Tips for accurate titration using the Multipurpose Titrator MPT3

Introduction

The stability of a colloidal sample will vary with pH, and as such, pH titrations are a common experiment to assess these changes and to determine over which pH ranges a sample is stable (or not). The MPT-3 Multi-purpose Titrator for the new Zetasizer Advance range, allows the user to do this. In addition to traditional size or zeta potential measurements across the pH range of choice, the MPT-3 allows the user to unlock a greater understanding of their samples by setting up pH titrations in different ways. These are discussed below.

Measurements Within a pH Titration

Multiple Measurements at Each pH Point

When running a pH titration, both size and zeta potential measurements can be conducted at each pH point defined in the method. This can be useful to determine whether the change in pH, or the application of the voltage during the zeta potential measurement, has caused the sample to aggregate. This can be done in two different ways when setting up a schedule in the ZS Xplorer software, as shown in Figure 1.

Figure 1: Different ways to measure both size and zeta potential at each pH point during a titration, where the method on the left will measure size, then three zeta potential, then another size measurement at each point, whilst the method on the right will measure size, zeta potential, size, zeta potential, size, and zeta potential at each pH point.

Both methods shown in Figure 1 will yield size and zeta potential results at each pH point in the pH titration. However, they will measure the sample in different orders. The ungrouped method (Figure 1; left) measures in the order of size, three zeta potential, size, whilst the grouped method (Figure 1; right) will measure size, zeta potential, size, zeta potential, size, zeta potential. The grouped method will also optimize before every size and zeta potential measurement, whilst the ungrouped method will optimize before each size measurement, and once before the three zeta potential measurements. Which method is used is down to the preference of the operator and illustrates the flexibility offered in method settings by the ZS Xplorer software.

Multiple Titrations

A step further from doing a single pH titration, multiple, consecutive pH titrations can be run to glean more information about a sample. Three possible uses of this are:

• To gain higher resolution pH steps over a pH range of interest.
• To determine whether there is a hysteresis in the zeta potential when the pH is adjusted back and forth.
• To determine whether a sample is stable over a fluctuating pH range. 

These are discussed in turn below.

Higher Resolution pH Ranges

In a traditional pH titration, the pH step size is set at the beginning and remains constant throughout the titration. This means that if the user wants to quickly gain an insight of the sample over a large pH range, then a larger pH step size of 0.5 or 1.0 pH units are generally used to reduce the time taken for the titration to run. 

However, if the user is interested in a specific pH range, for instance around the isoelectric point, then they will often use a smaller pH step size to gain a higher resolution trend, but at the expense of the titration taking longer overall to complete. 

With the MPT-3, the overall titration time can be reduced by covering the full pH range and still have higher resolution data around the region of most interest. This is achieved by setting up a method with three consecutive pH titrations in the ZS Xplorer software, as shown in Figure 2. The interface where the pH sequence is set up is shown in Figure 3, whilst an example of the results which can be obtained via this method is shown in Figure 4.

Figure 2: Three consecutive titrations which will yield zeta potential measurements from pH 11 to 2, but with more data acquired in the pH 5 to 4 range to give higher resolution results.

Figure 3: pH sequence parameter interface found in the pH titration properties when setting up a pH titration.

Figure 4: Higher resolution titrations of a titania sample, where the sample was titrated from pH 11 to pH 5 with a step size of 1.0 pH units, from pH 5 to pH 4 with a step size of 0.2 pH units, and from pH 4 to pH 2 with a step size of 1.0 pH units.

In the example shown in Figure 4, the pH range of interest is between pH 5 and pH 4, where the pH step size is 0.2 pH units, whilst the rest of the pH titration between pH 11 and 2 has a pH step size of 1.0 pH units. This allows the user to understand the properties of their sample across the entire pH range from pH 11 to 2, whilst also gaining higher resolution data around the pH range of greatest interest to them, in this case the region around the isoelectric point, with an overall reduced measurement time.

Titration Hysteresis

The zeta potential of a sample will change as the pH of the dispersant changes. If this change in pH is then reversed, then the zeta potential of the sample will not necessarily follow the same trend as in the initial pH titration, resulting in a hysteresis. An example of how this can be set up in the ZS Xplorer software is shown in Figure 5, whilst an example dataset is shown in Figure 6. Running inverse, consecutive pH titrations in this manner can yield information on the pH range where a sample is stable in both directions (green), stable in one direction but not in the other (yellow), and unstable in either direction (red).

Figure 5: Two consecutive titrations with identical settings, apart from the direction of the pH titration, with one running from pH 10 to 2, whilst the other runs in the opposite direction, from pH 2 to 10.

