The document provides guidelines for best practices in pH measurement, including: - How a pH electrode works by measuring the voltage difference between a reference and sensing half-cell. - Key electrode components like the glass sensor, reference electrode, electrolyte, and junction that impact performance. - Factors to consider when selecting an electrode like the application, built-in features, and electrode materials. - Proper calibration including using fresh buffers, ensuring full immersion, and recording calibration data for quality control. - Best practices for taking accurate measurements, maintaining electrode performance through cleaning and storage, and extending electrode life.

1. pH Measurement Principles and
Best Measurement Practices
HOW TO GET THE BEST pH DATA
Laura St. Pierre
Product Manager
Chris Cushman
Asst. Product Manager

2. Outline
• How a pH electrode works
• Electrode design
- Important structures of a pH electrode (glass sensor, junction, electrolyte, etc.)
• Selecting the best electrode
• Calibration and measurement
- Buffers and temperature
- Calibration frequency, number of points, and data
- Placing an electrode in solution
• Electrode maintenance and storage
• Selecting the best meter
• pH resources from YSI
2
IoLine in immunoassay production

3. How a pH Electrode Works
3

4. How a pH Electrode Works
Relies on measurement of a voltage, which
requires two points with different electrical
potential values
• Reference electrode (i.e. reference half-cell)
- Maintains constant potential (signal) independent of sample
• Hydrogen ISE (i.e. sensing half-cell)
- Electrical potential depends upon the activity of hydrogen
(H+) ions in solution
• Difference between these potentials (mV on display) is
correlated to pH based on the result of calibration
- Nernst equation establishes relationship between mV output
and pH
E = E
0
+ (2.303 RT/nF) log a
H
+
• Reference and ISE are often combined into one
electrode for convenience – combination electrode
4
A pH electrode has sensing
and reference half-cells
Combination electrode

5. pH Electrode Design
5

6. pH Electrode Design
Choosing the best electrode
begins with understanding the
main electrode structures.
• Electrode body
• Glass pH sensor
• Reference electrode
• Reference electrolyte
• Reference junction
6 Typical combination pH electrode

7. pH Electrode Design – Electrode Body
• ‘Glass pH electrode’ – sensor portion of
the electrode is made from glass
• Glass pH electrodes have either a glass
or plastic electrode body
• Plastic body electrodes are more
rugged; less likely to crack from impact
- Used more often in the field
• Glass body electrodes are more fragile,
but have wider range of operating
temperature and are commonly refillable
- Used more often in the lab
7
YSI TruLine pH 17 with glass body and
conical sensor shape
YSI TruLine pH 25 with plastic body and
cylindrical sensor shape
YSI ProDSS pH/ORP sensor with plastic
sensor module. The rest of the sensor
body is constructed of titanium for field
durability

8. Dissecting the Glass pH Sensor
• Glass used for sensor contains alkali ions
(Na+ or Li+)
• Alkali ions and H+ ions from solution form a
very thin ‘gel layer’ on the inside and outside of
the glass
- Sensor is filled with buffer, so inside has constant
binding of H+ ions
- Outside is exposed to sample with variable H+ ions
• Layer acts as ion exchanger; alkali ions are
exchanged for H+ ions from solution
• Movement of H+ ions into or out of gel layer
depends on pH
• Difference in pH between the inside and
outside creates a voltage (potential) read by
the meter
8
H+
H+
H+
H+
External
Aqueous
Solution
Li+
Li+
H+
Glass sensor (light blue) and
gel layer (dark blue)

9. pH Electrode Design – Glass pH Sensor Shapes
• The shape of the sensor varies to meet application needs
• Sphere (bulb) and cone glass pH sensor shapes used for most applications
• Specialized shapes are available – spear tip and flat
9

