Support Home > Technology Document  
Q: pH Info
A: pH Info
Basic Theory
When measuring pH, a user is not determining direct concentration of the hydrogen ion (H+), but the negative log of hydrogen activity. This gives us the effective concentration of the hydrogen ions in a solution.
pH = -log [H+activity]
[H+activity] = 10-pH

Thus, a pH of 7 is equivalent to a hydrogen ion activity of 10-7 M (molar). Since water dissociates into hydrogen ions (H+) and hydroxide ions (OH-) in an aqueous solution, the following equilibrium reaction would be used to describe pH:
H2O = H+ + OH-
Most pH readings range from 0 to 14.
Solutions with a higher hydrogen ion activity than water (pH less than 7) are acidic; solutions with a lower hydrogen ion activity than water (pH greater than 7) are basic or alkaline.

pH is measured potentiometrically when using an electrode. In other words, your measurement is based on an electrical signal. A potential develops across the glass membrane of a pH electrode when it comes in contact with a solution. This potential varies as the pH varies, but requires a constant second potential to compare the changes against. This is the function of the reference electrode, to provide a constant potential, regardless of pH.
In acidic or alkaline solutions, the voltage on the outer membrane surface changes proportionally to changes in hydrogen ion activity by the Nernst equation:
E = E0 + (2.3RT/nF) log [H+]
E = total potential difference (measured in mV)
E0 = standard potential
R = gas constant
T = temperature in Kelvin
n = number of electrons
F = Faraday’s constant
[H+] = hydrogen ion activity
The pH of any solution is a function of temperature. Voltage output from the electrode changes linearly in relationship to changes in pH. The temperature of the solution determines the slope of the response.
One pH unit corresponds to 59.16 mV at 25°C, the standard voltage and temperature to which all calibrations are referenced.
A pH electrode consists of two half-cells; an indicating electrode half-cell and a reference electrode half-cell. Most applications today use a combination electrode with both half cells in one body.

Buffers are solutions of known pH value that allow standardization of the electrode and meter. For best accuracy:
• Standardization should be performed with fresh buffer solutions
• Buffers used should frame the range of pH for the samples being tested
• Buffers should be at the same
temperature as the samples Since electrodes are the most sensitive pH instrument in your system, calibration, handling and maintenance are very important. Correct calibration procedures combined with proper maintenance will provide years of reliable readings.

Since glass pH electrodes measure H+ activity relative to their reference half-cells, they must be calibrated periodically to ensure accurate, repeatable measurements. Although calibration against one pH buffer (one-point calibration) typically ensures accurate pH measurements, frequent two-point or three point calibrations ensure the most reliable results. Make sure that your pH system includes calibration buffers for a range of pH values.

Handling, Using and Storage
When handling pH electrodes, rinse the electrodes with distilled water before and after measuring a sample. Blot the end of the electrode with lint-free cloth to remove excess water. Never wipe the electrode to remove excess water – wiping can create static charges that interfere with correct pH measurement.

When storing your electrode, always keep your pH electrode moist. We recommend that you store your electrode in either commercially prepared storage solution, or a 1:1 solution of pH 4 buffer and 4M KCl.
Do not store the electrode in distilled or deionized water – this will cause ions to leach out of the glass bulb and render your electrode useless. After storage, you may notice white KCl crystals deposited on your electrode. Such salt formation will not interfere with measurements, simply rinse the electrode with distilled water to remove the crystals and blot dry before use.

When using a refillable pH electrode, the solution should be filled up to, but not past, the refill hole. The refill hole must be open when measuring to ensure the fill solution flows properly through the reference junction. pH electrodes are shipped with the pH bulb moist. Prior to using your electrode for the first time, follow these three steps to condition
your electrode:
1. Remove the protective cap or boot from the bottom of the electrode and rinse the electrode with distilled or deionized water. (NOTE: Keep the protective cap or boot for use later during storage).
2. Place the electrode in a beaker containing one of the liquids listed below (in order of ionic ability to condition the electrode). Soak for 20 minutes.
• 4.0 M KCl
• 4.0 pH buffer
• 7.0 pH buffer
3. After conditioning the electrode for 20 minutes, rinse the electrode with distilled or deionized water. The electrode is now ready for calibration and to measure pH.

