 0.5 mL of titrating solution, waiting only about 10 s after the addition for the stabilization of potential, and to add an excess of titrant to obtain a plateau sufficient for carrying out the extrapolation of E,,.
Volume 67 Number 6 June 1990
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