Chulalongkorn University Faculty of Science 29 Aug 2016 Vipawee Banditwaytikul
Presentation Title pH: Theory, calibration & measurement
Agenda
1
Theory of pH
2
Checking of pH Electrode
3
Calibration pH electrode
4
Troubleshooting
2
To shift Highlight Click the arrow, press shift key and move top or bottom
For internal use - Confidential
pH measuring range: 0 - 14
3
Food & Beverages / Household products Orange juice Egg white Coca Cola
Cheese
Lemon juice
Milk
Beer
0
1
3
2
Hydrochloric acid 0.37% (0.1 M)
Sulfuric acid 4.9 % (1 M)
4
Antacida ( Mg(OH)2 )
Water
5
6
Borax
7
8
10
9
12
11
13
14
Caustic soda 4%
Hydrocyanic acid 0.27% (0.1 M)
Calcium carbonate (sat)
Acetic acid 0.6% (0.1 M)
Ammonia sol.1.7% (1 M)
Ammonia sol. 0.017% (0.01 M) Potassium acetate 0.98% (0.1 M)
Chemicals
Sodium hydrogen carbonate 0.84% (0.1
Principle of pH measurement
4
The pH is measured by potentiometry The measuring system consists of:
pH Meter
pH glass electrode reference electrode
The potential difference (mV) is measured:
Reference electrode
Glass electrode
E = EGlass - EReference
Cl-
H +
H
+
+
K+ Cl-
Cl-
++
+
H
K+
H
H
+
Cl-
H H +
K
ClK+
K+
Principle of pH measurement
5
Potential at the glas membrane
Gel layer
pH-sensitive glas
constant charge of the inner gel layer (inner buffer with constant pH) negative or positive charge of the outer gel layer depending on the pH of the solution
Inner buffer
H+
constant
H+ 0.2 - 0.5 mm
positive
H+
negative charge
H+
H+
H+
H+ H+
Gel layer approx. 0.0001 mm
Acidic solution
Types of glass electrode membranes
Classical types of membranes Round
Spherical
Cylindrical
H+
H+
Basic solution
6
Types of Membranes
7
Special types of membranes Sharp (puncture electrode) for medium solid products
Micro for small samples
Flat for surface
Nernst equation
8
E = Eo + 2.3 RT/F • log cH+ E = Eo – 2.3 RT/F • pH
Slope = 0.198 * T E
Measured potential
Eo
Standard potential
R
Universal gas constant
T
Temperature (Kelvin)
F
Faraday constant
mV
pH
1
7
2,3 RT/F Nernst potential (slope) cH+
H+ concentration of solution
Potential difference of 1 pH unit = 59.16 mV (at 25 °C)
14
อาการที่พบบ่ อยๆ
8
ตัวเลขไม่ นิ่ง ตัวเลขนิ่ง (ไม่ ว่าวัดในสารละลายใด) ใช้ เวลาในการอ่ านค่ านาน วัดซ้าแล้ วไม่ ได้ ค่าเดิม Calibrate แล้ วขึน้ ERROR
pH performance check
9
- Measured 2 points in buffer solution pH 4.01,7.00 - Record mv value - Calculate Slope of sensor
pH 7.00
pH 4.01
Checking of Electrode Working Std. Buffer (Value)
Read Value Temp ( C) pH
4.01 7.01
25 25
0
mV
4.01 7.00
MV …170…………………
11
175 5
MV PH
PH …3…………………
170 3
56.67 mv/pH
56.67 X 100 59.16
% Slope …95.8%……………
Checking of Electrode Working Std. Buffer (Value)
Read Value Temp ( C) pH
4.01 7.01
25 25
0
mV
4.01 7.00
MV …143…………………
12
PH …3…………………
% Slope …80.6%…………… Performance Limit of electrode: % Slope >85% Zero Point (mv at pH 7.00) ± 30mv
MV PH
155 12
143 3
47.67 mv/pH
47.67 X 100 59.16
Calibration
Conditions
Use always fresh buffers.
–
Use same buffer solution only once, otherwise no guarantee for correct value of buffer solution. –
Clean electrode after measurement.
–
Measure the temperature. All METTLER TOLEDO pH meters and titrators have automatic temperature compensation.
Calibration and adjustment
14
1. Offset adjustment mV After adjustment
15 Offset
pH0
0
pH
7 Offset -15
„Shifting" of the slope to the real zero point
Before adjustment (theoretical offset)
Theoretical offset: 0 mV at pH 7 pH0: Zero point
Calibration and adjustment
15
2. Slope adjustment mV 177
Adjustment of the slope to the real slope of the electrode through „turning“ the slope in the zero point
Before adjustment (theoret. slope)
theoretical slope: 59.16 mV/pH at 25°C
After adjustment
0
pH 4
7
Two Point calibration +mV 175 -
Calibrated
Before calibration 4
pH 7
175 mV = theoretical slope at 20oC -mV
9
Three Point calibration +mV 175 -
*
4
Calibrated
*
pH
7
9
175 mV = theoretical slope at 20oC
*
-mV
What is the meaning of the sensor icon in the instrument’s display? 18
Calibration
How frequently should electrode be calibrated? •
Depends on type of sample. Dirty and non aqueous sample ... frequent calibration.
