How to Achieve Accurate Titrations

A common method of quantifying the presence of an analyte in a sample is through titration, remaining relevant for centuries thanks to the simplicity and accuracy of the technique. However, there is always room for improvement, and the results of titrations are often variable between the chemists performing the analysis. Several factors affect the accuracy of the results:

1. Standardize the Titrant

“When is the last time you’ve standardized your titrant?” This is often the first question asked when troubleshooting a questionable result. Standardization is a process that normalizes the titration system and provides the most accurate concentration of the titrant, critical in calculating the analyte content of a sample.

If not known exactly, results can off disastrously. For example, sodium hydroxide can absorb CO2 from the surrounding air, causing the true concentration to decrease over time. Using a CO2 absorption tube filled with soda lime can help to lower the influence of this effect, but performing titrant standardization is still best practice.

Standardization in triplicate, and a %RSD. Values of less than 1% in GLP laboratories. Standardization methods are preprogrammed for most common titrants (NaOH, HCl, H2SO4, AgNO3, and many others) in SI Analytics titrators, making it easy to implement the procedure. Figure 1 demonstrates the standardization procedure of a strong base.

Figure 1. Titer determination of 0.1 mol/l NaOH with potassium hydrogen phthalate

Standardization should each time a fresh titrant to the reservoir, or otherwise periodically (monthly or bimonthly). If results of a titration appear questionable, performing a titrant standardization procedure is a sensible place to begin troubleshooting.

2. Homogenize the Sample

Homogenization should be considered, depending on the type of sample in question. It is a mechanical process that ensures uniformity in a sample, pertaining to titration as it can help to ensure analyte molecules are completely accessible to react with the titrant.

For example, when titrating for the salt content of cheese, it is hard to break down the sample and release the salt using just a magnetic stirrer. Homogenization can help to break down the sample, releasing the salt, which is then free to react with the sample. Without homogenization, the salt concentration may appear lower than expected.

3. Sample Measuring Techniques

However a sample is measured, whether by volumetric pipettes, or using proper weighing techniques, it is of great importance that it is done accurately. Looking at the basic titration calculations, it is clear why this is so:

                                                                       C2 = C1 * V1

                                                                                    V2

C1 = Concentration of titrant
V1 = Volume of titrant consumed at end point
C2 = Concentration of analyte in sample (UNKNOWN)
V2 = Volume (or weight) of sample

The above calculation shows that the volume of sample is employed to calculate the amount of analyte in the sample. Any inaccuracies in the measurement of volume at this stage will negatively affect results.

For liquid measurements glassware accuracy is usually determined in terms of tolerance, which is the uncertainty of the measurement. Graduated cylinders generally have a measured volume uncertainty of 0.5 to 1%. Volumetric pipettes usually bear a lesser uncertainty of ± 0.1%, a significant improvement. Sample volumes are usually relatively small in titration experiments, so using volumetric pipettes is advised.

Solid samples should be weighed using a calibrated balance, using one that will reflect the desired number of significant figures in the final result. For example, for a result that must be displayed to two significant figures, a balance capable of detecting hundredths will suffice. A calibrated and stable analytical balance is highly recommended to determine the weight accurately.

4. Electrode Calibration

Obvious, but an important reminder. If working with an electrode for titrations, the success or failure of the measurement depends largely on the condition of the probe.

pH electrodes should be calibrated every day to achieve the most accurate reading. Two-point methods are typically sufficient, when using appropriate buffers. Appropriate buffers should be selected based on the measurement range likely to be expected from the titration. For example, calibration buffers for titrations taking place in the pH 5-6 range should be pH 4 and 7. It is also important to use clean, fresh buffers.

The response time of an will increase over time, indicating that calibration should more frequently. The offset and slope may also change, which also indicates that calibration should more frequently. The following limits are commonly used to indicate that an electrode is trustworthy:

Slope 95% to 102%

Zero point pH 6.5 to pH 7.2

Electrodes that are properly maintained are likely to last an average of 1.5 – 2 years, depending on the frequency of use.

5. Air Bubbles in Burette

Air bubbles can form in the burette, whether performing titrations manually or by. Air taking up space in the burette can lead to false since it appears that the volume of titrant measured is more than was consumed.

When using manual stopcock burettes, air bubbles can form at the tip of the burette. These can usually be removed by flicking the burette or purging several of titrant. When using an or piston burette, air bubbles may form in the titration lines or the burette cylinder. Flushing titrant through the tubing and burette will remove these air bubbles.
 

This information has been sourced, reviewed and adapted from materials provided by OI Analytical.

For more information on this source, please visit OI Analytical.

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