Measuring the pH of small-volume samples is often necessary in life science research. Yet small-volume samples typically represent costly experiments, and sample manipulation can introduce error into pH results.
With the right pH micro sensor it is possible to minimize sample manipulation, even for low-volume samples, and achieve accurate pH measurement results. METTLER TOLEDO offers the micro pH sensors InLab Nano, InLab Ultra-Micro-ISM and InLab Micro Pro-ISM, as well as InLab Surface Pro-ISM for measurements of droplets or flat surfaces.
To discover the benefits of these sensors specialized in measuring the pH of small samples, download a free application note.
Below, we explore the challenges that may result when measuring the pH of small samples, and provide guidelines for measuring procedures and sensor selection.
How can I measure the pH of small samples?
Limited-volume samples are often kept in vessels too small for the electrode tip, requiring a sensor with appropriately sized and placed sensing membrane and junction. Typically, a semi-micro sensor displaces sufficient solution in a narrow container to cover the sensitive membrane and reference junction for an accurate measurement. However, sample volumes of a few microliters require specialized micro sensors with a very small membrane and low junction.
How can I measure the pH of flat surfaces?
Some samples are too small even for a micro sensor to measure accurately. In such cases, a surface sensor is the optimal configuration.
Surface pH sensors prevent sample contamination
Direct contact of the pH sensor with the sample during measurement can be a critical source of contamination. Reference electrolyte may flow into the sample; in addition, there is a risk of carryover from the rinsing solution, and residues may be present on the sensor. Pipetting at least 10 μL of sample onto a flat, clean surface and measuring with a flat membrane sensor can forestall such problems.
Measure sample, not sensor, temperature
pH is a temperature-dependent value, and sensor temperature may wrongly be interpreted as that of a sample when sample volume is limiting. Whenever possible, sensor and sample should be stored together to prevent temperature differences. No temperature probe is required for samples of known temperature; in this case the temperature can be entered manually during measurement.
Accurate measurement of pH in small samples relies on careful sensor selection and choice of measurement techniques. These guidelines can help you to successfully overcome the challenges of small volume measurements and achieve accurate, repeatable results.
Download an Application Note to learn about pH measurement in small samples: