SOP for Operating of Potentiometer

1.0 OBJECTIVE

To lay down the Procedure For Operation of Potentiometric Titration Apparatus.

2.0 SCOPE

This SOP shall be applicable to Quality Control Dept.

3.0 RESPONSIBILITY

Supervisor Quality Control

4.0 ACCOUNTABILITY

Sr. Manager Quality Assurance

5.0 PROCEDURE 

5.1 Operation of the Apparatus

5.1.1 Before connecting the mains keep the controls as under
(a) SELECTOR switch to ‘O’.
(b) STIRRER switch to ‘MIN’. (to its built-in ‘OFF’ position)
(c) SET POINTER to about the middle of its full range.
5.1.2 Connect the mains supply (230 Volts 50 HZ) and switch “ON” the MAINS switch. The red pilot lamp will then light up and the display will show some reading near about to O mv.
5.1.3 Rotate the STIRRER control knob and adjust the stirrer speed.
5.1.4 Connect the pins of the sensors to the sockets provided on the machine on both sides of the plated rod. (Calomel to black socket) and lower the sensors with the spring clamp. So that the tips of the sensors are well immersed in the solution in the beaker. Care should be taken to see that the tips may not get smashed by the rotating magnetic rotor. Turn SELECTOR to ‘POT-G’ or ‘POT-M’ as the case may be. To ‘POT-G’ if Class/Calomel pair is used and to ‘POT-M’ for Metal/Calomel pair.
5.1.5 The display will now show some positive or negative reading depending on the nature and strength of the solution. Adjust it near to ‘O’ by SET POINTER knob.
Note: The instrument is provided with ‘COARSE’ and ‘FINE’ SET POINTER controls. Fine control can be used to adjust the milli-volts reading precisely to O on the digital display. While coarse control is used for major change in the milli-volt reading.
When “SELECTOR” is in ‘O’ or ‘O-R’ position, both FINE and COARSE “SET POINTER” will have no control on Digital display.
5.1.6 Add some drops of solution from the burette and note the reading on the digital display.
Add fixed volume of solution from the burette (say 1ml, 0.5 ml OR 0.1 ml ) every time and note the reading of “CHANGE IN MILLIVOLTS” for each addition. It will be found that for the same addition every time the display will show larger and larger change in millivolts and at the END POINT, the change will be suddenly large. With further additions, the change will be progressively smaller and smaller. Thus the change in millivolts will be maximum at the END POINT. The reading corresponds to the maximum change in millivolts should be considered for calculations.
A special feature of this instrument is that, one can bring the display reading back to O mv and add the further solution from the burette. Since only change in millivolts is to be noted after each addition the shifting of reading to O mv makes no difference. Thus by adjusting display reading to O mv with each addition highest accuracy can be obtained by skillful handling. If the changes in millivolts are very small a graph of the change in millivolts “v/s” volume added may be drawn for accurate location of “END POINT”.

5.2 Precaution

While changing the sensors or solutions or whenever the titration is not in progress the SELECTOR must be in ‘O’ position.

5.3 Procedure for using the instrument with glass/calomel sensors

1) Keep ‘SELECTOR’ switch to position ’O’
2) Fit the Glass & Calomel sensors in the clamp carefully. Attach the lead of the Calomel sensor to the black terminal and the lead of the Glass sensor to the special connector fitted on the rear plate of the instrument.
3) Take about 60 ml Glacial Acetic Acid in the special teflon beaker supplied and lower the sensors. Fill the right hand side burette with 0.1N Perchloric Acid.
4) Start the magnetic stirrer and adjust its speed suitably. Turn the SELECTOR to ‘POT G’ and adjust the display near about O mv.
5) To neutralize the acetic acid add 0.1ml Perchloric Acid from the burette. The display will show a large change in millivolts (more than 50 to 80 mv).
6) Add 100 mg of the sample in the beaker and dissolve it completely. (If necessary the solution may be heated to dissolve the sample completely).
7) Go on adding a fixed volume of solution from the burette (0.5ml or 0.1 ml, depending on the strength) and note the corresponding change in millivolts. It will be found that the change in millivolts will be increasing with each addition and at the END POINT, it is maximum. Thereafter the changes are lesser with each addition.
8) If necessary, draw a graph of the change of millivolts v/s Volume added. The peak of the graph indicates the END POINT.
9) Again keep the ‘SELECTOR’ to ‘O’. While changing the solution or electrodes the ‘SELECTOR’ switch must be in the ‘O’ position.
In the same way, other samples can be tested by following the proper method.
Note: For efficient observations, the normality of the solution in the burette should be higher than that in the beaker.

