pH Measurement Principles and Best Measurement Practices | Webinar
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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.
pH Measurement Principles and Best Measurement Practices | Webinar
- 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
- 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
- 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
- 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