by Mark Heyda, MBH Engineering Systems

Hanna pH and ORP Meters

 

This monograph is your guide to using a new pH meter.  If you have purchased an ORP electrode, this article will address that as well. Your new pH meter may be very simple with automatic features, or a bit more complex with many advanced features.    Either way, the same basic procedures need to be followed.  These instructions are tailored to Hanna Instrument pH meters but can be applied to most meters.

Understanding  pH and ORP electrodes and calibration

The electrode itself or sensor sends a signal to the digital meter based on the acidity, alkalinity or ionic potential (ORP) of the solution.  The signal is a very weak voltage measured in millivolts.  The meter converts the signal to pH and displays the result.  However the signal can and will change over time for all electrodes.  This is a good analogy: Think of the electrode as a chemical battery.  Over time the battery no longer develops the same voltage and the attached flashlight bulb light weakens.  Eventually the light fails totally.  The same is true for an electrode.  Because the signal for the same pH value changes, the meter needs some way to assign the proper pH value to it.  This is the reason why pH meters need periodic calibration.  Each time the meter is calibrated, the meter assigns a new set of values to the incoming signals.  For instance, a new pH electrode when placed in a known standard solution (commonly referred to as a “buffer”) with a value of pH 7.01 may have an output signal of -5.1mV.  The meter after calibration will show on its display a reading of 7.01 (+/- the accuracy of the meter’s electronics).  Let us assume that after a month the same electrode now sends a signal of -5.5mV to the meter for the same buffer.  Without recalibration, the meter will show the pH incorrectly.  The error of 0.4mV is the drift of the signal.  In this case the error is small so the offset from the true value will not be very much (0.01pH).  However, as the electrode ages, the error will increase and the rate of the change will increase as well.  That is why electrodes need more frequent calibration as they age.  At the end of their useful life, the electrode may exhibit an offset of -30mV which is equivalent to 0.5 pH units or more depending upon where on the pH scale the reading is occurring.  To the user, all of this happens automatically each time the meter is calibrated.  (No math required!)  However, when the drift exceeds about -30 to -35mV, the electrode becomes unstable.  The user will find that the pH value on the display changes for no reason with the same solution.  And, the meter more than likely will not calibrate at all.

 

Two or More Point Calibration

 

Now that we understand the basics of calibration, why should a meter be calibrated at more than one pH value?  Most pH meters should be calibrated at a minimum of two points.  The most common points are values of pH 7.01 and 4.01 or 7.01 and 10.01 if you are going to be reading alkaline values.  The more advanced Hanna meters will let the user calibrate at three, four or five pH values.  These meters will also let the user select which buffers are to be used.  While the majority of meters have memorized values of 4.01, 7.01, 10.01 as well as European buffer values (1.68, 6.86, 9.18), custom buffers are available in pH values from 1.00 to 13.00.  To achieve the best accuracy, the calibration should cover the range of the desired measurement values.  For instance if the user wanted to measure a solution which had values expected to range between 3.00 and 4.00, one would want to calibrate at 3.00 and 7.01 rather than 4.01 and 7.01.  This eliminates the uncertainty associated with trying to extrapolate a value beyond the calibrated values.  Some very accurate Hanna meters (both bench top and portable) will be able to read and calibrate (using special buffers) to a thousandth of a pH, i.e.  3.204 +/- 0.002 pH.  Getting back to the science behind a multi-point calibration requires an understanding of “slope”.  Earlier we talked about offset.  If the pH values were linear, i.e. a pH value could be calculated by the meter using 59.16mV per pH unit.  The meter would display the proper value once the offset error was adjusted by the calibration.  However, to make life more complex, the slope of the pH line when plotted (pH vs. mV) may be altered at pH values which vary from pH 7.  Hence the error in the reading is larger as the reading increases or decreases from neutrality (pH 7).  The good news is that the multi-point calibration also corrects for this deviation.  The resulting readings then are correct at all values between the calibration points.  If the readings go beyond the calibrated range, the meter assumes linearity and simply extrapolates the value to be displayed.  The true value may be slightly different.  Much depends upon the users need for accuracy at the measured value and the need for a more advanced meter.

Temperature Compensation

 

One more point needs to be considered.  The temperature of a solution when depressed or elevated from 20°C will also contribute an error to the pH reading if the meter does not have a temperature sensor built in to correct for this variation.  The larger the divergence becomes, the greater the error will be.  The error only becomes significant if the pH value diverges from neutrality but increases as the pH values become more alkaline or acid.  In other words at pH 9, the error increases with the same temperature offset versus pH 8.  Most pH meters today include a temperature compensation function and will automatically correct for the error caused by temperature variations.

Technique for Calibration

 

If the user follows these simple steps, the meter will be calibrated properly.

1.      Rinse the pH electrode with tap water.

2.      Dry the outside of the electrode but blot the bottom of the sensitive glass with a paper towel.  This is done to avoid abrasion of the glass measuring end and to prevent dilution of subsequent use.

3.      Place the electrode into a pH 7.01 buffer solution making sure that the junction (located on the bottom or side of the electrode) is wet.  For most electrodes this means an immersion of only 1 to 2 inches.

4.      For more precise lab work, use a magnetic stir plate.

5.      Turn the meter on and select the calibration mode.

6.      The meter will normally default to asking for a pH 7.01 buffer.  If not use the up and down keys to select pH 7.

7.      The meter’s display will ask to “CONFIRM or CFM” when the value in the display has stopped changing.  If the electrode is in good condition, the value will jump to near the buffer value very rapidly.  As the electrode deteriorates or if dirty/clogged, this process will take longer.

