As with equivalent conductivity, there is equivalent resistance

Conductometry

Conductometry is another method in titration to identify the equivalence point. You can find everything else interesting about the topic in the article.

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Conductometry simply explained

With the help of Conductometry you can often find the equivalence point significantly in a titration more exact determine than by using an indicator. With this method you constantly measure the conductivity your sample solution while you continuously add the measure solution. Since most of the substances to be titrated are present in the sample solution as ions, these also carry as movable load carriers contributes to conductivity. However, if these ions are neutralized by adding the standard solution, the number of charge carriers and thus also the conductivity is reduced. If all ions of the sample solution are neutralized, the conductivity reaches a minimum in your measurement curve. This minimum corresponds to your equivalence point.

If you would like to know everything you need to know about titration in general, have a look at our own article on this topic!

Conductivity measurement

Now that you have understood the basic principle, you definitely want to know how to put it into practice. To do this, you can look at the following schematic experimental set-up.

The basic components of the other titration methods can also be found in conductometry. Here you also use a burette with which you can keep track of how much standard solution you have already added and a vessel in which there is a known volume of sample solution.

What makes the difference to other types of titration in this case is the Dive cell, also Conductometer or Conductivity measuring cell called. It consists of two parallel electrodes. During the measurement a Alternating current created. This can now be passed on through the ions present in the sample solution. A circuit is also attached to the two electrodes, which measures the resistance or conductivity across the two electrodes. Since the resistance also depends on other parameters, such as the area of ​​the electrodes and temperature, this conductivity measuring cell must be calibrated before it can be used. If the conductivity changes during the titration, this can be done with the Conductometer follow.

Conductometry process

As already said, one tries to reduce the number of ions in the sample solution by adding a standard solution. In an acid-base titration, for example, the conductive oxonium ions of an acidic HCl solution can be neutralized by adding sodium hydroxide ions:

The resulting water itself is uncharged and can no longer contribute to the charge transport.If you want to know more about acid-base titration, then click here.

Of course you get individual conductivities for each solution, as these depend on the ions present. The following three parameters determine the conductivity of the ions:

  • agility of the ions in the liquid
  • of ions carried cargo
  • concentration the ions in the solution

The respective conductivities of the individual ions are practically already as Conductivity equivalents tabulated, you can also analyze the measurement curve of solutions with several types of ions.

In the previous example of the titration of HCl with NaOH, you have probably already noticed that the addition of sodium hydroxide ions neutralizes the oxonium ions, but also new charge carriers in the form of -Add ions. Now the mobility of the load carriers comes into play. Oxonium and hydroxide ions can transfer charge much faster than simple ions by breaking hydrogen bonds.

Therefore, with an acid-base titration, a minimum of conductivity results when the pH value 7 is reached. With further addition of the basic standard solution, the concentration of the hydroxide ions would increase significantly, which instead of the oxonium ions then take over the charge transport to a comparable extent.

Conductivity titration evaluation

If you have now continuously followed the course of the conductivity during the titration, you now plot the conductivity in a diagram against the added volume of standard solution. This results in the following for the Conductivity titration typical course:

You can see it very quickly characteristic kink in the curve. This represents the minimum of the conductivity and is the point at which the complete sample solution neutralized has been. Therefore this kink point is also the Equivalence point. Now you can use this equality of substance quantities between standard solution molecules and sample solution molecules to infer the initial concentration of the sample solution.

Conductometry variants

There are several ways to do conductometry. The approach you have learned so far is called I.indication procedure. The conductivity of the sample solution is measured with the aim of being able to determine the equivalence point at the end.

In addition to this approach, there is also that Determination procedure. With this one also measures the conductivity, but at the end one does not calculate the equivalence point, but uses some calibration curves.

Conductometry compared to other titration methods

Finally, you might ask yourself what advantage conductometry has over other methods. To answer that, you have to compare the typical titration curve of conductometry with the classic titration curve.

If you choose the tangent method for a classic titration curve to determine your equivalence point, you can small deviations in the choice of the tangent point big mistakes lead in the equivalence point. You don't need this graphical determination method for conductometry. Here the equivalence point is already given by the characteristic kink in the curve. When determining from the graph, there can be hardly any errors.