10. pH Electrode Design – Reference Electrode
There are different types of reference electrodes available,
each with different characteristics
• Mercury chloride - Hg/Hg2Cl2
- In use for the longest time
- Very stable potential, but mercury is very toxic
- Narrow temperature range
• Silver/silver chloride - Ag/AgCl
- Most common
- Can be used in medicine and food applications
- Disposal less critical
- Wide temperature range
• Iodine/iodide
- Metal-ion free; can be used in Tris buffer and protein solutions
• Metal ions will interact with these solutions, causing the junction to clog
- Rapid response time
- Increased stability, even at changing temperatures
10
YSI TruLine pH 17 (left)
and IoLine (right)

11. pH Electrode Design – Reference Electrolyte
The reference electrode is immersed in a
solution of reference electrolyte that has contact
with the sample through the reference junction.
The electrolyte is needed because is closes the
electrical circuit in the electrode.
Electrolyte will leak into sample during
measurement; rate depends on form of
electrolyte and junction type
Good electrolyte must have certain properties…
• Good electrical conductivity
• Be chemically neutral
• Ions that are equally mobile
11
Potassium Chloride (KCl)
Others (potassium sulfate (K2SO4))
if allowed by electrode mfr.

12. pH Electrode Design – Reference Electrolyte
Reference electrolyte is offered in different forms
• Gel electrolyte
- Still consists of KCl, but gelling agent added
- Virtually no loss of electrolyte; easy to maintain
- Theoretically shorter life
• Liquid electrolyte
- Electrode can typically be refilled; longer electrode life
- Can be drained and replaced if it becomes contaminated
- Fast response time and used in wide range of applications
12
Tip: The refill hole must be
open during measurement and
calibration
Electrodes with liquid (3 on left) and
gel electrolyte (1 on right)

13. pH Electrode Design – Reference Junction
The reference junction creates contact between
the reference electrode and the sample.
The reference junction should be permeable and
allow consistent outflow of electrolyte.
Several types exist:
• Ceramic Junction
- Uses the porosity of unglazed ceramic; can easily become
blocked
- Outflow rate is low; diffusion potentials can develop
- Useful in solutions with oxidizing substances
• Platinum
- Fine, twisted platinum filaments between which electrolyte
flows
- Constant outflow of electrolyte and does not easily become
blocked
- More sensitive to mechanical stress
- Can be used in many applications
13

14. pH Electrode Design – Reference Junction
• Ground-Joint
- Much faster outflow of electrolyte
- Two ground glass surfaces mated to one another
- Surfaces are tightly fit together, but electrolyte can
permeate between them
- Best for solutions with a lot of suspended particles; also
useful in high and low-ion solutions
• Pellon Strip Junction
- Junction used in YSI field electrodes
- Utilizes pellon: non-woven nylon-like material
- Strip of pellon held in place by a silicone bushing
- Gel electrolyte, so outflow is nearly zero
• Variety of Other Junctions are Offered
14
YSI Science pHT-G with ground-joint junction
Before first use, the shrinkable
tubing needs removed
Outflow rate is adjusted by
moving the outer glass sleeve
up or down
YSI field pH electrode
Pellon junction

15. pH Electrode Design – Additional Junctions
Some electrodes feature additional
reference junctions
• Reference electrode is immersed within an
additional chamber; acts as additional
barrier to contamination
• Additional junction(s) used to ensure
contact with sample
• Reference can still become contaminated,
but solution must first pass thru these
additional junctions
15

16. Selecting the Best Electrode
16

17. Selecting a pH Electrode
Can a single pH electrode be used in every application?
• No, sampling conditions can vary dramatically between applications
The most important consideration is your application
- Will help determine the glass pH sensor shape, reference junction, reference
electrode, and other electrode characteristics you will need
17
YSI TruLine 21 for penetrating semi-solids
YSI TruLine 27 for surface measurements
YSI IoLine is ideal for measurements in
challenging solutions such as proteins and Tris
buffers

18. Selecting a pH Electrode
Also consider convenience…
• Built-in temp sensor prevents need for separate
temp probe
• Combination electrode
- Sensing half-cell (i.e. hydrogen ISE) and reference
half-cell in one body
- 3-in-1 electrodes: sensing half-cell, reference half-
cell, and temp sensor in one body
• Plastic body electrode if concerned with
electrode breaking
• Glass body for wider temperature range
• Liquid or gel electrolyte
- Refillable electrodes have longer life and faster
response time
- Gel are easier to maintain
18
It is best to order replacement 3 M KCl when
ordering a refillable electrode – you will need it!

