Guide to Potentiometric and Karl Fischer Titrators

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Please check out our  Potentiometric Titrators or KF Titrators sections for more information or to find manufacturers that sell these products.

Titration is the process of determining the concentration of a chemical substance in a sample. Because there are many kinds of substances and samples, there are also several methods of titration, and, accordingly, several types of titrators to accomplish this. The first consideration in choosing a titrator is to define the type of titration that is required. This may include acid/base, Karl Fischer (KF), potentiometric, redox, gas phase, complexometric, or precipitation titration. This article focuses on potentiometric and Karl Fischer titrators.

Karl Fischer titrators

KF titrators measure the amount of water in a sample. Besides being used in research laboratories, these widespread titrators are used in the food, pharmaceutical, and semiconductor industries because the amount of water in their products greatly influences product quality. Key considerations in selecting a KF titrator are the approximate amount of moisture expected in the sample; the samples and solvents that may be used; and whether occasional, manual measurements are needed, or large numbers of measurements are required regularly. “It first starts with the researcher deciding that moisture determination is necessary,” according to David Minsk, President of Hanna Instruments (Woonsocket, RI). “Then it comes down to the projected range of moisture. If those questions put you into the Karl Fischer area, then explore the relationship between sample and solvent to determine if there is a readily available solvent that can break down the sample.”

Moisture range

The first step in choosing a type of KF titrator is to determine the approximate moisture range of the samples. There are two types of KF titration—volumetry and coulometry. Volumetric titrators are usually designed for moisture levels ranging from 100 ppm to 100%, whereas coulometric titrators measure water content in the range of 1 ppm to 5%. Volumetric titrators measure the water in a sample according to the amount of titration reagent used, while coulometric titrators measure the water in a sample according to the amount of electricity used in an electrolysis reaction. “A volumetric Karl Fischer titrator adds the iodine-containing titrant with an accurate, mechanically driven burette to the solution of alcohol (typically methanol), and also contains sulfur dioxide and imidazole (both key to the reaction),” explained Matthew Eby, Product Manager for Titration at Mettler-Toledo (Columbus, OH). “A coulometric Karl Fischer titrator is used to electrochemically generate iodine to react with the water from the sample from iodide already in the specialized coulometric titration solution.”

Mettler-Toledo offers two models of each kind of titrator. The volumetric V20 and coulometric C20 are simpler, entry-level instruments that are suitable for quick and easy results, but have less flexibility and fewer opportunities to manage other external devices. In contrast, “the V30 [and C30] offer full flexibility with templates for external extraction, full FDA-compliant user management, and more methods and user shortcuts possible,” said Eby. “The V30 models also offer full titrant traceability with intuitive reagent management.” The coulometric titrators C20 and C30 also come with a diaphragmed (D model) or diaphragmless (X model) iodine generator.

Metrohm (Riverview, FL) also offers both types of titrators—the coulometric instruments include the new, portable 899 Coulometer model from the Coulometer series, as well as the Coulometric Titrando line. The volumetric instruments are three models in the Titrando line, and other instruments offered by the company have added volumetric KF functions built-in. Hanna Instruments also supplies a volumetric KF titrator that is completely customizable.

JM Science (Grand Island, NY) offers two models each of coulometric and volumetric titrators manufactured by Hiranuma Co. (Mito, Japan). The AQUACOUNTER™ AQ-2200 Karl Fischer Coulometric Titrator and AQV-2200S Karl Fischer Volumetric Titrator are designed with automation and upgrade capabilities. The AQ-300 Karl Fischer Coulometric Titrator includes six built-in calculation modes for evaluating solid, liquid, and gas samples. The AQV-300 Karl Fischer Volumetric Titrator is an easy-to-use, more affordable system with a small laboratory footprint.

Another volumetric option is the Karl Fischer Moisture Titrator MKA-610 from CANNON Instrument Co. (State College, PA). This titrator measures moisture in the 10 ppm to 100% range, and includes several convenient features, such as a large (8-in.-wide) LCD touchscreen, and the ability to control two simultaneous titrations individually. The measuring unit can also be detached from the operating unit, in case this is required for titrating in nontraditional sample locations. Another useful feature is automatic adjustment of electrode sensitivity and endpoint detection for solvent and sample types, based on patented technology from the manufacturer, KEM.

