Automated, High-Precision Quantitation of Known Chemicals

Titration is a common laboratory operation for quantifying chemicals or reagents, usually in aqueous solution. In a typical setup, the titrand—the solution containing the unknown— is treated with precise volumes of a standard solution of the titrant— the reagent of known strength. The titration end point is reached when a chemical balance is achieved between the titrant and titrand. The concentration of the unknown in the original sample is calculated through simple equations related to the applicable chemical stoichiometry and any dilutions that may have occurred during sample preparation.

Many scientists first experienced titrations during acid-base laboratory exercises in first-year chemistry class. While pH determination remains a popular application, the range of chemicals amenable to analysis by titration is huge and includes many metals and nonmetallic elements as well as specific compounds. Titrations may be based on oxidation-reduction (redox), metal complexes, zeta potential (for measuring colloids), amperometry, and other modalities.

Unlike spectroscopy, which can help identify (and often quantify) unknown compounds through their absorption of electromagnetic radiation, titration is limited to measuring concentrations of known materials. “Titration is used to quantify something you already know is there,” says Tore Fossum, technical director at Mettler Toledo (Columbus, OH). Nevertheless, the technique has become a mainstay in analytical laboratories serving practically every industry, including chemicals, materials, environmental, and foods, to support quality control, process monitoring, or regulatory requirements.

Common titration analyses include measurement of salt in potato chips, moisture in pharmaceuticals (e.g. the Karl Fischer redox method), heavy metal content of water/wastewater, iodine levels in solution, biological activity of enzymes or substrates, peroxide solution strength, wine acidity, pH of biological buffers, and many others.

All titrations require some type of indicator to alert the analyst that titrant and titrand are stoichiometrically balanced. Chemists are familiar with the range of colorimetric indicators for various pH ranges and the colorchanging reagents or signals in redox titrations. Another common visual indicator is precipitation. Analysts also use instrumentation to detect end points by changes in solution conductivity, heat absorption, and appearance of species that absorb specific wavelengths of light.

Although titration can be extremely precise, numerous sources of error exist, for example, measurement of the titrant aliquot, indicator variability, operator fatigue and computation error, anomalies in composition of the standard solution, accuracy of the delivery system, and other systemic and nonsystemic errors. Titrators are specialized instruments that perform titrations with minimal operator intervention and can thus minimize errors, improve throughput, and facilitate documentation.

Enter automation

Fossum notes from his own experience that manual titration takes time—up to 30 minutes for measurement of acids in fuel oils, for example. “Today you can put your sample in a beaker, push a button, and walk away to do something more productive.” He recalls that one company where he worked employed as many as a dozen operators who performed routine titrations for quality control. Despite their experience, systematic errors crept into the workflow but were eliminated when the company switched to automated titration.

“Compared with manual titration, a titrator will save time and improve the quality of the result,” he says, “while freeing workers to do things that are more productive.”

Mettler Toledo sells most of its titrators as stand-alone instruments, but add-ons such as sample changers can improve throughput dramatically for high-volume labs. Data management is another important feature, particularly for regulated industries that must document laboratory activities in formal reports. Titrators in pharmaceutical and environmental labs are increasingly connected to LIMS (laboratory information management systems), which tabulate and output data in approved formats.

Advanced titrators also incorporate onboard computers for storing and accessing methods, and that may record results locally. Touch-sensitive flatpanel displays allow users to select and deploy methods through visual menus.

Additional differentiators, according to Robert V. Menegotto, business development manager at Mandel/ Man-Tech (Guelph, ON), are software ease of use, user-configurable functions and routines, flexibility of automation (“how many samples can run unattended?”), the ability to run several titrations for multiple analytes in a single sample, and integration of spectrophotometric or other measurement techniques for advanced analysis and end point determination.

The importance of accuracy in delivering titrant cannot be underestimated. “Unlike human operators, a titrator equipped, for example, with a pH electrode automatically senses the end point approaching,” Menegotto says. “It will automatically inject smaller quantities than an operator could with a traditional stopcock burette. And there’s no operator bias, which means you get more accurate results.” Menegotto notes that automated titrators can deliver titrant aliquots as small as 0.2 microliters, which is impossible from a standard burette.

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