19. pH Electrode Guides
19
bit.ly/pHselectguide bit.ly/pHappguide

20. Calibration & Measurement Tips
20

21. What Calibration Changes
The first step to an accurate measurement is calibration
-Your results are only as good as your last calibration-
Calibrating a pH electrode establishes system:
• Plot Intercept or Zero Point
- Sensor’s mV value when placed in pH 7 buffer
- Theoretical is zero (0) mV
- Practical can be -50 to +50mV
- If outside of this range, electrode may need cleaned or replaced
• Electrode Slope
- Theoretical Nernst slope is -59.16 mV/pH unit at 25 oC
- Actual slope will differ from theoretical and will change over time
21 -200
-100
0
100
200
4 7 10
mV pH
The difference between each pH unit is 59.16 mV

22. The Importance of Temperature
pH is Temperature Dependent
• Nernst slope (i.e. electrode response): S = -2.303 RT/nF
• Measurements must be completed with an accurate measurement of temperature
Options for Measuring Temperature
- pH electrode with built-in temperature sensor
• 3-in-1 sensors have reference electrode, hydrogen ISE, and temperature sensor within one
electrode body
- External temperature sensor connected to meter
- Apply temperature reading from another connected sensor with built-in temp
• Not all meters have this capability
- Manual input of temperature
22
YSI external temperature sensor

23. Calibration – pH Buffers
• Calibration is completed using standard buffers
- Solutions with a pH that doesn’t easily change
- Often colored to differentiate
- Should be NIST (4.01, 6.86, 9.18) or NIST-traceable
(YSI buffers with values 4, 7, 10)
• Buffer tips
- Fresh buffer solutions should be used – don’t reuse
buffers
- Don’t leave buffer bottles open
- Ensure the correct buffer set is chosen on your meter
- pH buffers are slightly temperature dependent
23
YSI buffers (4, 7, and 10)

24. Number of Calibration Points
The number of calibration points is a common question. Most
instruments will allow at least a 3 point calibration.
1 Point Calibration
• Must be completed with pH 7 buffer
• Zero point is determined but theoretical slope is used
• Range of use is limited; only within a range of 6.5 to 7.5 pH
2 Point Calibration
• Best to perform at least 2 point cal; pH 7 must be used (6.86 for NIST)
• Buffers must differ by at least 2 pH units
3 Point Calibration
• Used when pH conditions of sample not well understood
24
The YSI MultiLab and TruLab instruments
will allow up to 5 point pH calibration

25. Additional Calibration Tips
• Enable mV values on the display during calibration
- Buffer 7: -50 to 50 mV
- Buffer 4: +177 mV away from the buffer 7 mV value (positive
change)
- Buffer 10: -177 mV away from the buffer 7 mV value (negative
change)
- Determine mV change between buffers. Ideal is 177, acceptable
is 165 to 180. Slope: 59 mV per pH unit.
25

26. Additional Calibration Tips
•Ensure the electrode and calibration cup are both
clean
•Make sure the sensing electrode, reference
electrode and temperature sensor are adequately
immersed in the buffer
•Triple rinse the electrode and calibration
container/cup with deionized water between
calibration points
26

27. The procedures for placing a pH electrode for calibration
and measurement are the same
1.Remove the storage cap
- Should contain 3 M KCl to prevent electrode from drying out
2.Refill opening (if equipped) must be open 
3.Level of reference electrolyte must be at least 2 cm (0.75
inches) above level of the solution. Refill with 3 M KCl if
needed
Placing Electrode in Buffer or Sample
27
Electrolyte level
Solution level

28. Placing Electrode in Buffer or Sample
4. Outer reference junction must be completely in solution
-Temperature sensor must also be immersed
28
YSI TruLine pH Electrode Junction Locations

