Abstract

Background

A quick and accurate way to determine the electrode potential of redox reaction in laboratory is to build a primary battery and in virtue of the test electrode’s potential and ammeter’s registration（EMF）to calculate (by Nernst equation and Kirchhoff equation). But in the work or study in lab sometimes, this general system may not work well in each situation.

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Keywords: redox reaction, potential, acidometer, concentration, Nernst equation

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Materials and method

Materials

Instruments:

pHS-25 acidometer?? ??????????????centrifuge tube (10ml)??

beakers (50ml,100ml) ??????????????saturated calomel electrode(type Lei ci)??

unglazed porcelain tube ????????????buffer solution??

conductor and conductor clamp ?????copper electrode??

zinc electrode? ????????????????????sandpaper

DC digital voltmeter(type PZ93A made by Shanghai ShangBiao Instrument Co., Ltd)

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Chemicals:

H2SO4 (2mol/L , 6mol/L) ??HCl (2mol/L , 6mol/L) ??ZnSO4 (0.1mol/L , 0.5mol/L)

NaOH (2mol/L)????????? CuSO4 (0.5mol/L)????? ?KCl (saturated)

Agar gel???????????????

Method

Redox reaction is a process which occurring by the loss of electrons from one substance and gain by the other substance.

Electrode potential is the criterion for judging oxidant and reductant relative strength, and can also be used to predict the direction of the redox reaction. In this experiment, we set the redox pair to be measured as one electrode. Using a DC digital voltmeter to determine the potential difference between two electrodes. Then subtract the potential difference from the potential of saturated calomel electrode (refer as +0.2415V). The number we gain is the potential of electrode to be measured.?

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Experiment design

1.???? Basic experiment

2.???? Redox reaction with changing of pH

Using two different concentration of HCl and the same concentration of KMnO4 in two different tubes , then observe whether the colour disappear to judge whether the reaction can spontaneous.

3.???? Redox reaction between the same substance but two different concentrations

Design a concentration cell (with standard electrode as one and change the H+? concentration to 2mol/L as the other) and determine the EMF of it.

4.???? Redox reaction with precipitate forming

5.???? Redox reaction with complexes forming

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Principle interpretation and experimental data

1.???? Basic experiment

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2.???? Data analysis

Through the file retrieval we can find the difference between them. For the reasons, there are obvious systematic errors in the pH meter(acidometer) method and voltmeter method we used in the past. The electromotive force is measured by potentiometer on the basis of the principle of the elimination method. An external potential difference of equal size and opposite direction is connected in parallel on the battery to be tested, which is generally measured by digital voltmeter. The so-called acidimeter is an instrument that uses the electromotive force of the battery measured by the voltmeter to directly express the pH value.

The measurement error of electromotive force of battery measured by potentiometer is composed of system error, accidental error and sensitivity. The value of electromotive force is determined by the grade of potentiometer, the grade of standard battery and the sensitivity of galvanometer. The accuracy of the potentiometer used in general experiments is 0.

In addition, the internal resistance of the battery composed of glass electrode is very high.

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Conclusion

The principle of pH meter for measuring electromotive force is: in the reaction between redox systems related to pH value, the electrode potential changes not only with the concentration and ionic strength of the solution, but also with the pH value of the solution. There is a large error in the electrode potential itself, so there is no current passing through the cell to be measured.

When the electromotive force is measured by acidimeter, the standard electrode after calibration can be used to measure the electromotive force of battery, which can reach several hundred megaohms at normal temperature. Therefore, it is impossible to measure the electromotive force of battery with ordinary potentiometer

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