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Practical pH Theory and Application
by Mark Heyda
Part I- Basics
A commonly used measurement in laboratories and in process control, pH is widely used but poorly understood. Simply put however, pH is measured as a voltage or potential of the Hydrogen (H3O+) ion activity in an aqueous solution. A pH meter or controller is just a fancy voltmeter. Well, yes- it is a very fancy one due to the fact that usually temperature influences the voltage produced. So, it is not that simple after all. And to make matters worse, there are other factors which can influence the measurement too. In any event a pH scale of 0-14 was developed with pH 7, the midway point as neutrality where a pH of 7 is equal to zero milli-volts.
Where do we start to find the beginnings of pH theory? Most chemists go back to the basics by referring to a pretty famous physical chemist by the name of Dr. W. H. Nernst. Back in the late 1800’s, Dr. Nernst applied principals of thermodynamics to explain the chemical reactions of a wet cell battery and the Nernst equation was born. By the way he ran in some fast company including: Max Plank and a fellow by the name of Albert Einstein. In any event his equation related the potential (voltage) of a battery cell to the logarithmic Hydrogen (actually hydronium H3O+) ion concentration with a temperature factor. His work continued and in 1904 he finally introduced the concept of the diffusion layer for pH electrodes. Without getting into the math, and a host of complicating factors, the bottom line is that the measured pH depends upon the degree of ionization of salts, acids and bases in solution with temperature playing a part. A very small voltage is generated by a pair of electrodes immersed in water and converted into a pH reading by a meter with temperature compensation.
Today most electrodes are actually a combination of two half cells in one body. Think of a pH electrode as a chemical battery and the meter as a microprocessor based voltmeter which can factor in the effects of temperature and the aging of the diffusion layer (glass membrane surface). One half of the electrode contains the glass, an internal solution and a wire and the other half (also called the reference) has another membrane typically made from a porous material, an internal solution and a second wire. While the probe is immersed in a solution, the meter measures the voltage between the two wires and translates this value into pH. Like an ordinary battery, an electrode has a limited time in which it will perform properly. Hence, all electrodes will eventually become unusable. In addition to the physical construction, other factors such as: the design of the glass surface, the protection of the wires, the reference junction half cell and the internal electrode chemistry are all very important in determining the life of the electrode. These design factors should be tailored to the electrode environment (chemicals, fluid flow rates, pressures and temperatures.)
In the world of process control, all of these factors must be considered when choosing an electrode. However, even in a laboratory setting, the type of fluids to be measured will influence the construction or type of electrode which should be used. All electrodes are not created equal. Some are designed only to work well for special applications and are not suitable for others. Other types will work well in a broad variety of conditions. We will discuss this in more detail in Part 2.
As a practical matter, we really don't care that the Nernst equation did not tell the whole story. We now rely on the measurement and need a stable and repeatable reading under a variety of conditions. The software contained in the meter or controller can provide the user with more information about: calibration data (including offset, slope and date), electrode cleaning and buffer contamination. It can also eliminate ground loop errors and provide general ease of use. The electrode design is critical for good performance. Picking the proper electrode to match the requirements of the application makes a big difference in the life and accuracy of the pH reading.
Part II- Practical Considerations
The body of the pH electrode can be made from a variety of materials including: glass, stainless steel or titanium, PVDF (KYNAR®), PVC, and other plastics. The effects of chemicals, solvents, pressure, rough handling, temperature, contamination and mounting all must be considered when choosing a body type. Users in different industries will find a particular body type more suitable for day to day use.
Most industrial users have found that PVDF construction with a polymer gel filled Teflon® reference junction to be the most versatile and long lasting for a pH probe. A matching pin feature is also very desirable to increase electrode longevity. The tip of the electrode should be flat to prevent breakage and ease the effects of abrasion due to flow rate across the surface. In many cases, a built in RTD sensor is also a welcome feature to eliminate temperature errors. However, the actual type of glass used in the sensor must be considered too. While some manufacturers simply change the thickness of the glass, that is not the answer. The actual type of glass and the resulting gel diffusion layer should be tailored to the temperature to which the glass is exposed. Low temperatures found outdoors in northern climates, high temperatures, changing temperatures and even fluoride applications all can be addressed by using specially formulated glasses.
Portable and bench-top meters tend to feature pH probe bodies made of plastic, metal or glass. The pH sensor tip geometry may also be important. The measuring glass can be conical, spherical, flat or have a very small micro taper. The type of junction (single, double, triple or open), geometry and material choice then become key factors in fitting the electrode to the application.
The selection process is all application driven. For instance, applications with high solids typical in the food and beverage industries require an open junction reference (using a viscolene electrolyte) to solve the problem of reference clogging. In other cases a triple junction may suffice to protect the junction. Measuring the pH of skin would require a flat sensor. There are also special purpose electrodes with extra features such as integrated temperature sensors, identification codes, and more.
The junction design will allow the contained electrolyte to flow very slowly or more rapidly. If the electrolyte is allowed to flow faster, the response rate of the sensor will increase. However, in such cases a provision for refilling the electrolyte is made. In some cases a polymer or gel will be used that will have a very slow leakage rate. These electrodes are designed for minimum maintenance.
And finally there is always a trade-off between response speed and electrode life. Even the type of electrolyte used internally may be a factor. If the user is uncertain about the electrode, it is best to consult with a knowledgeable source to select the best option. After all the sensor design will play a large part in the overall performance of the attached meter.