8.      Press the proper key to CFM or other key as stated in your manual.

9.      The meter will now ask the user to place in another buffer (usually pH 4.01).  The user can accept this value or change it to the desired second calibration point.

10.   Rinse the electrode as before in tap water and dry as in step 2.

11.   Place the electrode in the buffer selected.

12.   Wait for the meter to accept the second value and press the confirm key when prompted.

13.   Repeat as needed for additional buffers if needed.

14.   If the meter indicates “Wrong buffer” it means that the value selected does not match the immersion solution or the electrode’s diverges too far from an accepted value.  To remedy, a new electrode must be used or the electrode must be cleaned prior to use.

15.   If the meter indicates “Wong electrode” it could mean that the attached electrode is of the wrong type (i.e.- ORP rather than pH).

16.   Once the calibration at two or more points has been completed, the meter will automatically return to the normal measuring mode.  It will show the temperature compensated value of the solution.  Some small variation may occur depending upon the inherent accuracy of the meter and the variation of temperature.  If the probe and the solution are at different temperatures, readings will continue to change slightly as the two reach equilibrium.

Electrode Fouling

 

If an electrode is dirty or clogged with oil, grease, debris etc., the meter will still yield a digital result.  The reading more than likely will not be correct.  Older electrodes which have been extensively used in solutions other than plain water or at elevated temperatures can be damaged.  Sometimes even an inspection of the electrode tip will not reveal any outward problem.  Cleaning solutions developed by Hanna Instruments can be used to rectify these problems.  Many solutions are available for different industries and maladies.  It is best to consult with a knowledgeable distributor such as MBH Engineering Systems to select the best solution(s) for your application.  The use of these products can easily extend the useful life of an electrode. 

Smart meters developed by Hanna can identify electrode age and fouling.  These instruments are referred to as “Calibration Check™” meters.  The CAL-CHECK feature shows the user the condition of the attached electrode after each calibration.  This allows users to know in advance whether a displayed value is correct or marginal.  It also should be a prompt to clean the electrode.  In fact some of the meters actually flash “clean electrode” in the display! 

Electrode Care

 

To prolong the life of the electrode, it should be immersed in a special solution when not in use.  The solution is simply called, Storage Solution (HI 70300).  The benefits are twofold.  First, the glass will remain hydrated.  This is very important as a dry electrode will degrade and must be rehydrated prior to use.  Second, the salts in the solution are at the same concentration as the salts inside the electrode.  Therefore no transportation of salts will occur to deplete the pH reference.  Technically one could say that there is an absence of osmotic pressure across the junction.  The life of an electrode will vary depending upon the type of solutions, temperatures, immersion times and cleaning cycles.  Typically electrodes will last about a year although some can remain functional for two years.  Rarely do electrodes exceed this life span.  In some cases with severe conditions of use, electrode life may be as short as one month or even a few days!   Selection of the proper electrode is important as hundreds of different types of probes are manufactured.  Each probe is designed to accommodate different conditions of use to maximize its useful life.

ORP Electrodes

 

ORP or oxidation-reduction potential electrodes look similar to a pH electrode but in fact function quite differently and are not interchangeable with a pH electrode.  Typically they have a platinum tip although sometimes a gold tip can be used.  The voltage developed by these electrodes does not drift nearly to the extent that their pH probe cousins do.  In other words these probes have a longer life.  While ORP probes are not calibrated as pH probes are, they should be checked to see if their error is unacceptable.  As a rule of thumb, a divergence of more than 50mV from a standard solution (HI 7022L 470mV) would show that the electrode needs replacement.  It is a good idea to condition an ORP electrode prior to use.  Hanna Instruments has two solutions which can be used to help the tip respond properly to ORP changes.  These pre-treatment solutions are: HI 7091 or HI 7092 (reducing or oxidizing respectively).  ORP electrodes also respond well to cleaning solutions and need to be immersed in “Storage Solution” when not in use.

Troubleshooting

 

What happens if my meter is not working properly?  Chances are that the electrode is no longer serviceable.  In the absence of a Calibration Check™ meter, a quick easy method to determine electrode failure is to look at the electrode offset.  Some meters and controllers have either a CAL Data key or a GLP (good laboratory practice) key.  Using this key, the meter will show the last calibration time and date as well as the offset, slope and buffers used with the calibration.  If the offset is numerically greater than [-30mV] i.e.: [-36mV], the probe should be replaced.  If your meter does not have that feature, but has a RANGE key, you can determine the offset as well.  Simple place the electrode in a pH 7 buffer and read the raw reading from the probe in millivolts by switching the reading from pH to mV.  The meter will show the offset.  In rare cases, the offset is acceptable but meter is still not functioning properly, the slope may be too large.  However, in that case the meter more than likely will not calibrate.

Final Thoughts

Hanna Instruments continues to innovate and build what many refer to as the "Cadillac" of pH meters and electrodes.  The choices available now are extensive for those in the market for a meter.  From the very excellent rechargeable portable field instruments which are waterproof-- of course-- to excellent research grade bench meters, Hanna manufactures all of them.  Because they build everything in their own factories, they continue to offer great prices as well.  Advanced features like: intelligent electrodes, ISE capabilities, Calibration Check™, millesimal pH ranges, GLP, data logging, graphical displays, inductive recharging, auto-end points and more are standard features on selected models.  I have seen meters in the field which are still in service after twenty years.  It is nice to know you are going to be getting a reliable instrument with an iron clad no nonsense warranty. 

The above article may not be copied or reproduced without consent of the author.  All rights reserved. Mark Heyda 2008