Sample Preparation & Processing
Manual vs. Automated Titration: Benefi ts and Advantages to Switching
Iris Kalkman, Metrohm AG
Titration is one of the most commonly used analytical methods. Manual, semiautomated, and fully automated titrations are well-known options and are examined in detail in several academic studies. This article summarises the advantages and benefi ts of automated titration in comparison to manual titration. The increase in accuracy and precision of measurements as well as signifi cant time and cost savings are discussed.
Beginnings of titration
Titration is a primary analytical method that is very easy to perform - a buret, a titrant, and a suitable endpoint indicator are the only required items. Therefore, it is not surprising that this is also one of the oldest quantitative analytical methods.
The fi rst buret was invented in the 18th century by Francois Antoine Henri Descroizilles [1], and was further developed by Karl Friedrich Mohr who wrote the fi rst book about titration titled ‘Instructional Book of Titration Methods in Analytical Chemistry’ back in 1855.
Principles of titration
Amedeo Avogadro fi rst proposed in 1811 [2] that the amount of molecules in a particular volume of gas always remains the same at standard conditions, no matter what type of molecules it contains.
A century later in 1909, Jean Perrin defi ned the term ‘Avogadro constant’
as exactly 32 grams of oxygen molecules (a ‘mole’ of O2) [3]. This constant meant that a defi ned amount of mass of a particular substance contains a specifi c amount of molecules; therefore, the amount of molecules in a volume of solution is also defi ned. Titration was born - it is just as simple as counting the molecules of a substance (the analyte) in a sample matrix.
The volume of titrant, which is used to facilitate a chemical reaction with the analyte, is determined via titration. All chemical reactions take place in defi ned stoichiometric ratios. Therefore, the concentration of the analyte can be determined by knowing the exact concentration of the titrant and calculating its consumption (volume) with a measurable endpoint. A typical benchtop setup for a manual titration is shown in Figure 1.
Figure 1. Typical setup for a manual titration. The buret is fi lled with titrant, and the Erlenmeyer fl ask contains the sample solution which includes the analyte to be measured.
Determination of pH with titration indicators
There are several variations of titration, with pH titrations being the most common. The pH value at the endpoint may vary, as the acid dissociation constant (pKa) differs depending on each individual acid. Figure 2 shows a selection of pH indicators that cover the majority of the pH range from 0-14.
Type of Titration Indicator
The colour change for each of these indicators, especially for the complexometric reactions, is diffi cult to accurately predict as it depends on both the pH value and the complexed metal.
Diffi culties of manual titration
Although manual titration is nearly 200 years old, it is still frequently used and mentioned in several standards and norms. However, manual titrations face some diffi culties.
Visual perception of the endpoint
Each person experiences colours and colour intensity differently. This leads to some deviations or bias depending on the individual analyst performing a manual titration. Figure 3 illustrates this point and the diffi culty in determining an endpoint visually.
The endpoint of other titration types, like redox, complexometric, or argentometric titrations, can also be determined by the colour change of a suitable indicator. Some examples of the indicators used for these different reaction types are listed in Table 1.
Table 1. Common endpoint indicators used for different reaction types.
Figure 3. Titration of c(HCl) = 1 mol/L with c(NaOH) = 1 mol/L and phenolphthalein as the indicator. Each of these images differs only in the addition of a single drop of the NaOH titrant.
The intensity of the colours obtained in Figure 3, 1-5 differ only by approximately 50 µL NaOH titrant in each case. The question arises regarding where the ‘correct’ endpoint should be chosen. If this is not handled in exactly the same manner by different analysts, then the precision of the measurement will suffer.
Drop size Figure 2. Colour changes of different pH indicators depending on the pH value.
With manual titration, results can only be as accurate as the smallest drop size from the buret. In the pharmaceutical industry, one drop is defi ned to be 50 µL. This means
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