Conductometric titration works where other methods struggle
2024. 2. 12.
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Conductometric titration, also called conductivity titration, is an analytical method based on the change of conductivity while adding a titrant. The change of the conductivity of the solution is measured after each titrant addition. This is done with a conductivity sensor. Principles, advantages, and some examples of conductometric titration are given in this blog article.
Introduction
Various industries, including the food and petrochemical sectors, utilize conductivity titrations. This method allows the determination of parameters in samples that are often difficult to quantify with other titration approaches. Conductometric titration offers a valuable solution to these analytical challenges.
Conductometric titration can be used for the following situations:
- Acid-base titrations: both aqueous and nonaqueous
- Precipitation titrations: Cl-, Br-, I-, SO42-, R–S–R, R–SH
- Complexometric titrations
What is conductometric titration?
The analytical method which is based on the change of conductivity in a solution while adding a titrant is called conductometric titration.
The overall conductivity of a sample is equal to the sum of the conductivities of the individual dissociated ions in the measurement solution. During the titration, the conductivity changes from the addition of the titrant and the reaction between titrant and analyte. The endpoint of the titration is indicated by a break in the titration curve. Examples of this are shown later in the article.
For more information about the determination of endpoints, read our blog post here.
What is the process of a conductometric titration?
Conductivity titration is a monotonic endpoint titration. This means that the titrant is added in fixed volume increments.
Executing this task involves employing a magnetic or overhead stirrer, a dosing tip, and the conductivity sensor. An important consideration when performing conductivity titrations is a fast sensor response time (see section on sensors for conductivity titrations for more information).
For all sensors with a removable sleeve, the sleeves are taken off. Additionally, the stirring is adjusted to a high rate. The limitation on the stirring speed is that there should be no air intake into the sample. Air bubbles on the sensor result in an unstable signal.
The OMNIS Software from Metrohm evaluates the typical conductivity titration curves as measured by the conductivity sensor in the solution.
Conductivity varies for each ionic species
The ions H+ and OH- both exhibit high ionic conductivity. The ions themselves do not move, but rather transport a proton or a proton gap via the hydrogen bond (Figure 1). Therefore, oxonium ions and hydroxide ions have a much higher ionic conductivity than most other ions.
Counting the ions
Consider the example of a conductometric titration of hydrochloric acid with sodium hydroxide. The chemical reaction equation is as follows:
Hydrochloric acid, being a strong acid, completely dissociates in water. Sodium hydroxide, a strong base, also dissociates completely in water. As stated previously, the measured conductivity is the sum of all dissociated ions in the solutions. To obtain the conductivity value of a sample, calculate the concentration of ions and their dissociation constants using the molar conductivity of each ion.
As shown in Figure 2, many H+ and Cl- ions are present at the beginning of the titration (on the left). The concentration of the Cl- ions does not change over the duration of the titration. The presence of the chloride ions contributes to the overall conductivity but remains unchanged during the titration.
Then the sodium hydroxide is added to the sample. This introduces Na+ ions to the sample, enhancing its conductivity. The amount of sodium ions increases continuously over the course of the titration. The hydroxide ions from NaOH also have an effect. The OH- ions neutralize the hydronium ions, forming water as shown in the equation above.
The conductivity value decreases significantly when hydronium ions are excluded from the total conductivity. The lowest conductivity is found at the titration endpoint where no hydronium or hydroxide ions are present (Figure 2, center).
Immediately after the endpoint, the conductivity rises sharply again. When more sodium hydroxide is added, OH- ions are present that no longer react with the hydronium ions (because none are left).
The following three examples explain different situations that are commonly encountered when performing conductometric acid-base titrations: titrating a strong acid with a strong base, titrating a strong acid with a weak base, and titrating a weak acid with a strong base.
This is a typical conductometric titration curve of a strong acid titrated with a strong base. The decrease in conductivity as H+ ions are neutralized follows the explanation given in the previous section. After the endpoint is reached (conductivity minimum), the OH- ions from the excess base contribute to the overall conductivity, making the curve rise again.