PH Glass
Sensing half-cells are the measuring portion of the electrode system and contain the pH sensitive membrane. Proper selection is important, please use the following guide to aid you in your decision or contact an ASI representative for more details:

Type II:
pH Range: 0 to 12 pH
Impedance Range: 45 to 360 MΩ
Temperature Range: -10 to 135 °C
Response Time: 15s
LIS Response Time: < 30s
Stability: ± 1mV (in 24 hours)
Type III:
pH Range: 0 to 12 pH
Impedance Range: 200 to 600 MΩ
Temperature Range: -10 to 135 °C
Response Time: 30s
LIS Response Time: 45s
Stability: ± 1mV (in 24 hours)
Type IV:
pH Range: 0 to 13 pH
Impedance Range: 25 to 360 MΩ
Temperature Range: -10 to 135 °C
Response Time: 10s
LIS Response Time: < 30s
Stability: ± 1mV (in 24 hours)
Type V:
pH Range: 0 to 13 pH
Impedance Range: 20 to 315 MΩ
Temperature Range: -10 to 105 °C
Response Time: 10s
LIS Response Time: < 30s
Stability: ± 1mV (in 24 hours)
Low Sodium Error
Type VIII:
pH Range: 0 to 14 pH
Impedance Range: 120 to 675 MΩ
Temperature Range: -10 to 135 °C
Response Time: 10s
LIS Response Time: < 30s
Stability: ± 1mV (in 24 hours)

• The impedance of a glass is given as a range of values @ 25°C. This is due to the different bulb shapes and the different glass stem sizes used in our products. Please ask your ASI account manager to determine the impedance of your glass.
• Temperature range is based on the abilities of the glass stem, not the overall combination electrode.
• Response time is the time to 95% of scale
• LIS: Low Ionic Strength Solution (< 100 μS)
• Response time is calculated on the pH stem level, not as a completed combination electrode.
• Response time in a combination electrode varies from design to design due to the various electrolytes and reference half-cells used in ASI sensors.

Reference half-cells provide the reference potential needed for pH measurement. Many options are available to you, please use the following guide to aide you in your decision or contact an ASI representative for more details:
• Single Junction – ideal for general purpose applications
• Double Junction – ideal for solutions that contain sulfides, heavy metals or tris buffers to prevent contamination of the reference cell
• Silver/Silver Chloride (Ag/AgCl) – the most common internal element, suitable for almost all applications (temperature limit of 80°C)
• Calomel (Hg/Hg2Cl2) – is recommended for use in solutions containing proteins, organics or heavy metals which could react with silver and clog the reference junction (temperature limit of 70°C)
• Refillable – economical and long-lasting style that allows the user to refill the reference chamber with reference solution as needed
• Sealed – rugged and requires virtually no maintenance; however, they must be replaced when the reference fill-solution level is low.
• Dynagen – a self pressurizing electrolyte used in many industrial/process applications that involve high pressure.

Extend the Life of Your Electrode
When an electrode ages, it may exhibit sluggish or noisy readings. You can attempt to improve performance with the following procedures:
Reference Problems:
A blocked reference junction is the most common problem of pH measurements. Symptoms include a slow response, offscale and noisy readings. The procedure is specific to the type of electrode reference.
• GEL FILLED (non-refillable)– Soak the electrode in a beaker of warm water (60°C) for 15 minutes to remove dried gel or salts from the junction. Then place in a beaker of warm 4M KCl solution. Set aside until it returns to room temperature. The gel should be moist and the junction flow should be restored.
• LIQUID FILLED (refillable) – Drain the electrolyte from the electrode; rinse the cavity with distilled water; and refill with fresh electrolyte. Then soak the electrode in warm water (60°C) for 15 minutes to restore flow.

Glass Bulb Problems:
A glass membrane will get dirty over time, we suggest the following solutions for cleaning:
• PROTEIN – Use ASI Protein Cleaning Solution or wash in a solution of liquid soap, (about ½ teaspoon per 200mL warm water), using a soft cloth to gently wipe the pH glass - remember that pH glass is extremely delicate and breaks very easily.
• INORGANIC SALTS – Wash in 0.1M HCl or EDTA (DO NOT SOAK); rinse with distilled water.
• GREASY FILMS – Wash in acetone or methanol (DO NOT SOAK); wash with liquid soap then rinse with distilled water.
After cleaning, place the electrode in a storage solution for 15 minutes prior to use.
The best storage solution for a combination electrode is available from ASI in several convenient sizes, however, a pH 4 buffer is also acceptable.

- Back -