Maybe you’ve read our earlier blog post on the main error sources in manual titration and are now wondering what you have to do in order to convert your manual titration to autotitration. In this blog entry, I want to give you a step by step guideline on how to proceed and what you have to consider.
Let’s jump right in with the following topics (click to go directly to each topic):
Choice of sensor
The first and most crucial step in transferring a manual titration to autotitration is the choice of the sensor for the indication of the equivalence point.
One of the simplest choices is to use a photometric sensor, effectively replacing the human eye with a sensor, especially when norms or standards stipulate the use of color indicators. It is often simpler to use a potentiometric electrode for indication purposes, because indicator solution is not needed and you can even combine multiple titrations into one to save time.
The electrode choice depends on the type of reaction, the sample, and the titrant used. Acid-base titrations require a different electrode than redox or precipitation titrations. Additionally, the sample matrix can have a significant influence on the electrode. The more complex the matrix is, the more crucial the choice. For example, you must use a different pH electrode for non-aqueous titrations than for aqueous titrations.
To help you select the best electrode for your titrations, we’ve prepared a free flyer, which you can download below. If you prefer, our Electrode Finder is even easier to use. Select the reaction type and application area of your titration and we will present you the best solution.
Optimizing the sample size and solution volumes
If you’ve performed manual titrations before, then you know that many of these methods have endpoints requiring the use of up to 30 mL or even 40 mL of titrant. Autotitrators are most commonly equipped with 10 mL or 20 mL burets. Since refilling the buret during the titration causes errors, you should reduce the sample size for autotitrations. In general, for autotitration it is recommended that the equivalence point lies between 10% and 90% of the total buret volume. The second step when transferring a manual titration to autotitration is thus the optimization of the sample size.
Don’t forget: decreasing the sample size additionally reduces the waste produced since you need less titrant, contributing to greener chemistry and cost savings with each titration.
When you transfer a manual titration to autotitration, it might be necessary to adjust the amount of used diluent (water or solvent) for the analysis. To obtain accurate results it is imperative that the glass membrane (for measurement) and diaphragm (for reference) of the sensor are fully immersed into the solution, as shown here.
Selecting the right titration mode
Depending on the reaction type, some titration reactions are completed faster than others. For this reason, autotitrators are equipped with different titration modes.
The two most often used modes (monotonic and dynamic mode) can be distinguished by the way the titrant is added. When using the monotonic mode, the same amount of titrant is always added with each addition. In the dynamic mode, the amount of titrant added differs depending on how close you are to the equivalence point. The closer you are to the equivalence point, the smaller the additions—similar to a manual titration.
As a rule of thumb, use the dynamic mode for fast titrations such as acid-base titration and the monotonic mode for slower titrations, where the equivalence point is suddenly reached (e.g. vitamin C determination).
Learn more about the different titration modes in this free webinar available on our website.
Optimizing the titration setup
The stirring speed and sensor placement within your titration beaker will influence the accuracy of your results.
Depending on the sample beaker and stirrer, choose the stirring speed in such a way that the mixing is fast enough but no splashing occurs. Also be sure that no vortex is created in the solution due to the speed of mixing, leaving your electrode hanging in dry air. Make sure to place the electrode close to the beaker wall and upstream of (behind) the buret tip as displayed in this example. This allows for an ideal mixing of the titrant with the sample and improves your accuracy.
Optimizing the titration method
In this last step, I want to present some options about how you can optimize your titration in regards to titration speed and titrant consumption.
One efficient way to speed up titrations, especially monotonic titrations, is to utilize a start volume. This would be similar to a case where you pre-dose titrant to your sample before you beginning the manual titration. Don’t forget to add a pause step after the addition! This way, the added titrant can mix well with the sample before the titration starts.
To save titrant and to reduce waste, I recommend to use stop criteria. The simplest stop criterion is the stop volume. If your equivalence point always occurs at the same volume, this is the easiest way to go. If the volume of your equivalence point varies, you can use the option to define a volume which should be added after the value for the expected equivalence point has been reached. In general, I recommend a stop volume of approximately 1 mL after the equivalence point.
- Select the right sensor for your titration
- Adjust sample size and diluent volume
- Select the titration mode depending on your reaction
- Optimize your titration for speed and titrant consumption
You see, changing from manual to autotitration is as easy as it sounds – maybe even easier!
If you want to learn even more about practical aspects of modern titration, have a look at our monograph.
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Practical aspects of modern titration
Post written by Lucia Meier, Product Specialist Titration at Metrohm International Headquarters, Herisau, Switzerland.