Peter Keusch, University of Regensburg
German version
- with the possible exceptions of handguns and tequila."
Mitch Ratliffe in Technology Review
 Photo 1: PC-Interface CHEMBOX | The measurement- and experimentation-system CHEMBOX / CHEMEX consists of the universal interface CHEMBOX and the data acquisition software Chemex. The sensor inputs represent a combination of analog and digital input. Signals recorded with the sensors, are converted by the interface CHEMBOX into computer-readable signals. Apart from the sensors, provided by IBK (conductivity sensor, pH electrode, redox electrode, temperature sensor, drop counter), all analog devices can be used, whose analog output signal is not larger than 32 V. These instruments can be connected to the sensor inputs 1 and 2 (identical in function) or also to the pH / mV-input. In the latter case an external input voltage may not have values outside the range - 6V ... + 6 V. |
The measuring principle of the conductivity-input is somewhat differently. According to its function the conductivity-input is in fact an output, because CHEMBOX applies automatically a voltage to the connected conductivity sensor. Very short voltage pulses and alternating polarity avoid electrolysis of liquids and polarization of the electrodes. Like the two sensor inputs 1 and 2, also the conductivity input is provided with a measurement-range switch.
Positive aspects:
· Problem free calibration of the sensors (Temperature sensor, conductivity measuring cell, pH sensor, redox probe)
· Simple defining of the channels
· Conversion of the measured values by entering of the appropriate formula (only allowed before starting the measurement process)
· Choice of data logging time intervals and any measuring ranges
· The measured values are listed in a data table.
· Main function of the program 'Chemex' is the 'Recorder' - the sensor readings are displayed on the screen as real-time graphs
· Presentation of four analog measuring instruments
· Optimization of the graph axes during the recording process
· Overlay of several curves in a multigraph screen
· Enlargement of a defined portion of a measuring curve
· Application of a straight line to the linear portion of a graph
· Determination of the x- and y-intercept
· Determination of the equivalence point in titrations
· Labeling of the diagram using 'Text tool'
· Use of commercial measuring instruments provided with a voltage-output (e.g. photometer)
 Photo 2: Diode photometer | IBK electronic+informatic provides a simple diode photometer, which is particularly suitable for the demonstration of photometric measurements, due to its easy handling. Three light emitting diodes with the wavelengths 660 nm (red), 565 nm (green) and 470 nm (blue), selectable using a switch, are available for this purpose. |
Critical aspects:
· While calibrating the conductivity sensor a temperature compensation is not allowed. Therefore the graphical overlay of the conductivity measurements carried out at different reaction temperatures, does not convince.
· After data logging, conversions of the measured data using the program Chemex are not allowed. Probably aware that the software is not designed for this purpose, the programmers allow the user to import the data into Excel for further processing.
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Conversion of the data logged file into a text file - Importing of the data into a Microsoft Excel spreadsheet:
The Chemex menu provides aids at the right border of the display area of the screen. Using the table function, data can be exported for further processing in a spreadsheet application like MS Excel. A click on the appropriate symbol (see figure to the left - mouse cursor: nw-resize) generates a table containing the data pairs of all defined channels. Depending on the computer velocity and the amount of the measured values, opening of the entire table will take one minute. By a click on 'file ® file export' in the menu of the table function the measured values can be saved. The file type should be *.txt, since this format causes the smallest problems. The file name may include max. eight characters. After successfully importing the data into Microsoft Excel, the table is available as .txt file on a data carrier. In the Excel 'File ® Open' dialog box this file appears only if the file type is specified as 'Text Files'. Alternative procedure: After highlighting the data in the Chemex value table, the data are copied and then inserted into an Excel spreadsheet. |  |
A dialog module programmed in Visual Basic permits the hiding of data pairs in the data sheet, so that e.g. only every tenth data point will be plotted on the appropriate chart. The hidden data pairs are not deleted. They are available in the background and will be taken into account in calculations performed in the Excel table.
References:
Manual Chemex Chembox (Download PDF-Datei)
Peter Keusch Computergestützte Experimente
Kinetics:
Alkaline Hydrolysis of Ethyl AcetateObjectives: Test for a Second Order Behaviour, Determination of Rate Constants and Activation Parameters | Experiment Description |
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Hydrolysis of Methyl Formate Objectives: Test for a First Order Behaviour, Determination of Rate Constants, Temperature Effect on Rate | Experiment Description |
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Hydrolysis of Benzoyl Chloride Objectives: Test for a First Order Behaviour, Determination of Rate Constants and Activation Parameters | Experiment Description |
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Hydrolysis of t-Butyl Halides Objectives: Effect of Solvent or Leaving Group on Rate, Determination of Rate Constants | Experiment Description |
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Bromination of reactive Aromatics Objectives: Test for a Pseudo First Order Behaviour, Determination of Rate Constants and Activation Parameters | Experiment Description |
| Fading of Phenolphthalein in Alkaline Solution Objectives: Test for a Pseudo First Order Behaviour, Determination of the Half-Life and the Rate Constant | Experiment Description |
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Reaction of Methyl Orange with Tin(II) Chloride Objectives: Test for a Pseudo First Order Behaviour, Determination of Rate Constants and Activation Parameters | Experiment Description |
| Fading of Triphenylmethane Dyes Objectives: Test for a Pseudo First Order Behaviour, Determination of Rate Constants and Activation Parameters | Experiment Description |
| Acid catalyzed Iodination of Acetone Objectives: Determination of Rate Constants, Test for a Pseudo Zero Order Reaction | Experiment Description |
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Iodination of Hydroquinone Objectives: Determination of Rate Constants and Activation Parameters | Experiment Description |
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Enzymatic Hydrolysis of Urea Objectives: Determination of the Temperature Optimum, the Michaelis Constant Km and the Maximal Velocity Vmax | Experiment Description |
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Decomposition of Hydrogen Peroxide catalyzed by Potassium Iodide
Objective: Dependance of the Reaction Rate upon the Concentration of the Catalyst | Experiment Description |
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Decomposition of Hydrogen Peroxide catalyzed by Dichromate Objective: Dependance of the Reaction Rate upon the Concentration of the Catalyst | Experiment Description |
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Addition of Bisulfite to Ketones Objectives: Nucleophilic Addition to Carbonyl Group of Ketones, Effect of Alkyl Groups Electron-Donating Effect, Steric Hindrance | Experiment Description |
Electrochemistry:
| Edison Cell (Iron-Nickel-Battery) Objectives: Dependence of Discharge Time on the Strength of Discharging Current, Capacity of the Cell | Experiment Description |
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LeClanché-Element (Zinc-Chloride Battery) Objective: Dependence of Discharge on the Strength Discharging Current | Experiment Description |
Titrations:
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Titration of Fumaric and Maleic Acid Objectives: Determination of the Equivalence Points and Dissociation Constants | Experiment Description |
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Redoxtitrations: Cerimetry, Dichromatometry, Manganometry Objectives: Determination of the Equivalence Point by Measuring the Redox Potential | Experiment Description |
