For compendial purposes,pHis defined as the value given by a suitable,properly standardized,potentiometric instrument (pHmeter)capable of reproducing pHvalues to 0.02pHunit using an indicator electrode sensitive to hydrogen-ion activity,the glass electrode,and a suitable reference electrode.The instrument should be capable of sensing the potential across the electrode pair and,for pHstandardization purposes,applying an adjustable potential to the circuit by manipulation of standardization,zero,asymmetry,or calibrationcontrol,and should be able to control the change in millivolts per unit change in pHreading through a temperatureand/or slopecontrol.Measurements are made at 25±2,unless otherwise specified in the individual monograph or herein.
The pHscale is defined by the equation:
in which Eand ESare the measured potentials where the galvanic cell contains the solution under test,represented by pH,and the appropriate Buffer Solution for Standardization,represented by pHs,respectively.The value of k is the change in potential per unit change in pHand is theoretically [0.05916+0.000198(t 25)]volts at any temperature t.
It should be emphasized that the definitions of pH,the pHscale,and the values assigned to the Buffer Solutions for Standardization are for the purpose of establishing a practical,operational system so that results may be compared between laboratories.The pHvalues thus measured do not correspond exactly to those obtained by the definition,pH=log aH+.So long as the solution being measured is sufficiently similar in composition to the buffer used for standardization,the operational pHcorresponds fairly closely to the theoretical pH.Although no claim is made with respect to the suitability of the system for measuring hydrogen-ion activity or concentration,the values obtained are closely related to the activity of the hydrogen-ion in aqueous solutions.
Where a pHmeter is standardized by use of an aqueous buffer and then used to measure the pHof a nonaqueous solution or suspension,the ionization constant of the acid or base,the dielectric constant of the medium,the liquid-junction potential (which may give rise to errors of approximately 1pHunit),and the hydrogen-ion response of the glass electrode are all changed.For these reasons,the values so obtained with solutions that are only partially aqueous in character can be regarded only as apparent pHvalues.
Buffer Solutions for Standardization of the pH Meter
Buffer Solutions for Standardizationare to be prepared as directed in the accompanying table.*Buffer salts of requisite purity can be obtained from the National Institute of Science and Technology.Solutions may be stored in hard glass or polyethylene bottles fitted with a tight closure or carbon dioxide-absorbing tube (soda lime).Fresh solutions should be prepared at intervals not to exceed 3months using carbon dioxide-free water.The table indicates the pHof the buffer solutions as a function of temperature.The instructions presented here are for the preparation of solutions having the designated molal (m)concentrations.For convenience,and to facilitate their preparation,however,instructions are given in terms of dilution to a 1000-mLvolume rather than specifying the use of 1000g of solvent,which is the basis of the molality system of solution concentration.The indicated quantities cannot be computed simply without additional information.
pH Values of Buffer Solutions for Standardization
Potassium Tetraoxalate,0.05m Dissolve 12.61g of KH3(C2O4)2·2H2Oin water to make 1000mL.
Potassium Biphthalate,0.05m Dissolve 10.12g of KHC8H4O4,previously dried at 110for 1hour,in water to make 1000mL.
Equimolal Phosphate,0.05m Dissolve 3.53g of Na2HPO4and 3.39g of KH2PO4,each previously dried at 120for 2hours,in water to make 1000mL.
Sodium Tetraborate,0.01m Dissolve 3.80g of Na2B4O7·10H2Oin water to make 1000mL.Protect from absorption of carbon dioxide.
Calcium Hydroxide,saturated at 25 Shake an excess of calcium hydroxide with water,and decant at 25before use.Protect from absorption of carbon dioxide.
Because of variations in the nature and operation of the available pHmeters,it is not practicable to give universally applicable directions for the potentiometric determinations of pH.The general principles to be followed in carrying out the instructions provided for each instrument by its manufacturer are set forth in the following paragraphs.Examine the electrodes and,if present,the salt bridge prior to use.If necessary,replenish the salt bridge solution,and observe other precautions indicated by the instrument or electrode manufacturer.
To standardize the pHmeter,select two Buffer Solutions for Standardization whose difference in pHdoes not exceed 4units and such that the expected pHof the material under test falls between them.Fill the cell with one of the Buffer Solutions for Standardization at the temperature at which the test material is to be measured.Set the temperaturecontrol at the temperature of the solution,and adjust the calibration control to make the observed pHvalue identical with that tabulated.Rinse the electrodes and cell with several portions of the second Buffer Solution for Standardization,then fill the cell with it,at the same temperature as the material to be measured.The pHof the second buffer solution is within ±0.07pHunit of the tabulated value.If a larger deviation is noted,examine the electrodes and,if they are faulty,replace them.Adjust the slopeor temperaturecontrol to make the observed pHvalue identical with that tabulated.Repeat the standardization until both Buffer Solutions for Standardization give observed pHvalues within 0.02pHunit of the tabulated value without further adjustment of the controls.When the system is functioning satisfactorily,rinse the electrodes and cell several times with a few portions of the test material,fill the cell with the test material,and read the pHvalue.Use carbon dioxide-free water (see Water in the section Reagents,Indicators,and Solutions)for solution or dilution of test material in pHdeterminations.In all pHmeasurements,allow a sufficient time for stabilization.
Where approximate pHvalues suffice,indicators and test papers (see Indicators and Indicator Test Papers,in the section Reagents,Indicators,and Solutions)may be suitable.
For a discussion of buffers,and for the composition of standard buffer solutions called for in compendial tests and assays,see Buffer Solutionsin the section Reagents,Indicators,and Solutions.
* Commercially available buffer solutions for pHmeter standardization,standardized by methods traceable to the National Institute of Standards and Technology (NIST),labeled with a pHvalue accurate to 0.01pHunit may be used.For standardization solutions having a pHlower than 4,a labeled accuracy of 0.02is acceptable.Solutions prepared from ACSreagent grade materials or other suitable materials,in the stated quantities,may be used provided the pHof the resultant solution is the same as that of the solution prepared from the NISTcertified material.