Charge Analyser II
The Rank Brothers Charge Analyser II is a completely automatic polyelectrolytic titrator. Polyelectrolytic titration enables determination of ionic charges of polymers in solution and surface charges of colloids. The system is microprocessor controlled giving the operator a user friendly interface that has been designed to ensure novice operators can quickly and easily carry out accurate titrations. The cell has been designed to measure the charge of liquid samples or liquid samples containing only small particles, thus paper stock for example, will need to be filtered to remove particles above 1 µm before a titration.
- Waste water treatment:
- Optimising dosage of flocculents.
- Paper manufacture:
- Determination of charge density of unknown polymer (e.g. fixative, wet strength agent or retention aid).
- Determination of the concentration of a known ionic polymer in solution.
- Determination of the concentration of anionic polymers (interfering substances in the water circuit).
- Determination of the charge demand of paper stock or wire water by back titration.
- Colloid science.
- Complete system supplied to carry out fully automatic titration consisting of detector cell, control unit, and precision metering unit (only standard anionic and cationic polymers required).
- Microprocessor controlled giving user-friendly interface via four front-panel keys and a liquid crystal display.
- Hard copy of the titration available via parallel printer interface.
- Many diagnostic and help messages displayed to aid novice operators.
Principle of Polyelectrolyte Titration
The principle of polyelectrolyte titration is based on the fact that the standard anionic and cationic polymers form 1:1 compounds with each other based on their charge and are thus directly equivalent to an acid/base neutralisation.
The titrator uses a standard anionic and/or a standard cationic polymer to determine the equivalent charge of the sample. Potassium Polyvinyl Sulphate (PVSK) is used as the standard anionic polymer and Hexamethrine Bromide (Polybrene) is used as the standard cationic polymer. The titration end point is determined using a streaming current detector cell or a photometer cell.
The Rank Brothers Charge Analyser II (CAII) will perform a polyelectrolytic titration automatically and consists of three main parts: the detector cell, the control unit, and the precision metering unit. The three parts are described in more detail below.
The Detector Cell
Two types of cell are available for use with the CAII, the streaming current cell and the photometer cell. The cells can easily be changed by switching off the CAII and connecting whichever cell is required.
The cell is easily detached from the standard assembly to enable cleaning after every titration. The titration is carried out in disposable plastic cups, the cup is placed on the stirrer unit (supplied with the metering unit) and the cell is partially immersed in the sample to enable a titration to be carried out.
Streaming Current Cell
The streaming current cell comprises of a stand (with incorporated drive motor) and the measuring cell. The cell consists of a cylinder closed at one end with a pair of electrode rings, one at the closed end and one in the middle of the cylinder. A piston oscillates axially inside the cylinder at a speed of approximately 250 rpm streaming sample past the two electrodes. Polymers have a tendency to adsorb onto interfaces and thus become fixed to the piston and cylinder walls. The mobile counter-ions of the fixed electrolyte create an electric current due to the partial charge distribution between the two electrodes in the cylinder wall. Thus, the cell produces a sinusoidal voltage of about 4 Hz.
The photometer cell carries out a photometric titration using Toluidine blue indicator to detect the end point. This method will be familiar to many people already carrying out manual titrations. The cell is a stainless steel cylinder containing the optical components plus associated electronics. The electronics measure the relative transmission of light through the sample, and thus the cell is able to detect when the indicator turns from blue to pink or vice versa.
Fixed doses of polymer (20 µl) are added to the sample until the unit has detected a significant colour change and that the colour has become stable, thus the dosing typically continues for over 1 ml after the colour change has occurred. The titration data is then scanned and processed to determine the end point of the titration.
Streaming Current Cell vs Photometer Cell
There are advantages to both types of cell as listed below.