Figure 6: Consecutive pH titrations of a sample from pH 10 to 2 (blue) and back again (orange), highlighting the hysteresis which can occur when the pH of a sample is altered and brought back. The colored regions represent the stability of the particles. The green region indicates where the particles are stable in both directions. The yellow region indicates where the particles are stable in the initial acidic titration, but not in the basic titration, and where caution should be taken when using this sample in this pH range. The red region indicates where the particles are unstable in both pH titrations.

Checking Stability Over a Defined pH Range

Multiple pH titrations can be used to check the stability of a sample over a fluctuating pH range. For example, if the system which a sample operates in is susceptible to some degree of pH fluctuation, then the pH of the sample can be repeatedly altered to go between the two extremes of the pH range it is exposed to. An example of how this can be set up in the ZS Xplorer software is shown in Figure 7, whilst an example dataset is shown in Figure 8.

Figure 7: Consecutive titrations, alternating in direction between pH 8 and 10 to allow the stability of a sample in a system with fluctuating pH to be assessed. Please note, the operator is not limited to four consecutive pH titrations.

Figure 8: Zeta potential (top) and z-average (bottom) of a titania sample as the pH is altered between pH 8 and 10.

The example shown in Figure 7 shows four titrations. However, the user can input as many as they wish, with 5 titrations used in Figure 8. The operator must be aware that, with the more titrations they run, the more likely the sample vial will overflow, and therefore will require greater supervision. Size and/or zeta potential measurements can be carried out as an assessment, with size measurements able to show aggregation very easily. The addition of the titrants will also lead to the conductivity of the sample increasing. This can be extracted from the Parameters Table and can be easily exported and plotted if of interest.

Plotting pH Titration Data

There are three plots specific to pH titrations built into the ZS Xplorer software; Titration Zeta Potential Trend, Titration Z-Average Trend, and pH Trend. The Titration Zeta Potential Trend shows the entire zeta potential results of the pH titration when the parent titration record is selected. This also enables multiple titrations to be selected and compared, as shown in Figure 9. The Titration Z Average Trend plot works in the same way except that it plots the z-average results instead of zeta potential.

Figure 9: Two pH titrations plotted using the Titration Zeta potential Trend plot in ZS Xplorer.

The pH Trend plot works slightly differently to the Titration Zeta Potential Trend and Titration Z-Average Trend plots and is populated when the individual child size and/or zeta potential measurements are selected. This has a benefit over the other two graphs if any of the following need to be plotted:

• Both size and zeta potential trends on the same plot.
• Average repeat pH titrations.
• Merge two titrations e.g. if different titrations have been run with different step sizes, as in the higher resolution pH range example explained above.
• To view a pH titration trend with outliers removed e.g. if a bubble gets stuck in the sample measurement area.

Figure 10 shows the size and zeta potential values for a latex sample plotted on the same pH Trend graph. Using the pH Trends graph in this way allows for the easy identification of correlations between the size and zeta potential results. In the dataset shown, once the zeta potential becomes less negative than -30 mV, the z-average begins to dramatically increase, due to aggregation occurring as the repulsive forces between the particles are reduced.

In addition to these specific titration plots, the Trend plot can also be used to plot other parameters, such as conductivity instead of pH. By selecting Measurement pH as the x-axis, then the y-axis can be configured for other measurement parameters, such as Polydispersity Index or Mobility, to be plotted as a function of pH. This plot essentially allows the user to customise which parameters are plotted, allowing them greater freedom to plot parameters from their titration data other than z-average, zeta potential, and measurement pH. To define which parameters to use on each axis, click the ellipsis on the top right-hand corner of the graph, and then click the cog. This will open a pop-up window where the desired parameters can be chosen. To populate the Trend plot, the child records of the titration need to be selected, as the parent titration records will not work with this plot.

Figure 10: Size and zeta potential measurements for a pH titration from pH 9 to 2, with step sizes of 0.5 pH units, plotted using the pH Trends graph.

Conclusions

The MPT-3 Multipurpose Titrator, in combination with a Zetasizer Advance instrument and the ZS Xplorer software, is a powerful tool to aid the understanding of a sample’s stability as pH changes. In addition to being capable of conducting traditional size or zeta potential measurements over a range of pH values, the MPT-3 can also be used:

1. to measure both size and zeta potential at each pH point
2. to gain higher resolution data over pH ranges of interest from consecutive titrations
3. to determine whether there is a hysteresis resulting from changing pH
4. to determine whether a sample is stable through smaller fluctuations in pH and conductivity. 

There are also four different plots which can be used to view pH titration data to best suit the requirements of the user and the information they are interested in. These are the Titration Zeta Potential Trend, Titration Z-Average Trend, pH Trend and Trend plots.