Conductometric titration of a strong acid with a weak base
In this example, a strong acid (e.g., hydrochloric acid, HCl) is titrated with a weak base (e.g., ammonia, NH3).
Initially, the solution's conductivity is high because the strong acid is fully dissociated. When adding the weak base, a reaction is triggered that forms ammonium ions (NH4+). As the weak base continues to react with the H+ ions, the solution's conductivity gradually decreases. This happens because the molar conductivity of NH4+ is much lower than that of H+.
The equivalence point occurs when all free hydrogen ions are neutralized. Afterward, the conductivity gradually increases again as the weak base titrant only undergoes partial dissociation.
Conductivity titration of a weak acid with a strong base
In this case, titration of a weak acid (e.g., acetic acid, CH3COOH) is performed with a strong base (e.g., sodium hydroxide, NaOH).
At the beginning of the titration, the conductivity of the solution is low. This is because the weak acid does not fully dissociate. When a strong base like sodium hydroxide is added, a reaction occurs with the undissociated acetic acid to form water. The release of sodium and acetate ions increases the conductivity.
The equivalence point is reached when the acetic acid has completely reacted with the sodium hydroxide. Once this happens, the conductivity considerably increases from the hydroxide ions (main contribution) and the sodium ions (minor contribution) after adding more sodium hydroxide.
Molar conductivity is the electrical conductivity of a fully dissociated ion in relation to molarity. As every type of ion conducts electricity differently, molar conductivity is a unique characteristic of each one (Table 1).
Table 1. Molar conductivity of different ions at infinite dilution.
Sensors for conductivity titrations
The most important parameter to consider when selecting a sensor for conductivity titration is its response time. As we are interested in the change in the conductivity, the absolute measurement value does not matter as much.
For this reason, the 4-wire conductivity measuring cell c = 0.5 cm-1 with Pt1000 (Figure 3, left) and the 5-ring conductivity measuring cell c = 0.7 cm-1 with Pt1000 (Figure 3, right) from Metrohm are most suitable. The sleeve of the 5-ring sensor is taken off before measurement.
Both sensors are ideal for conductometric titration – they are highly durable and exceptionally robust.
Advantages of conductometric titration
Conductivity titration has several advantages. First, no color indicator is needed, and therefore it is possible to titrate colored and turbid samples. Second, a single sensor can be used for all titrations. Third, even weak acids can be titrated since this method gives sharp endpoints for these kinds of samples, as shown in the example above.
Advantages of using conductometric titration
- Easy handling
- Maintenance-free electrode
- No reference electrode needed
- No indicator required
- Possibility to titrate very dilute solutions down to 0.001 mol/L
Conductometric titration in OMNIS is simple to run. Depending on the reaction, the user can readily adjust the titration parameters to achieve reliable results.
If a curve is difficult to evaluate, OMNIS offers users a comprehensive toolbox. The software allows for the addition of optimal tangents (straight lines) to the curve. Also, users can establish a measuring window to pinpoint the specific area where the endpoint should be identified. This can be flexibly adjusted for both the conductivity and the volume.
Conclusion
Overall, conductivity titration is a valuable analytical method for determining the concentration of ionic compounds in solutions. It offers a fast and precise alternative to conventional titration methods and allows the examination of many samples for their ionic content.
It also offers some advantages over classical potentiometric titration. The conductivity sensor exhibits high durability, requires no preconditioning, and it can be effortlessly cleaned with a cloth. The sensor requires no maintenance. Since the ions involved in the reaction are measured directly, there is no need for an indicator.
OMNIS software from Metrohm improves the efficiency and accuracy of conductometric titration.
The user has ultimate control and precision over the analysis with functions like smoothing, defining linear range, adjusting weighting factors, and flexible endpoint evaluation. This capability ensures reliable performance of conductivity titrations and the acquisition of accurate results. OMNIS software is therefore a valuable solution for laboratories that want to perform conductometric titrations.
