Comments on “Design and Construction of a Personal‐Computer‐Based Automatic Titrator”

As stated by Kinniburgh et al. (1995), the flexibilit y desired in titrators for researc h purposes is difficul t to find in most commerciall y availabl e titrators. In response , Kinniburgh et al. (1995) described the design and construction of a personal computer (PC)-base d automati c titrator designed to overcome many limitations by connecting multiple burettes (each with a RS-232 interface) and multiple ion-specifi c electrodes to a single interface unit that provides both analog-digital conversions for the electrodes and GPIB (IEEE-488) bus interface to a PC programmed in Microsoft Fortran (Microsoft Corp., Redmond, WA). Other technical solutions to the problems of obtaining flexible, research-oriente d titrations are possibl e and within reach of many laboratories . For example, we have used bidirectional RS-232 (serial ) connections to interface a Metrohm 665 Dosimat (Metrohm AG, Herisau, Switzerland) burette and an Orion pH/ISE meter Model 720A (Orion Res. Inc., Boston, MA) with a PC programmed in QuickBasi c 4.5 (Microsoft Corp.). An IBM-XT compatibl e computer is in dedicated use as both master for the slave devices and as a hard disk storage device. Both the burette and the pH/ISE meter may be commande d and queried with a simple set of proprietar y ASCII commands for each device. Computer programming with QuickBasi c allows for programming modules, which permit generi c subroutines (e.g., checking pH readings for stability) to be shared among similar programs. Alternatively, QBasi c (standar d with DOS v. 6.0+) or VisualBasi c for Windows (Professiona l Edition, Microsoft Corp.) can be used for the same purpose. We found that a number of pH meters are availabl e that have unidirectional RS-232 connections for reporting values to a computer or printer but are unabl e to suppor t true remote control and are not recommende d for this purpose. We have used the equipment configuration described above for a number of researc h purposes , including (i) titration of KCl-extractabl e acidity, (ii ) automati c Gran titrations, (iii ) acid titrations of metal chelates , and (iv) kinetic studies of acid consumption at fixed pH. Although the GPIB is generall y considered a more rigid standar d than RS-232 (Gates and Becker, 1989; Johnson, 1994), experience has shown that both burette and pH meter serial interfaces may be addresse d in a straightforwar d manner through the serial port as intended by the manufacturers . The principal limitation that we forese with this configuration is that the number of unique serial ports available under DOS is typically limited to two. For example, the pH meter described above permits two electrodes to be read and reported through a single RS-232 connection; the other RS-232 connection is used for a single burette. This configuration may be adequat e for many researc h needs. However, multiple serial port boards and DOS-Windows drivers are commerciall y availabl e to offer four or more true serial ports suitable for additional burettes. On-off control over additional mechanica l devices such as a stirrer is relatively easil y added through the use of a digital input-output interface either through an inexpensive computer board or parallel port device. The configuration described above was constructed from laborator y equipment already in house and, when not required, the component pieces are returned to their general tasks of benchtop pH meter and commercial titrator (with Metrohm 614 Impulsomat and 632 pH meter).