29. Calibration Record
• Many meters save calibrations to a GLP file (Good Laboratory Practice)
- Zero point (mV or pH), slope, mV value in each buffer, temp
• Some meters evaluate the calibration result and display/store an electrode
efficiency
• Some may show a percentage
Electrode efficiency = Observed slope *100
-59.16 mV/pH
29
YSI MultiLab and TruLab
Calibration Evaluation

30. Calibration Frequency
Over time the electrode slope and zero point (i.e. electrode
response) drifts, so we must recalibrate
Does a strict guideline for frequency exist?
• Simple answer is no
• Best practice - calibrate each day you use it until you better grasp the needed
frequency
• More frequent calibration will be needed as the electrode ages!
• Frequency depends on application
• More frequent calibration when electrode is used in
- Heavily contaminated, low-ion, strongly acidic, high temperature – these age the electrode faster
(slower response, change in slope and zero point)
Some instruments feature calibration timers
30

31. Maintenance Tips
31

32. Electrode Cleaning
Clean the electrode if response time is poor (slow)
or the calibration result is not acceptable
General Guidelines for Cleaning
• Recalibrate after cleaning
• Rinsing with DI water is OK, but don’t store in DI water
• It is best to chemically, rather than mechanically
(physically), clean the electrode
• Check for physical damage – cracked membrane
YSI does have different recommendations for field
and lab pH electrodes
32

33. Lab pH Electrode Cleaning
Inorganic Debris
• Place in 0.1 M HCl or 0.1 M NaOH for ~5 mins
• If buildup not removed, heat the solution up to
50 oC before increasing the acid or alkaline
concentration
Organic Debris
• Rinse with organic solvents (e.g. ethanol or
acetone)
• Ensure solvent doesn’t damage plastic-body
electrode
• Glass bulb can be carefully wiped with damp,
lint-free, soft cloth
33
Ensure proper safety
precautions are used

34. Lab pH Electrode Cleaning
Proteins
• Place the electrode in a pepsin/HCl solution for at least 1 hour
Sulfides on Ceramic Junction
• Place in thiourea/HCl solution (7.5% in 0.1 M HCl) until the discoloration on the
junction has disappeared
After Cleaning (any procedure)
• Rinse with DI water
• Soak in electrolyte (3 M KCl) for at least 1 hour
34
The YSI IoLine can be used in
protein solutions

35. Lab pH Electrode Cleaning
Debris in Reference Electrolyte
• Remove the old reference electrolyte
• Replace with fresh electrolyte (don’t use
other chemicals)
• May need to repeat several times until
debris is gone
• Crystals of KCl in electrolyte can be
dissolved by heating electrode in water bath
to 45 oC; replace electrolyte afterwards
Note: KCl crystals may develop on the
watering cap and electrode during storage
and shipment (new electrodes!). Clean with DI
water and a lint-free cloth.
35
YSI Science pHT-G features liquid
electrolyte
Don’t forget to order 3 M KCl when ordering a
refillable electrode – you will need it!

36. Storage Tips
36

37. Electrode Storage
Storage is perhaps the main contributor
to the life, speed and precision of a pH
electrode
General Guidelines for Storage
• pH electrodes can be permanently damaged
if improperly stored
• Storage recommendations vary based on
length of intended storage
• Never allow a pH electrode to dry out
• Never store a pH electrode in DI water
(rinsing with DI is OK)
37
Proper storage is
critical for field and
lab pH electrodes

38. Lab pH Electrode Storage
Between Measurements
• Place in solution of 3 M KCl
• Rinse with test sample or DI water prior to next sample
• Refill opening can be kept open
Overnight or Longer
• Ensure electrode is clean
• Fill plastic storage cap with 3 M KCl and install over sensor
• Close refilling opening
38
IDS 4110 with storage
cap for longer term
storage

