A simple, low cost & easy to build DIY apparatus is presented for measuring enzyme reaction rates that uses solution conductivity as the detection method.

 

Novel AC bridge with two conductivity cells operating in differential mode - gives additional sensitvity & stability - a sample cell & a reference cell

 

The experimental device, its design, construction and use.

 

Don't expect it to look too tidy - it's simple breadboard DIY at present - but it works very well! Could easily be miniaturised

 

CONTENTS (from Menu):

 

Intro (Basic & General) - can be skipped if you know about conductivity & enzymes

 

Intro (Specific & Technical)

 

The Kit (Equipment Design & Electronics)

 

Some Example Results - urease catalysed hydrolysis of urea

 

Potential Applications: enzyme assays & kinetics, clinical tests, school/university projects - works for most enzyme reactions, see list on subpage to RHS of Home

 

                         

If you are working on this project or propose to do so please contact me [conductivityassays@gmail.com] if you have any problems or have any comments to make about the design or construction & use. Also I would be pleased to hear of any improvements you may suggest, particularly to the electronics, as I am no expert in this area.

 

                              NEWS & UPDATES

 

 

ELECTRODES

 

Recently added is a new section detailing the construction of low cost electrodes made from pencil leads. Use of these in place of the original commercial (RS) electrodes saves some £50 in the overall cost of the apparatus. They are easy to construct, work very well and are robust & reliable in use.

 

ELECTRONICS

 

OOps!! - The 1M ohm feedback resistor was missing from the thermostat circuit - has now been included! Apologies.

 

Following a re-examination of the use of a 741 OPAMP in place of the long-tailed pair as the differential amplifier, it has been found to be better, particularly in the removal of common mode signals that (inevitably) arise from transients on the input square wave signal. A lower balance null voltage* is achieved and the noise is lower.

* Never quite zero owing to residual ac components that arise from electrode polarisation (see next section). NEW CIRCUIT SHOWN IN ELECTRONICS SECTION

 

The effects of electrode polarisation can be reduced by increasing the frequency of the input square wave. By changing just one resistor the frequency has been raised from 10.3 kHz to 27.8 kHz. A transient on the rising edge of the input waveform is effectively attenuated by the use of the 741 (above).

 

CALIBRATION, Linearity etc. [see sub-page to RHS of Results]

 

Following detailed studies of the above new information is being added to the calibration section. Guidelines have been evolved for allowing very sensitive measurements over a 100,000-fold range of conductivity.

 

                                  COOLING SYSTEM

 

 

I am currently working on a thermo-electric cooling sytem, which may prove important for application of the kit in hot climates. This will, of course, require more power but I will work to minimise this.

 

               PRECISION AT HIGH SENSITIVITY

 

Also working to eliminate very small offset voltage in the precision rectifier that prevents a very precise ZERO balance (i.e. balance is at 20-30 mV rather than zero). This can affect the most sensitive measurements, particularly when a sub-optimal pot is used relative to the background ionic strength (see Calibration).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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