23 Sample Preparation & Processing
that a maximum accuracy regarding a drop size of 50 µL can be achieved. Assuming a consumption of about 5 mL titrant, this may lead to an error of up to 1%.
Error of buret
As with all glassware, the buret itself has a specifi c tolerance. In case of a 50 mL buret, the allowed tolerance is 50 µL. However, this is not the only error source resulting from handling a buret, such as the Parallax error. This error occurs if the analyst does not view the meniscus horizontally, but from an angle. Meniscus readings are different depending on the viewing angle, as shown in Figure 4.
precision can be further enhanced by using a motor-driven buret with a smaller volume. Visual perception no longer matters
Automated titration could not have been established without the development of sensors. In 1909, the fi rst glass electrode for potentiometric titration was built. Potentiometric sensors allow an endpoint or equivalence point determination independent from a colour change or bias from an analyst.
As various samples require different electrode properties, there are currently a large number of sensors available. Table 2 lists some examples for the different types of titration.
One sensor in particular must be mentioned when discussing automating titrations: the Optrode. As titrations with colour indications are still common, the Optrode was developed - a sensor which is able to detect minute colour differences by determining the change in absorbance at a specific wavelength, then converting the absorption into a measureable potential. In such a way, also colorimetric titrations become more accurate and precise, as they no longer depend on the visual perception of laboratory analysts.
Traceable results for data integrity
During manual titration procedures, all results are read from the buret and written into the lab journal or typed manually into software. This is an error-prone process, with a high likelihood of the values transferred incorrectly. To solve this issue, automated titrations record the measured values into a measuring point list and the result calculation is performed automatically on the device. These results can be exported as a PDF fi le or printed with date and time stamp.
Less sample necessary
Figure 4. Parallax error occurs if the user reads the meniscus values from different visual angles.
All of the previously discussed points show that manual titration is easy to perform but is infl uenced by the user. Also, manual titration is not very advantageous cost-wise due to the high amount of time needed for cleaning, refi lling the buret, as well as manually calculating the results. Safety of the lab personnel is not guaranteed, as chemicals can be spilled easily while refi lling the buret. Data integrity is another point to consider for manual titrations, since all data must be transferred manually into a notebook or computer. The calculations are not carried out automatically, increasing the risk of errors. Human error is a likely possibility in this situation.
Development of semi-automated titration
To overcome the limited accuracy and precision of manual titrations, an electronic buret can be used. It consists of a motor-driven spindle and a glass cylinder, which is fi lled with titrant. Furthermore, recent generations are equipped with a built-in stirrer and allow the user to carry out calculations automatically and save results on a storage device (e.g., USB stick or PC), or print the results directly after analysis.
Semi-automated titration: Even more accurate
When switching from manual methods to semi-automated titration, the main improvements relate to the accuracy and precision. Electronic burets can dose as precisely as 2.5 µL for a 50 mL cylinder unit, which enhances the accuracy by a factor of 20 compared to a manual titration. However, the biggest disadvantage still persists: the subjectivity of visual perception. Therefore, the next step in the evolution of titration is automated potentiometric titration.
Automated potentiometric titration overcomes the remaining disadvantages
Automated potentiometric titration was developed in the mid-1960s. The collection of data, equivalence point determination, and evaluation can be done automatically. The benefi ts are tremendous:
• improvements in both accuracy and precision • reduced time for tedious manual processes • minimised potential for human error
Improved accuracy and precision
A resolution of 10,000–100,000 steps can be achieved with modern auto-titrators. This corresponds to a precision of 5 µL down to 0.5 µL for a 50 mL motor-driven buret. The
Titrations become more cost-effective
The most expensive part in any manual titration is the cost of labour. Lab personnel must be properly trained and are also fully occupied during titration work. This changes completely with automated titrations. The method on the autotitrator is programmed once and can be recalled at any time by any user simply by pressing a button.
A comparison was made to determine the analysis time needed for a series of fi ve titrations performed both manually and automatically. The results are summarised in Table 3.
In this case, manual titration is faster than the automated method, but with much less accurate results. The mean analysis time per automated titration was measured at 6 min 12 s, meaning that over the series of fi ve determinations, the presence of the lab analyst only accounted for
To achieve acceptable accuracy and precision values with manual titration, a large volume of sample is used to obtain a titrant consumption within a certain range, to overcome the error of the volume addition which might be up to 0.05 mL.
On the contrary, the volume addition by a motor-driven buret might go down to 0.5 µL. This enables the operator to reduce the sample size and to use less chemical reagents. It is recommended to utilise between 10-90% of the cylinder volume of the buret and to adjust the sample size accordingly.
Table 2. Specifi c electrodes for different applications.
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