•
Depends on wanted accuracy. For very accurate measurements... minimum daily.
•
Depends on quality of electrode. Old electrodes ... more frequent calibration.
•
After change of electrolyte.
•
After cleaning a blocked diaphragm.
•
After long or wrong storage.
Relation between pH and temperature
20 Error from sensor behavior - Membrane activity in each temp not same response - Reference electrode activity of KCl and AgCl reaction - Diaphargm potential at different pH value will cause electrolyte different flow rate This type of problem need to correct by high quality of pH sensor or calibration at application temperature
Three major issue for temperature need to be correct will help to let right trend between each temperature
Temperature compensation
21
The T-compensation in a pH meter doesn’t compensate for: pH value of a solution at different temperature Samples
20°C
30°C
HCl 0.001 mol/L
pH
3.00
pH 3.00
NaOH 0.001 mol/L
pH 11.17
pH 10.83
Phosphate buffer
pH
7.43
pH 7.40
TRIS buffer
pH
7.84
pH 7.56
Each solution has its own temperature dependence! It is NOT possible to compensate for this effect with a pH meter. pH measurements can only be compared at the same temperature Always measure the temperature with each pH measurement
Special Course
Storage and cleaning
pH electrodes
Storage
25
Storage 1.
Rinse the electrode with DI water.
2. Recap the electrode with the watering cap or place the electrode in a beaker with storage solution. 3.
Close the filling hole.*
*not applicable for electrodes with polymer electrolyte.
Always store in: 3 mol/L potassium chloride (3 M KCl) Or buffer solutions pH 4 or 7, Or diluted HCl (approx. 0.1 mol/L)
Never store the electrode in distilled water or dry! Otherwise the pH sensitive glass membrane will be affected which shortens the lifetime and the electrode must be reconditioned before use.
Reference Electrolyte
Which electrolyte? - for aqueous solution - for non aqueous solution
3 mol/L KCl 1 mol/L LiCl in ethanol
Replace/refill reference electrolyte regularly. E.g. every 2 weeks
Electrolyte level in electrode must be higher than in sample solution. If electrolyte level lower than sample solution, then sample will contaminate reference system.
No air bubbles behind junction. Vertical shaking of electrode.
Maintenance: Cleaning
27
Cleaning the diaphragm blocked with AgCl -
with concentrated ammonia
blocked with Ag2S -
with 8 % thiourea in 0.1 molar HCl
(ME-51 340 070)
blocked with proteins -
with 5 % pepsin in 0.1 molar HCl
(ME-51 340 068)
other contaminations -
in ultrasonic bath with water or 0.1 molar HCl
Treat one hour, rinse with distilled water Over night 3M KCl ,rinse with distilled water and perform a new electrode adjustment.
Lifetime of a pH Electrode
Approximate lifetime of pH electrode under ideal conditions (good storage, maintenance, and measurement in aqueous solutions, pH range 1 to 12).
at ambient temperature at 90°C at 120 °C
1 - 3 years 3 - 9 months few weeks
Reconditioning the Electrode
Dry electrode Dry membrane: Re-conditioning in 0.1 mol/L HCl during 12 hours.
Electrode out of specs Reactivation of membrane: Place electrode in reactivation solution (NH4HF2) for 1-2 minutes. (ME-51340073)
Summary: pH Electrode
If an electrode is not performing well:
Membrane dehydrated, contaminated or damaged
Electrolyte contaminated or missing
Air bubbles behind ceramic junction or membrane
Diaphragm (junction) contaminated
Crystallized KCl blocking outlet of electrolyte
Electrode worn out (out of specs)
Troubleshooting
31
Measurement problems can have many reasons: Symptoms
Possible reasons
Solution
• Slow response • Decreasing slope • Increasing offset
Age of electrode (glas membrane)
Regenerate/ change electrode
• Drifting result • Result depends on
Diaphragm potential
Clean diaphragm (change electrode) Check if other diaphragm type should be used (Sleeve- oder Polymer electrode)
• Increasing offset • Drifting result • Wrong results
Reference electrolyte is contaminated / diluted
Replace the electrolyte
• Result is too high/ too
No temperature compensation Calibration with wrong buffers All points from above
Use electrode with temperature probe
stirrer speed • Increasing offset • Deacreasing slope
low
Thank you
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