1.0 PURPOSE : To provide a procedure for periodic calibration of pH meter.
2.0 SCOPE : Covers procedure for the calibration of pH meter (daily and monthly) and also for the activation of electrode and maintenance of electrode.
3.0 DEFINITION :
3.0.1 pH: pH value conventionally represents the acidity or alkalinity of an aqueous solution.
3.0.2 CALIBRATION : The set of operations which establish, under specified conditions, the relationship between values indicated by a measuring instrument or measuring system or values represented by a material measure and the corresponding known values of a reference standard. 4.0 PROCEDURE :
4.1 PREPARATION OF BUFFER SOLUTION
Buffer solution D: (pH 4.01) Dissolve 10.21 g of potassium hydrogen phthalate, previously dried at 110 degree to 130 degree for 2 hours in sufficient carbondioxide free water to produce 1000mL.
Buffer solution E: (pH 6.87) Dissolve 3.40 g of potassium dihydrogen phosphate and 3.55 gram of anhydrous disodium hydrogen phosphate previously dried at 110 degree to 130 degree for 2 hours both in sufficient carbondioxide free water to produce 1000mL.
Buffer solution G: (pH 9.18) Dissolve 3.814 gram of borax in sufficient carbondioxide free water to produce 1000.0 ml.
Store all the above solutions in alkali free glass. Label the bottle as follows:
Frequency of preparation : Monthly
Every week transfer 250ml of this buffer to the secondary bottle after checking for clarity and label the bottle with details as under :
Solution showing precipitation, haziness or growth of fungus shall be discarded.
4.2 CALIBRATION OF Ph METER :
Use the service beaker provided for each buffer and do not transfer the buffer from service
beaker back to the bottle. Discard after every use.
Calibrate the apparatus using 25ml from the stock solution of buffer solution D as the primary
standard adjusting the meter to read appropriate pH 4.01 Plus or minus 0.05 corresponding to the temperature of solution (25degreeCPlus or minus 2 degree C). To set the scale use a second reference buffer solution
either E (6.87) or G (9.18).
If the difference between this reading and the original value is greater than 0.05, the set of
measurements must be repeated.
When measuring pH values above 10.0 ensure that the glass electrode is suitable for use under
alkaline conditions and apply any correction that is necessary.
IMPORTANT CONDITIONS :
a) All solutions of substances being examined must be prepared using carbon dioxide free
b) pH determination is carried out at a temperature of 25degreeCPlus or minus 2 degree C.
c) Buffer solutions should be stored in bottles made of alkali free glass.
5.0 ACTIVATION OF ELECTRODE :
FREQUENCE : EVERY 15 DAYS
5.1 COMBINED GLASS ELECTRODE.
Soak the electrode overnight in 6 M Hydrochloric acid, next day wash repeatedly with distilled
water and immerse the electrode in distilled water.
5.2 GEL FILLED ELECTRODE
5.2.1 Between measurements (upto 1 hour)
leave the electrode in an open air lab environment not in distilled water.
5.2.2 Short term storage (upto one week)
Soak electrode in 200ml pH 7 buffer added with 1 g potassium chloride.
5.2.3 Long term storage (over one week).
Cover the electrode tip with a protective cap used for shipment.
6.0 MAINTENANCE OF ELECTRODE :
6.1 GLASS ELECTRODE
6.1.1 General : Soak electrode in 0.1 M HCl or 0.1 N HNO3 for 15 minutes.
6.2 GEL FILLED ELECTRODE :
6.2.1 Removal of Deposits
6.2.2 Protein : Digest with 1% pepsin in 0.1 M HCl.
6.2.3 Inorganic : Rinse with 0.1 M tetra sodium EDTA solution.
6.2.4 Grease and oil : Rinse with mild detergent or methanol solution.
7.0 RECORDS :
7.1 Record the pH calibration & activation of electrode in form F:Sxxxx/DPHR/A.
7.2 Record the monthly calibration in form F:Sxxxx/PHR/A
7.3 Record the pH of all solutions checked in daily pH log book.
8.0 DOCUMENTATION :
8.1 F:Sxxxx/PHR/A pH Meter calibration record
8.2 F:Sxxxx/DPHR/A Daily pH checking record.