Nonaqueous acid-base titrations are widely used in several industries, including the petrochemical and pharmaceutical sectors. Whether you are determining the acid or base number (AN or BN) in oils or fats, titrating substances that are insoluble in water, or quantifying products with different strengths of acidity or alkalinity separately, nonaqueous acid-base titration is the method of choice.
If you already have some experience performing nonaqueous acid-base titrations, you may remember that there are several challenges to overcome in comparison to aqueous acid-base titrations.
In this blog post, I would like to cover some of the most typical issues that could pop up during nonaqueous acid-base titrations and discuss how to best avoid them. An important point to note is that there is no single solution regarding how to perform any nonaqueous acid-base titration correctly. The right procedure depends highly on the solvent and titrant used.
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- What is a nonaqueous acid-base titration?
- Electrostatic effects
- Blocked diaphragm
- Choice of electrolyte and storage solution
- Checking the electrode according to ASTM D664
- Proper rinsing and cleaning
- Conditioning the glass membrane
- Maintenance of burets
- Thermometric titration as an alternative
Before discussing nonaqueous titrations, first let’s talk a little bit about aqueous acid-base titrations.
Here, a sample is dissolved in water, and depending of the nature of the sample (whether it is acidic or basic) a titration is performed either using aqueous base or aqueous acid as titrant. For indication, a glass pH electrode is used.
However, sometimes due to the nature of the sample, aqueous titration is not possible. Nonaqueous acid-base titration is used when:
- the substance of interest is not soluble in water
- samples are fats or oils
- components of mixtures of acids or bases have to be determined separately by titration
In these cases, a suitable organic solvent is used to dissolve the sample instead of water. The solvent:
- should dissolve the sample and not react with it
- permits the determination of components in a mixture
- if possible, should not be toxic
The solvents that are most often used include ethanol, methanol, isopropanol, toluene, and glacial acetic acid (or a mixture of these). Titrants are not prepared with water but rather in solvent. Frequently used nonaqueous basic titrants are potassium hydroxide in isopropyl alcohol or sodium hydroxide in ethanol, and a common nonaqueous acidic titrant is perchloric acid in glacial acetic acid.
Due to the nature of nonaqueous solvents, they are normally poor conductors and do not buffer well. This makes indication a bit challenging because the electrode must be suitable for such sample types. Therefore, Metrohm offers the Solvotrode which is developed specifically for nonaqueous titrations.
This pH electrode offers the following advantages over a standard pH electrode:
- Large membrane surface and a small membrane resistance for accurate reading, also in poorly buffered solutions
- A flexible ground-joint diaphragm which can easily be cleaned even when contaminated with oily or sticky samples, additionally it offers a symmetrical outflow for outstanding reproducibility
- The electrode is shielded and is therefore less sensitive to electrostatic interferences
- It can be used with any nonaqueous electrolyte such as lithium chloride in ethanol
In the following sections I will discuss the most common mistakes when performing potentiometric nonaqueous acid-base titrations and how you can avoid them.
This is then an indication of an electrostatic effect. However, where does it come from and how can we overcome this?
Electrostatic charge can be generated from many sources, such as friction. For example, while walking across a surface you will generate an electrostatic charge which will be stored in your body. You have probably touched the doorknob after walking across a carpeted space in your socks and obtained a small electric shock—this is the discharge of built up electrostatic charge. If we now assume that you are electrostatically charged and then you approach an electrode that is currently measuring (in use), this will result in a spike (Figure 1). Therefore, it is essential to make sure that you are either properly discharged or that you do not approach the electrode during measurement. You can avoid this issue by wearing the appropriate clothes. ESD (electrostatic discharge) clothes and shoes are mostly recommended when performing nonaqueous titrations.
A blocked diaphragm is another point which occurs more regularly during nonaqueous titrations. Due to the oily and sticky sample, you might have seen that the electrode diaphragm is clogged and cannot be opened anymore. What should you do then?
In most cases, you can place the electrode in a beaker of warm water overnight. This treatment often helps to loosen the diaphragm. To completely prevent the diaphragm from clogging, a Solvotrode with easyClean technology should be used. With this electrode, electrolyte is released by pressing the head ensuring that the diaphragm is not blocked.
We recommend two types of electrolyte for nonaqueous titrations.
For titrations with alkaline titrants: tetraethylammonium bromide c(TEABr) = 0.4 mol/L in ethylene glycol
For titrations with acidic titrants: lithium chloride c(LiCl) = 2 mol/L in ethanol
Please make sure to store the electrode in the same electrolyte with which it is filled.