Streaming Current Cell
|Greater accuracy than photometer cell.||Needs careful cleaning for maximum accuracy.|
|Not affected by calcium ions.||Cannot cope with low molecular weight materials.|
|Titration not complicated by addition of cationic indicator.||Heavier and larger than photometer cell.|
|Lighter and smaller than the streaming current cell therefore more portable.||Less accurate than the streaming current cell.|
|Easily cleaned.||The indicator is cationic; allowance must be made for this in the titration.|
|Uses a familiar technique.||The Toluidine blue indicator is affected by various metal ions, particularly calcium and aluminium.|
|Titrates low molecular weight chemicals.|
The Control Unit
The control unit houses the interface electronics and power supply for the cell, plus a microprocessor card to control the complete system. The front panel consists of a 2 line by 24 character liquid crystal display (LCD), four keyboard switches, two LEDs, and a BNC socket. The top line of the LCD shows current values of the charge of the sample and the total volume of polymer dosed (when this information is available). Note: the value of the charge is not calibrated to any specific units, but is proportional to the value of the signal from the cell. The sign of the charge is also displayed so that the user immediately knows whether the sample is anionic or cationic.
The second line of the display indicates the status of the unit, for example: Titrating or displays useful help information such as: Press <TITRATE> to start. The four keys are labelled <MOTOR>, <TITRATE>, <PRINT>, and <STOP>, with the first three keys having built-in LEDs to indicate their status. The two main LEDs are used to indicate when the metering unit is dosing and when the titration has finished. The BNC socket connects the unit to the cell and is on the front panel to enable easy connection during cell cleaning.
The rear panel has connections for the power inlet lead, the cell motor, a printer (with parallel port), and the metering unit. The printer socket allows connection to a printer with a parallel port and will print the charge value, the amount of polymer dosed, and the phase angle of the charge signal. This information is printed after every measurement cycle (10 30 times a minute depending on whether dosing occurs or not).
The microprocessor has complete control of the system, including the cell and the metering unit. Once the operator has determined the correct standard polymer (anionic or cationic) to use, the system is able to carry out a fully automatic titration. The control unit determines the volume of polymer to dose into the sample from the magnitude of the charge and then proceeds to titrate until the phase of the streaming current has inverted. The end point dose will then be calculated from the minimum charge value during the titration and will be displayed on the LCD.
The microprocessor is monitoring various functions; if problems occur it will display diagnostic information relating to the printer, the cell connections, etc. It also has an internal buzzer to warn the operator of faults, invalid key presses, and the titration end.
The Metering Unit
The supplied metering unit is a Metrohm 876 Dosimat plus complete with remote keypad, magnetic stirrer and stand, and two 10 ml exchange units complete with 1 litre reagent bottles. The Dosimat is connected to the control unit enabling the control to dose exact volumes and to refill an exchange unit at the beginning and end of a titration. The Dosimat is a versatile programmable metering unit and can be used completely independently from the CAII for other laboratory applications.
|Printer Interface||25-pin parallel port|
|Power Supply||AC 110–120 V or AC 220–240 V switch selectable (power cord supplied)|
|Size||245 mm (width) × 245 mm (depth) × 135 mm (height)|
|Size||142 mm (width) × 310 mm (depth) × 164 mm (height)|
Streaming Current Cell
|Size||130 mm (width) × 280 mm (depth) × 375 mm (height)|
|Material||Stainless steel with glass optical window|
|Repeatability||Approx. 50 µl PVSK vs Polybrene titration|
|Size||146 mm (length) × 28 mm (diameter)|
Rank Brothers Ltd reserves the right to change specifications in the light of continuing development.
|CHA007A||Charge Analyzer II with streaming current cell excluding Dosimat|
|CHA008A||Charge Analyzer II with photometer cell excluding Dosimat|
|CHA009A||Charge Analyzer II with streaming curent cell and photometer cell excluding Dosimat|
|CHA020A||Dosimat 876 complete system with stirrer and two 10 ml exchange units|
|CHA100A||Streaming current cell for Charge Analyzer II|
|CHA101A||Photometer cell complete with clamp for Charge Analyzer II|