5.4 Oxidation/ Reduction Titrations

Note : For Oxidation / Reduction Titrations connect Calomel sensor in red socket and Metal sensor in the black socket.
For Oxidation/ Reduction Titrations assemble the apparatus in the same way as for Potentiometric titrations. The Calomel/ Platinum Sensor is preferred but the Platinum/Platinum Pair can also be used which may not give sharp readings. In any case, the Platinum Sensors in use should be perfectly clean and without any scratches on the metal surface. Before connecting the mains keep the controls in the same way as for Potentiometric Titrations. (Follow steps 1 to 9 for assembling the instrument).
1) Before connecting the mains keep the controls as under :
(a) SELECTOR switch to ‘O’.
(b) STIRRER switch to ‘MIN’. (to its built in ‘OFF’ position)
(c) SET POINTER to about the middle of its full range.
(d) The tips of the sensors above the level of the solution in the beaker. (kept raised by the spring clamp)
2) Lower the sensors in beaker and dip tips of Sensors in the solution. Then turn “SELECTOR” to ‘O-R’. “SET POINTER” control will now have no effect on the digital display. It will show negative or positive reading depending on the nature of the solution.
3) Add solution from the burette say 0.1 or 0.5 ml, every time. At the “END POINT” the digital display will show a sudden large deflection from positive to negative or vice versa.
Note: When “SELECTOR” is in ‘O’ or ‘O-R’ position, both FINE and “COARSE SET POINTER” will have no control on digital display.

5.5 Charging the Calomel Sensor

Prepare some saturated Potassium Chloride solution in a small beaker by adding some Analytical Reagent Grade Potassium Chloride to distilled water. Dissolve the KCl till some Crystals remain at the bottom. Add some KCl (A.R.) to the reservoir of the sensors, (about one gram) and fill up to about half the level of the capacity of the reservoir of the sensor with the saturated KCl solution made in the beaker. Wipe out the sensor from outside with a clean cloth and allow it to stand in a test tube 24 hours. The calomel sensor will then become ready for use.

5.6 Care and Maintenance of the Instrument and Sensors

1) Keep the instrument clean and covered when not in use. It is always a pleasure to work on a good clean instrument.
2) This equipment is highly sensitive and should be operated always keeping this point in mind. The controls fitted are all very smooth and delicate and no force whatsoever is needed to operate them. Undue force will damage the controls and the initial settings done at the factory may get disturbed.
3) For all Potentiometric titrations, the SELECTOR switch must be in the POT position and should not be moved to O-R position even for a short duration. If it is done unknowingly then the cleaning of Metal sensors may become necessary.
4) The sensitivity of the Metal sensors gets reduced very quickly due to the accumulation of deposits on their surfaces. The sensitivity can be restored as before by lightly rubbing the metal tip wires on very on very soft emery paper. It is necessary that the surface of the metal wires of the sensors should be clean and at the same time quite smooth. It should not be therefore scratched with a blade or knife for cleaning. Washing the sensors (only wire tips) with dilute Nitric Acid and thereafter with distilled water often restores the sensitivity. Of course, this method is good for Platinum- sensors but for other Metal sensors, the action of the acid on metal must be taken into consideration.
5) Always make sure that the Potassium Chloride solution in the Calomel sensor is well saturated. For this, it is necessary that some crystals of Potassium Chloride are seen at the bottom of the sensor reservoir.
6) Do not allow any unauthorized person except Trained Servicemen to repair the instrument if it becomes necessary. The circuits are protected under tamper-proof seals.

6.0 ABBREVIATIONS

6.1 SOP: Standard Operating Procedure
6.2 HZ: Hartz
6.3 mv: Milivolts
6.4 Dept.: Department

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