Karl Fischer titration is one of the few assay techniques recognized by the US Food and Drug Administration for the determination of residual moisture in lyophilized pharmaceutical products (1). Because the iodine–water reaction is specific and quantitative, this technique provides high accuracy and precision. However, conventional Karl Fischer titration is not considered a high-throughput assay and requires significant manual intervention. This test can present a challenge when many samples must be analyzed in a short time. Furthermore, most lyophilized products are hygroscopic and must be processed promptly after removal from the primary container.
The few off-the-shelf automated setups offered by Karl Fischer titrator vendors use a carousel onto which several samples, preweighed in individual containers, can be loaded. The titration is performed directly in these containers as the carousel rotates each sample through the titration station. Unfortunately, no satisfactory means has been devised for sealing the samples from the environment while they await analysis. Hence, hygroscopic lyophilizates cannot be processed on these types of automated titrators.
This article describes the authors' development of a reliable system for performing Karl Fischer titration automatically on numerous samples. It explains the rationale for selecting instrumentation and discusses sample-preparation strategies that affect the overall design of the automated system.Titration-instrument selection
The Karl Fischer titrator is the heart of an automated moisture-analysis system. First, a sample is loaded into a conditioned water-free titration cell. Iodine (I2) from the titrant is then metered in and reacts with water present in the sample. The titration end point occurs when all the water in the sample has reacted away. The end point is determined by detecting free iodine at a double-pin platinum electrode. Because this reaction is quantitative, the amount of water can be obtained by measuring the amount of titrant required to return the titration cell to an anhydrous state. The literature contains a comprehensive treatise about Karl Fischer titration (2). Two types of instruments are available to perform titration: coulometric titrators and volumetric titrators.
Coulometric titrators. Coulometric titrators generate iodine by the in situ electrochemical oxidation of iodide (I–) contained in the reagents. The total amount of iodine generated is proportional to the total charge passed in the electrochemical cell. This allows for sensitive detection of water: from ~1 to ~50,000 ppm (5% w/w).
Volumetric titrator. In a volumetric titrator, the iodine is already present in the titrant, and the titrator simply measures the volume of reagent dispensed into the cell from a precision burette. Sacrificing sensitivity for range, volumetric titrators can measure water content from ~0.1 to 100 % (w/w).
For the current application, the high sensitivity of Coulometric titration was not necessary, but it was desirable to be able to analyze samples with high moisture content (>5 wt %). Hence, the authors decided that a volumetric Karl Fischer titrator was more suitable.
Any titrator that allows operations to be controlled from an RS-232 interface could have been selected. The authors considered only Metrohm (Herisau, Switzerland) and Mettler–Toledo (Greifensee, Switzerland) instruments, however, because these two companies' titrators were already used and supported at their site. When the project began, Metrohm offered the 700 series of titrators, which included models 784, 787, and 795. The models had similar capabilities, but the 784 and 795 provided more flexible interconnectivity with optional accessories and additional internal memory for method storage. Mettler–Toledo offered the DL31 and DL38 titrators; the latter model had more internal memory and slightly more data-processing functionality. No significant operational differences existed between these five instruments for the authors' application because all data treatment was intended to be performed on a separate computer. Ultimately, the authors selected the Mettler DL31 because its setup included a higher-volume titration cell with a larger sample port than the Metrohm models. In addition, the Mettler titrator's stir-bar driver could be controlled remotely.