Samples and solvents

The solubility and pH of a sample are important considerations, and determine in part what types of solvent might be needed. Eby advised asking whether the sample is soluble in methanol. “If so,” he said, “direct injection of the sample to the titration solvent can typically be achieved, and if not, co-solvents can be used to help bridge the solubility gap to the sample.” Extremely acidic or extremely basic samples can be challenging, since they can disrupt the proper functioning of the KF reagent, which functions best in the range of pH 5 to 7.“This is not a limitation as much as a consideration where extra buffers may be needed in the solvent to negate these effects,” explained Eby. “For example, salicylic acid [may be] added to the solvent when testing strongly caustic amines with direct injections.”

Potentiometric titrators

In potentiometric titration, the titrator measures the potential between two electrodes (one reference, one measurement) as reagent is added. The use of electrodes makes this method more precise, so it is preferable when accuracy is especially important. Potentiometric titration is also quite amenable to automation, where the titration reagent can be autopipetted in discrete amounts by robotic systems, while the electrodes take regular measurements. This method is commonly used for acid-base titrations.

Figure 1 – HI902C potentiometric titrator (courtesy of Hanna Instruments).

Hanna Instruments offers a broad line of potentiometric titrators, including mini, automatic models. “The [line] includes mini titrators that do specific titrations for acidity and alkalinity in water, acidity in juice and dairy, and SO2 in wine,” said Minsk. “These affordable automatic titrators allow customers to take an ‘affordable step’ from manual titrations.” On the other end of the spectrum is Hanna’s HI902 multiparameter potentiometric titrator (see Figure 1), which includes a USB interface and the ability to link multiple titrations. Its functionality can optionally be expanded to include ion-selective electrodes as well.

Other options for potentiometric titrators include offerings from JM Science and CANNON Instrument Co. The AQUACOUNTER COM-300A Potentiometric Titrator from JM Science is capable of many types of measurements, including pH, acid/base, complexometric, redox, Karl Fischer, and nonaqueous titrations. This model can also be upgraded if a titrator needs to be changed. CANNON Instrument Co. offers the Automatic Potentiometric Titrator AT-610, which includes a large (8-in.-wide) LCD touchscreen, and can simultaneously run two types of titrations independently. This instrument can be combined with a coulometric or volumetric KF titrator.

Accessories and features for titrators

Titrator manufacturers make available accessories and extra features to help users get the most out of their purchase (see Table 1). Many models of titrators offer automatic filling and emptying of the titration vessel, which is not only a convenience, but is also a safety feature. Some offer one-touch or touch-screen operation for convenience, such as Mettler-Toledo’s One-Click® interface (see Figure 2). In addition, LabX® titration software helps store data, produce reports, and automatically export results to other systems (such as a laboratory information management system).

Table 1 - Titrator manufacturers

Metrohm’s Dosino™ reagent dosing technology promotes precision in titration. “The Dosino™ sits on the top of the reagent bottle and doses reagent from the top down, preventing leaks and air bubbles,” said George Porter, Titration Product Manager for Metrohm USA. The company’s titrators are also designed to use more environmentally friendly KF reagents.

Figure 2 – T90 Titrator with One-Click titration, and Rondo automated titration system (courtesy of Mettler-Toledo).
  Figure 3 – 874 Karl Fischer Oven Sample Processor (courtesy of Metrohm).

Most titrator manufacturers also supply accessories for preparing difficult samples. Samples that do not dissolve easily, for example, may require external extraction in different solvents. Another option for stubbornly insoluble samples is a KF oven, in which the sample is heated until the water it contained is released. A stream of dry air or gas (such as nitrogen) carries the water to the KF titration cell, where it is measured. For additional convenience, ovens can also be coupled with automatic sample changers. The Mettler-Toledo DO308 drying oven, along with the Stromboli oven sample changer, can run up to 14 samples hands-free. The Metrohm 874 KF Oven Sample Processor (see Figure 3) automates moisture measurements using titrators such as the Metrohm Titrando 890 or 901.

No matter what types of samples need to be analyzed, prospective purchasers will most likely find the technology they need. Today’s titrators include many more safety and convenience features to free up researchers’ time. In addition, the incorporation of automated functions, robotic elements, and more accurate dosing make titration easier and, most importantly, more precise than ever.

Key purchasing considerations for a Karl Fischer titrator or potentiometric titrator

  • Type of titration required
  • Moisture content: volumetric or coulometric (KF titration)
  • Types of additional modalities desired: pH, acid/base, redox, etc. (potentiometric titration)
  • Sample solubility
  • Complexity of features required: entry level or customized/flexible
  • Degree of automation required.

Caitlin Smith is a freelance science writer who has a Ph.D. in Neuroscience from Yale University and postdoctoral work in Electrophysiology and Synaptic Plasticity; e-mail: [email protected]