39. Conditioning Dried Electrodes & Replacement
If your electrode has dried out, you MIGHT
be able to rehydrate it
• Place field electrodes in pH 4 buffer and lab
electrodes in 3 M KCl for at least 24 hours
• Clean and recalibrate afterwards
If all else fails (cleaning and rehydrating), you
will likely need to replace your electrode.
Electrodes generally last 18-24 months, but
electrode life can vary greatly based on
electrode quality, application, and storage
practices.
39
YSI pH 4 buffer
YSI 3 M KCl solution

40. Selecting the Best Meter
40

41. Selecting a pH Meter – Questions to Ask
There are many pH meters available, so how do you choose
the right one? Begin by asking these questions…
1. Do I need to measure other parameters in addition to pH?
41
YSI TruLab 1310: pH and ORP
MultiLab: pH, ORP, conductivity, DO/BOD, ISEs

42. Selecting a pH Meter – Questions to Ask
2. How many measurement channels do I need?
• Some instruments only allow 1 sensor to be connected
and displayed, while others allow more
42
YSI MultiLab 4010-2, a two
channel instrument
YSI MultiLab 4010-3, a three
channel instrument
YSI MultiLab 4010-1, a one
channel instrument

43. Selecting a pH Meter – Questions to Ask
3. What type of sensor input do I want?
• BNC: Very common connector; other mfr’s BNC electrodes can be used on meter
• Digital: Mfr’s call them different names (YSI MutliLab IDS)
- Specific to meter
- Typically store calibration data and serial number; auto-recognized by meter
43
YSI TruLab instruments feature
BNC input
Electrode with BNC
YSI MultiLab instruments
feature digital sensors
MultiLab IDS pH
electrode
MultiLab 4010-2/3
BNC adapter –
converts one digital
channel to BNC

44. Selecting a pH Meter – Questions to Ask
4. How will I be recording data?
• Some instruments simply display the pH value – no data management
• Instruments may have printer option – optional accessory or built-in
• Some advanced instruments have a large memory, ability to backup data to USB
flash drive, and the ability to send data straight to a PC
44
YSI TruLab 1110 – no memory,
perfect for a simple pH readout YSI MultiLab 4010-2/3 – large
memory, backup to USB stick, PC
software included
YSI TruLab 1320P – built-in printer

45. Selecting a pH Meter – Questions to Ask
5. How important is the display quality?
• Some instruments feature a very basic, segmented display
• Others may have a graphic display, color screen
• Easy to see - consider the display size, font size, and if display is backlit
45
YSI TruLab 1110 – segmented
display
YSI MultiLab 4010-2/3 – large, easy to
read, backlit, color display

46. Selecting a pH Meter – Other Considerations
Important Features
• Auto-buffer recognition: meter recognizes the buffer based on
mV values and selected buffer set – speeds up calibration
• ATC (automatic temperature compensation): meter automatically
compensates pH for temp – accurate pH measurements
• GLP (calibration) record: allows user to review detailed calibration
information
• What’s included with purchase
46
YSI MultiLab 4010-1 pH kit

47. Selecting a pH Meter – Other Considerations
Important Features
• Number of stored buffer sets
• Warranty period and after sales support
• Available pH electrodes and other sensors
• Easy to use; auto-stable feature
• Ability to display mV values
47
YSI pH electrodes with BNC connection
YSI MultiLab 4010-3 displaying in mV

48. Available pH Resources
48

49. YSI pH Handbook
The YSI pH Handbook covers:
• Basic concepts of pH measurement
• pH electrode design
• Measurement, calibration, and cleaning tips
• Selecting an electrode
• Lab pH ‘Quick Tips’
• And much more...
49
bit.ly/YSIpHbook

50. Other Resources
50
Electrode Selection Guide
bit.ly/pHselectguide
Electrode Application Guide
bit.ly/pHappguide
Electrode Calibration Guide
bit.ly/pHcalguide
YouTube.com/ysiinc
YSI.com/blog
YSI Laboratory Newsletter
YSI.com/subscribe

51. 51
Thank you!
1-937-767-7241
ccushman@ysi.com
lstpierre@ysi.com
info@ysi.com – Tech Support