pH METER Principal & pH Chemistry by dasharth.pdf

pH Meter Principal & pH Chemistry by

1. 10/12/2025Parmar Dashrath

2. pH & pH MeterWorking
principal
01.What is pH ?
02.pH Meter working principal?
03.Type Of pH electrode ?
04.pH Meter Calibaration& pH Slop.
05.Inhouse pH BuffarSolution preparation
Parmar Dashrath
[email protected]

3. 01.What Is p
H
?
•
Hydrogen potential, or pH, isa measure
of the chemical activity of hydrons (also
known as protons or hydrogen ions) in
solution
•
pH = Power Of Hydrogen Ion Or
Potential Of Hydrogen.
•
pH isa logarithmic measure of the
hydrogen ion (H⁺) concentration in an
aqueous solution

4. •
A pH meter works on the principle ofpotentiometry, measuring
the electrical potential difference between a glass electrode and a
reference electrode.The glass electrode's special membrane
develops a voltage based on thehydrogen ion
concentration(pH) in the solution, while the stable reference
electrode provides a baseline.This voltage, related by theNernst
equation,is converted into a pH reading by the meter's
circuitry.
•
NarnestEquation:-
E=Eºcell-
6.747??
??
log
4
??????+
E = Eºcell-
6.747??
??
pH
E-Eºcell=
6.747??
??
pH
pH =
??(
E-Eºcell
)
6.747??

5. •
The Nernst equation describes thecell potential(E) of an electrochemical cell under non-standard conditions, relating it to
thestandard electrode potential(E⁰), thetemperature(T), the number ofmoles of electronstransferred (n), and thereaction
quotient(Q).The general formula isE = E⁰-(RT/nF)log[H+], which can be simplified at 25°C (298 K) to E = E⁰-(0.0592
V / n)log₁₀Q.This equation is crucial for calculating cell potentials when concentrations or partial pressures deviate from
standard conditions, allowing for the determination of cell voltage in real-world scenarios.
pH =
????????????(
E−Eºcell
)
??????.??????????????????????????????
Where,
E-Eºcell=difference Milivoltbetween Ref.electrode&Intarnalelectrode= V volt
F=faradeconstant ( 96,500 C/mol)
n= Number Of Electrone
R=Universalgasconstant(8.314 J/K.mol)
pH =
????????????
??????.??????????????????
at 25ºC

6. •
The Nernst equation is used for pH calculation by relating the electrode potential of ahydrogen
electrodeto the hydrogen ion concentration, or pH.After measuring the cell's electromotive
force (EMF) and applying the Nernst equation, the hydrogen ion concentration is found by solving
for[H+].The pH is then determined using the formulapH = -log[H+].
Where [H+] = Molar Conc.forH+ Ion
Example:0.01N SulphuricAcid solution pH
Normality H
2
SO
4 =
0.01N
Molarity= Normality X Basicity = 0.01 X 2 = 0.02M
pH = -log10[0.02] = -log10[ 1 X 10-²] = 2
So, 0.01N H
2
SO
4
pH = 2
10/12/2025Parmar Dashrath 6

9. pH Meter BuffarSolution Slop
•
To calculate a pH meter's slope,find the millivolt (mV) change between two different pH buffer
solutions, then divide the mV change by the difference in their pH units.The ideal theoretical slope is
approximately 59.16 mV per pH unit at 25°C, according to theNernst equation.The measured slope
is then compared to this theoretical value to determine the percentage slope, which indicates electrode
health.
•
Steps to Calculate the pH electrode slope :-
1.0)Obtain two buffer solutions :-
Choose two standard pH buffer solutions, for example, a pH 7.0 buffer
and a pH 4.0 or 10.0 buffer
.
2.0)Record the millivolt (mV) readings:
Place the pH electrode in the first buffer and record its millivolt reading.Then, rinse the electrode
and place it in the second buffer, recording its millivolt reading

10. •
Calculation the diffrancein millivolt : -SubstractmV reading from the first buffer from the mV
reading of the second buffer solution .•
CalculationthediffrenceinpH:Find the diffrancebetween the pH Values Of the two bufferused.•
Calculate the electrode Slope : Divide the differncein millivolt by difference pH.Thisgivisyou the
measured slop in mV per pH Unit.
Example pH 4.0 = -10mV & pH 7.0 = 165mV
Diff mV = 165-(-10) = 175mV
DiffpH=7-4=3.0 pH
▲mV / ▲pH = 165/3 = 58.33 mV/pH
▲mV / ▲pH = Slope = 58.33 mV/pH
All pH Instrument Theoraticalslope = 59.16 mV/pH
% Of Slope = 58.33 x 100/59.16 = 98.60%

11. Why the Slope is Important in pH Meter ?
•
TheSlopeOfthepHelectrode indicates its sensitivity.•
Aslopeclose to the ideal theoretical value (59.16mV/pH unit at
25ºC) means the electrode is working efficiently and providing
accurate readings.
•
Thepercentageslopeisusedtoassesstheelectrode’s performance
its falls within and acceptable range (e.g;90to105%asperUSP
chapter791).
•
Anacceptableslopeansurethereliabilityofthecalibration and
subsequent meaurements.
•
LowSlope(e,g;<90%)-A very low slope mean the electrode is
defective and need replacement.
•
HighSlope(e.g; >105%)-A slope higher than the theoretical value
can be caused by problem with the calibration process,suchas using
conatmintedor expire buffer solution.

12. Inhouse pH Buffer Solution preparation
•
Follow these steps to prepare the pH 4.0 buffer:-
•
Weightexactly5.259g of potassium citrate and transfer that amount to the glass bottle
•
Weightexactly6.309g of citric acid and transfer the solid to the same glass bottle
•
Fill the bottle to around 250mL using distilled water
•
Mix the solids using a glass rod or any other inert mixing utensil until fully dissolved
•
Fill the bottle to 500mL using distilled water.
•
Label the flask clearly so that you know this is the pH 4 buffer.
•
Follow these steps to prepare the pH 7.0 buffer :-
•
Weightexactly3.369g of potassium dibasic phosphate transfer that amount of second glass bottle.
•
Weightexactly4.172g of potassium monobasic phosphate transfer that amount of second glass bottle
•
Fill the bottle to around 250mL using distilled water
•
Mix the solids using a glass rod or any other inert mixing utensil until fully dissolved
•
Fill the bottle to 500mL using distilled water.
•
Label the flask clearly so that you know this is the pH 4 buffer

13. pHbuffer Chemistry
•
CompostionOf buffer solution :
•
Acidicbuffer: These have a pH bellow 7.0 and are made by mixing a week acid with its salts (which contain the conjugate base)
Example :AmixtureOftheaceticacidAndsodiumacetate
•
Basic buffer : : These have a pH above 7.0 and are made by mixing a week acid with its salts (which contain the conjugate acid)
Example : A mixture Of the ammoniumhydroxideAnd ammoniumchloride

14. WhatIspHBufferSolution?
Definition : A buffer solution resists a change in pH when small amount Of acid or alkali are added
Composition:AnacidicbufferismadebymixingaweekacidwithasolutionofitssaltOfastrongalkali.
Example:CH3COOHwith CH3COONa
CH3COOH + NaOH CH3COONa + H2O