To check whether the Solvotrode is still in good working condition, perform a test according to ASTM D664 using aqueous buffer solutions of pH 4 and 7. The procedure is as follows:
- Measure the potential of buffer pH 4.0 while stirring and note the value after 1 minute
- Remove the electrode and rinse it well with deionized water
- Measure the potential of buffer pH 7.0 while stirring and note the value after 1 minute
- Calculate the mV difference between the reading of buffers 4.0 and 7.0
- The difference must be larger than 162 mV (20–25 °C) to indicate an electrode in good shape
If the measured potential difference is less than 162 mV, the electrode requires maintenance. Lift the flexible sleeve of the ground-joint diaphragm to let some electrolyte flow out. Repeat the measurement according to the steps above. If the value is still less than 162 mV, clean the electrode or replace it.
Proper rinsing is essential if you want to obtain reliable results. Otherwise, the curve might flatten and the equivalence points are no longer recognizable. Figure 2 illustrates this phenomenon well.
The sample is the same, however, you see that the equivalence point and starting potential begin to shift and the curves become flatter. This indicates an improper cleaning procedure between measurements. The corresponding electrode is shown in Figure 3.
This electrode was certainly not cleaned properly! Anyone who performs a nonaqueous titration must consider which solvent might best dissolve the residue—this is not an issue that other analysts can easily solve due to the nature of each individual sample. However, do not ignore an electrode with such an appearance.
As you may remember from our previous blog post about pH measurement, it is essential that the hydration layer of the glass membrane stays intact. Nonaqueous solvents dehydrate the glass membrane rather quickly. A change in the hydration layer can have an impact on the measured potential, therefore it is important that the hydration layer is always in the same state before starting a titration to achieve the most reproducible results.
This can be established with a conditioning step of the glass membrane to rebuild the hydration layer. However, if the solvent is able to remove the hydration layer faster than it takes to perform a titration, this can lead to ghost equivalence points. Therefore, the electrode should be completely dehydrated and kept like this for all further titrations.
|Polar solvents (e.g., ethanol, acetone, isopropyl alcohol, or mixtures with toluene)||Water-free solvents (e.g., dimethylformamide, acetonitrile, acetic anhydride, or mixtures of these)|
|Preparation of electrode||
Store only the pH membrane (not the diaphragm) in deionized water overnight to build up a proper hydration layer.
Lift the flexible sleeve to allow some electrolyte to flow out.
Dehydrate the pH membrane by placing only the pH membrane (not the diaphragm) in the solvent you will use afterwards for titration.
Lift the flexible sleeve to allow some electrolyte to flow out.
|Conditioning of glass membrane||Place the pH membrane (bulb only) into deionized water for 1 minute.||Place the pH membrane (bulb only) into the corresponding solvent for 1 minute.|
|Rinsing procedure||Rinse electrode with 50–70% ethanol. If this does not help, use a suitable solvent to rinse the electrode and then clean afterwards with 50–70% ethanol.||Rinse electrode with glacial acetic acid. If this does not help, use a suitable solvent to rinse the electrode and then clean afterwards with glacial acetic acid.|
|Remarks||Make sure to always keep the bulb of the electrode in deionized water for the same time duration, otherwise the thickness of the hydrated layer (and therefore the response) may vary.||Avoid any contact of the electrode with water as this can induce a reaction with the solvent causing ghost equivalence points and irreproducible results.|
It is not only the electrode that needs some special attention when performing nonaqueous titrations, but also the electrical buret. Some special maintenance is required since alkaline nonaqueous titrants are especially aggressive and they tend to crystallize, therefore leakage of the buret is likely.
The buret must be maintained on a regular basis according to the manufacturer’s instructions. Metrohm recommends the following procedure:
- For shorter titration breaks, it is recommended to refill the cylinder with titrant (especially with OMNIS)
- Clean the buret with deionized water at the end of the day
- Lubricate the cylinder unit on the centering tube and on the cylinder disc
Also check the corresponding manual of the buret. The most important points are mentioned there which will lead to a longer working life of the buret.
One alternative to using potentiometric nonaqueous acid-base titration is thermometric titration (TET), depending on the sample and analyte to be measured. Thermometric titration monitors the endothermic or exothermic reaction of a sample with the titrant using a very sensitive thermistor.
The benefit of TET over potentiometric titration is clearly the maintenance-free sensor which does not require any conditioning nor electrolyte refilling. More information about thermometric titration can be found in our previous blog posts below.
Hopefully this article has provided you with information about the main problems encountered during nonaqueous titrations. First, make sure that all electrostatic influences are eliminated. This will save a significant amount of troubleshooting. Then prepare and treat your electrode correctly before, during, and after titration. Make sure to condition the electrode right before your first measurement!
Of special importance here is the solvent you plan to use. If it is a polar solvent, the electrode should be conditioned in deionized water. If nonpolar solvents like acetic anhydride are used, the electrode should be dehydrated first. Between measurements, the electrode should be cleaned with a suitable solvent and the diaphragm should be opened on occasion.
Last but not least, take care of your buret. Maintain it regularly and replace it whenever necessary. With this advice, performing nonaqueous titrations should be a breeze!