It is important that the measuring tip of the pH probe remain hydrated. Drying out the tip (gel layer of the glass) will prevent the ionic exchange between sodium and lithium. The glass will need to become re-hydrated to restore proper function to the sensor. Also drying the electrode will cause the reference junction to become crystallized. The junction is a critical part of an electrode and perhaps its weakest link. Depending upon the junction type, it can be re-hydrated so that the function of the electrode can be restored. But, in general it is best to store the electrode in a solution which is identical in concentration to the reference solution itself. Without any ionic osmotic pressure between the solution and the reference, no interchange of materials will occur. This is why a “storage solution” is commonly used when the pH electrode is not in use. Electrodes should normally be shipped with this solution to keep the glass and reference hydrated. Although some manufacturers insist that their pH probes can remain "dry" for a period of days, the fact remains that all pH probes have the same properties. The glass needs to be hydrated in order to respond to changes in pH quickly. Although the glass may be left dry for a short period of time, eventually the active glass membrane must re-hydrate in order to achieve a proper reading. Leaving the pH probe in a storage solution of the same ionic concentration as the reference (liquid or gel) will not deplete the solution and will allow a faster response when the electrode is actually called into service.
Part III- Voltage Measurements
Milli-volt potentials at various pH levels: the pH scale
Theoretically, the voltage developed between the two internal electrode wires is 59.16mV per pH unit. The total pH scale is 0-14 with a neutral pH of 7.000 measured voltage of zero milli-volts. The pH scale is negative above seven and positive below seven. A pH of 4.000 would have a voltage which corresponds to three pH units from neutrality or 3 x 59.16mV= +177.48mV. A pH of 10.000 also has a value of three pH units from neutrality, but the resulting voltage would be negative (-3 x 59.16mV or -177.48mV). By using the mV key or Range key on your pH meter, one can directly view the voltages produced. This can be a very desirable diagnostic feature. When buying a new meter, always make sure to get this feature even if you are not measuring ORP. More on this later.
Electrodes seldom produce a zero voltage at pH 7.000. Typically electrodes produce a voltage slightly less than or greater than zero. This difference is referred to as an offset. The offset will be as much as 15mV in some new electrodes. This is quite normal. However, electrodes with larger offsets will have a shorter useable life, since as electrodes age, the offset will increase over time. This aging effect and the corresponding change in offsets must be corrected in order to read the proper pH. Each time a pH meter is calibrated, the meter is correcting for a change in offset. For example: a typical new pH electrode is manufactured with an offset of -12mV. In a pH buffer of 7.00 or 7.01 the meter will correct for this error and memorize that -12mV not 0.0mV should correspond to a displayed value of 7.000 (7.010). While conditions of use will dictate the rate of aging, each time the meter is calibrated the stored offset value will change so that the meter will be able to display the proper value. This is why pH meters need to be calibrated frequently. Practically, an electrode whose offset is greater than 1/2 a pH unit (or about +/-30mV) will be start to become unstable. Calibration will become difficult and the electrode will not correctly indicate the proper pH value. If cleaning or reconditioning of the electrode does not correct the problem, the electrode should be replaced. The milli-volt scale on a pH meter as stated above is extremely useful for this exact reason.
Typically a meter should be calibrated at more than one pH value. After calibrating at a value of pH 7.000, a second value should also be entered. For example, a value of 4.000 (or 4.010) would be also used for a calibration point. One would expect that a voltage of 177.48 plus -12mV or 165.48mV should produce a pH reading of 4.000 (4.010) in the display of the meter. However, the second point may not correspond perfectly to the expected value. This is typically represented graphically and the change in the curve represents a change in slope. We simply refer to this difference in readings at the second value as a slope. Theoretically the predicted value would have a voltage corresponding to 59.16mV/pH unit. The slope may be different and only be 58.16mV per pH unit. The slope may be expressed either as an absolute value or as a percentage: 58.16/59.16 or 98.3%. Slope values normally should be greater than 95% and less than 102% but a probe can still be useable with slopes as low as 90%.
For special applications such as winemaking, for example, it is better to use a slope buffer solution closer to the actual use point. In this example a second buffer would have a value of 3.00pH. For other applications one should consider using so called "Technical buffers" rather than the standard 4, 7 and 10 common buffers. The technical buffers are available in a wide range of values from pH of 1.000 to 13.000.
Part IV- Complications in pH Measurements
In certain instances, pH readings can be difficult. Obviously, solutions with heavy solids loading or with oils can clog or coat the glass sensor and or junction. This makes the reading slow and unresponsive. These problems can be cured with a simple cleaning using a number of specialized cleaning solutions. pH meters with a "Cal-check" feature often prompt a user to perform an electrode cleaning in order to improve the response and accuracy. (See the additional technical article: "Improvements in pH Measurement.")
Other problems are typically due to glass breakage or clogging problems. By using more advanced controllers, an alarm condition can be created to indicate sensor failure. Some advanced controllers like the Hanna 504 use a cleaning cycle and have relays which when coupled with solenoid valves can direct a clean water jet to impinge upon the sensor on a periodic basis. This can be very helpful in difficult applications to minimize the necessity of electrode removal and manual cleaning.
Other problems are more mysterious. When the pH probe is inserted into a tank, the readings can fluctuate wildly or provide generally unstable results. This is typically due to electrical interference from a mixer motor or some other device grounded through the tank. Remember that the pH signal is only measured in thousandths of a volts. It would only take a small transient voltage to be the cause of interference. Depending upon the severity of the problem, several solutions are possible. The most simple is simply to take a sample of the tank in a small plastic cup and measure the pH of the contents.
If the sensor is connected to a continuously reading pH controller, then an industrial sensor equipped with a matching pin or solution ground may be the best answer. The stray interfering voltage can be filtered out using circuitry in the controller. Not all controllers have this advanced feature, so it is advisable to be sure the controller will accommodate a matching pin sensor.
In more extreme cases, combining matching pin technology with an actual grounding rod connected to a good earth ground should be used to eliminate this problem. Extending the service life of the electrode itself is a corollary effect of using a matching pin sensor.
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