What is a potentiometric transducer
Potentiometry is a form of titration in which you use a Voltage measurement determine the concentration of your sample solution. You can find all the details here in our article.
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Potentiometry simply explained
Since potentiometry is a form of titration, the basic titration process is the same. The only difference to traditional titration is the recording of the equivalence point. You no longer determine this via an indicator, but by measuring the electrochemical potential difference between Customized solution and one Reference electrode on constant potential.
You always use this titration method if you cannot find a suitable indicator or if the strong color of the sample solution makes it impossible to see the color change. If you would like to have the basics of titration explained again before reading on, then take a look at our article on titration.
Potentiometry experiment setup
If you do the titration potentiometric want to perform, then you need both a reference electrode and a measuring electrode. The reference electrode is usually an electrode of the 2nd type. These are characterized by the fact that their potential constant and not concentration dependent is. The measuring electrode, on the other hand, has a potential that depends on the concentration in your measuring solution. So if you change the concentration of the sample solution through neutralization, the potential of your measuring electrode also changes. In potentiometry, you record this change by continuously measuring the voltage of the constant potential against your reference electrode.
If you are already well versed in electrochemistry, then you probably also know that a current flow would also change this voltage difference over time. But since you want the neutralization of the sample solution to be the only source of your potential changes, you block this current with a Voltage compensation circuit. You can use this to measure the voltage without the risk of falsifying your results.
Now that you have understood the basic processes, you are probably wondering how you can tell from the recorded voltage curve when you have reached the equivalence point. To answer that, you have to look at the Nernst equation look at. If this equation doesn't mean anything to you, then take a look at the article here to do this.
The Nernst equation describes the voltage curve of an electrode that is in contact with an electrolyte solution, depending on the concentration.
Using this equation, you can convert the voltage of your measuring electrode into a Normal electrode comprehend. Usually, however, your experiment setup consists of both a measuring electrode and a reference electrode. So you record the voltage difference between these two electrodes. But since the voltage of the reference electrode is always constant, you can easily calculate the voltage curve. For this, one also assumes that the substance to be titrated is present as an ion in a solution and, by adding the standard solution, afterwards equation will react.
Ox = oxidized form; Red = reduced form n = whole number; e = electron
If this is the case, you can easily calculate the voltage by subtracting the constant potential of the reference electrode from the concentration-dependent measuring electrode.
Equivalence point determination
The potential of the reference electrode varies depending on the material used. Frequently used electrodes are the Ag / AgCl electrode or the Calomel electrode (Hg / HgCl).
If you now look at this equation, you can see that the voltage value will change significantly if the concentrations of the oxidized and reduced form are the same. As the reaction progresses, the ln function falls below the value 1 and its slope increases sharply as a result. This increase is then clearly visible in the measurement curve. Within this jump of tension you will then also find your equivalence point. If you would like to take a closer look at the equivalence point, then click here.
Potentiometry evaluation of the measurement curve
If the measured voltage is now plotted against the added volume of standard solution, the result is again a for the titration typical course.
As you can see here, there is again a course with a steep climb. Similar to acid-base titration, this increase occurs when the equivalence point is reached. You can also go through this curve graphical determination of the turning point the curve to find out the equivalence point. This is an example of this Tangent method or that Circular process at.
If you want to use the tangent method, you only have to add two tangents to the Breakpoints of the curves. These should form an angle of approximately 45 ° with the x-axis. Then you just have to draw a third straight line that is parallel to the other two tangents. This should be exactly in the middle of the two. The Intersection of this third straight line is then your equivalence point.
If you know the voltage at which the equivalence point has been reached, you can rearrange the Nernst equation and thus calculate the concentration of the sample solution at the equivalence point. In order to determine the original concentration, you now have to add the amount of substance that has already reacted with the standard solution when the equivalence point is reached. Since you can read from the diagram how much standard solution has already dripped into the sample solution at the equivalence point, you can also calculate the amount of reacted substance:
- n = amount of substance of the measuring reagent in the sample solution at the equivalence point
- = added volume of standard solution at the equivalence point
- = Concentration of the standard solution
You can then use the reaction equation for the titration to calculate the amount of substance used in the sample solution. You then only have to add this to the already known equivalence point amount of substance. You will then receive the initial concentration based on the volume of the sample solution.
Potentiometry fields of application
Since potentiometry is a very versatile method, it can also be found in many fields of application. You can use it for example Solubility products measure or determine the concentrations of any solution in the classic way. A very important area of application is above all that Acid-base titration.
Acid-base titration using potentiometry
Since the use of indicators often leads to somewhat inaccurate titration results, alternative measurement methods were sought early on and found in potentiometry. A special one is normally used for the measurement even during the titration Glass electrode.
This electrode is designed so that it can both electrodes, so reference and measuring electrode, contains at the same time. However, these two are spatially separated from each other and only connected by a conductive wire to measure the voltage between the two. As before, the reference electrode is an Ag / AgCl or a calomel electrode. The measuring electrode itself is immersed in a solution with the pH 7 a. In addition, the solution contains a bufferto keep the pH stable. If you want to repeat the term pH value again, then click here.
The crucial part of the glass electrode is one Glass membraneimmersed in the sample solution. One side of the glass membrane is immersed in the sample solution. The other side, on the other hand, is in contact with the Buffer solutionwhich also contains the measuring electrode. Now you have to know that ions are stored in most glasses, like -Ions. But these are due to the amorphous structure of the glass very agile. In addition, the oxonium ions can adhere to a solution Surface of the glass membrane attach, but this not penetrate.
Due to the additional charge on the surface, the Li ions in the glass are pushed back from the surface. How many oxonium ions accumulate depends on the respective concentration of the oxonium ions in the adjacent solution. The higher it is, the more will accumulate. If the pH value on the two sides of the glass membrane is different, then the tendency of the oxonium ions to attach to the membrane is also different. Accordingly, there is also a charge difference between the two sides of the glass membrane and a resulting voltage above it. This voltage can then be recorded using the two electrodes.
Since there is no redox process per se, the Nernst equation cannot be used without problems to calculate the voltage. Instead, you have to Donnan's equation draw in. But this looks very similar to the Nernst equation:
- = Activity of the oxonium ions outside
- = Activity of oxonium ions inside
If you now want to titrate a solution, you can easily measure the pH value changes with this method, since the activity of the inside always remains the same due to the buffer and the impermeability of the membrane. A change in the measured voltage on the glass electrode can only come about through a change in the pH value of the solution on the outer layer. If you now want to track the pH value, which is necessary for an acid-base titration, you can easily change the above equation according to the activity / concentration of the oxonium ions on the outer layer.
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