pH analyzer working principle and Calibration

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pH is the measurement of acidity or alkalinity of an aqueous solution and its hydrogen ion concentration. The pH scale is a logarithmic scale and ranges from 0 to 14.Neutral solutions, and pure distilled water, which have neither acidic nor alkaline content, have a pH value of 7, acids have values below 7 and alkalis have values above 7

In aqueous solutions, a compound dissociates into charged particles called ions. Any strong acids, when dissolved in water will produce positively charged hydrogen ions and negatively charged acid radical particle ions as shown below

1

As all strong acids are fully ionized in aqueous solutions, a measure of hydrogen ion concentration will give the strength of acid

One of the many methods used to fine the pH value is electrometric method which can be explained as below

This method is used to continuously monitor the degree of acidity or alkalinity of the process liquid. Basically an electrode immersed in a liquid acquires an electric potential, the value of this potential varies with the value of the pH of the liquid. As it is impractical to measure a single potential, we use two electrodes and a potential difference is measured. The different types of electrodes used are glass measuring electrode, calomel reference electrode and submersible electrode assemblies.

In a glass measuring electrodes which is most widely used one,2

This comprises of a bulb made of special glass which is fused into the end of a glass tube to form a membrane. This membrane is sensitive to hydrogen ions. This assembly contains a solution of HCl of known pH value and a silver wire coated with silver chloride is dipped into HCl. This silver wire is connected to a screened lead and the glass tube is sealed in the top cap

When the electrode is placed in a solution, a potential difference is established across the membrane. The value of this potential depends on the pH of the solution This potential difference from the pH sensor is then converted to mA in the transmitter and transmitted to the control room.

Calibration Procedure of pH analyser

1.Obtain necessary Permit to Work. 2.Override the trip if it is connected to any trip system.

3.Isolate the sensor from process & remove the sensor. Place it into the insulated container of water along with a calibrated thermometer.

4.Be sure the glass bulb and the reference junction are completely submerged. Stir continuously.

5.Allow the sensor to reach thermal equilibrium.

6.Press “MENU” key on the transmitter, then menu screen appears.

7.Chose “Calibrate”.

8.Chose “Temp”.

9.To calibrate the temperature, change the number in the second line to match the temperature measured with the “Standard Thermometer”.

10.Press “ENTER”.

11.Press “MENU” then “EXIT” to return to the main display.

12.For pH AUTO calibration go to the

     step13 & for MANUAL calibration go to       the step 35.

Auto Calibration

13.Obtain two buffer solutions of 7.00 & 10.1 pH.

14.Press “MENU” key.  The main menu appears. 

1

15.Choose “Calibrate”.

2

16.Choose “Sensor 1” and then “pH”.

3

4

17.Choose “Buffer Cal”.

5

18.Choose “Auto” and then enter “Start Auto calibration”

6

7

19.Choose “Buffer1”.

20.Rinse the sensor with water and place it in buffer 1.  Ensure that the glass bulb and the reference junction are completely submerged.  Swirl the sensor

8

21.The screen on transmitter is displayed with “Wait” flashing until the reading is stable. The default stability setting is <0.02 pH change in 10 sec.

22.Display shows the actual reading.  The transmitter also identifies the buffer and displays the nominal buffer value9

23.If the display value is not correct Press “UP ARROW KEY “or “DOWN ARROW KEY “to alter it to the correct value.

24.Press “Enter” to store.

25.“Cal in progress” screen will appear momentarily.

26.Remove the sensor from Buffer 1, rinse it with water and place it in Buffer 2. . Be sure the glass bulb and the reference junction are completely submerged.  Swirl the sensor.

27.Choose “Buffer2”.

28.The screen on transmitter is displayed with “Wait” flashing until the reading is stable.

9a

29.Display shows the actual reading. The transmitter also identifies the buffer and displays the nominal buffer value.

9c

30.If the display value is not correct Press “UP ARROW KEY “or “DOWN ARROW KEY “to alter it to the correct value.

31.Press “Enter” to store.

32.“Cal in progress” screen will appear momentarily.

33.If the calibration was successful, the transmitter will display the offset and slope

9d

34.To return to the main display, press “MENU” then “EXIT”.

9e

9f

 

                        MANUAL CALIBRATION

35.To perform the MANUAL calibration follows the steps from 36.

36.Press “MENU” key. The main menu appears. 

37.Choose “Calibrate”.

38.Choose “pH”.

39.Choose “BufferCal”.

40.Choose “Manual”.

41.Choose “Buffer1”.

42.Rinse the sensor with water and place ii in buffer 1. Be sure the glass bulb and the reference junction are completely submerged.  Swirl the sensor.

43.The reading in the top line is the live pH reading. Wait till the reading is stable.  Then, use the arrow keys to change the reading in the second line to the match the pH value of the buffer.

44.Remove the sensor from buffer 1 and rinse it with water. Place it in buffer 2.  Be sure the glass bulb and the reference junction are completely submerged. Swirl the sensor.

45.Choose “Buffer2”.

46.The reading in the top line is the live pH reading. Wait till the reading is stable.  Then, use the arrow keys to change the reading in the second line to the match the pH value of the buffer.

47.“Cal in progress” screen will appear momentarily.

48.If the calibration was successful, the transmitter will display the offset and slope.

49.To return to the main display, press “MENU” then “EXIT”.

50.Rinse the sensor with clean water.

51.Restore the sensor in process.

52.Observe the reading in HMI and ensure that the reading is as per the process. Inform production department and normalize the override is applicable.

53.Close the Work Permit.

 

For analyser related topics

https://kishorekaruppaswamy.wordpress.com/2017/08/13/oil-in-water-analyzer/

 

https://kishorekaruppaswamy.wordpress.com/2017/08/13/bsw-analyzer/

 

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One thought on “pH analyzer working principle and Calibration

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