Guide to pH Meters
Need to test pH levels but don't know what testing tools to use? This PE Fact article clues you in on all of the ways you can successfully measure pH and how each instrument is used.
It is vital to maintain and control proper pH levels in several agricultural, industrial and environmental processes. In farming, the appropriate pH level of the soil assures the best possible crop. Also, the pH in our drinking water is controlled in processing plants in order to produce water that is safe to drink. Without the proper monitoring and control of pH levels, deterioration of the quality of all plant, animal and human life would be imminent.
The pH scale is a numeric scale used to show the acidity or alkalinity of a solution. The amount of hydrogen ions in a solution determines if it is an acid or an alkaline. A more acidic solution would have a large concentration of hydrogen ions. A solution with fewer hydrogen ions would be alkaline.
The range of the pH scale is 0 to 14. A solution that has a pH level of seven (distilled water) is not an acid or an alkaline; it is neutral. Solutions that have pH levels lower than seven are more acidic such as vinegar (3.0 pH), sulfuric acid (1.2 pH) and orange juice (3.7 pH). Alkaline or "basic" solutions have pH levels that are greater than seven such as sodium hydroxide (pH 13), ammonia based household cleaners (pH 12), and potassium hydroxide (pH 14).
Testing Tools
In order to properly control pH, you must correctly measure it. There are various ways that pH can be measured including pH paper, digital readout pocket testers, portable and bench top pH meters.
A fast and affordable way to measure pH is by using pH paper. Strips of paper are soaked with a chemical indicator. This indicator, when placed in a sample liquid, turns the paper a certain color. The liquid's pH is found by comparing the color of the paper to a standard chart that is provided. Although these indicators are offered for every pH level, they have limits to their accuracy. It is very difficult to decipher the change of color when working with colored or cloudy samples.
Another way to determine the pH level of a solution is with the pocket tester. They are extremely handy and easy to carry. pH tests can be made very fast and there is no need to take a sample back to the lab. Pocket pH testers are accurate enough for common applications and are reasonably priced.
If your measures need to be more accurate, portable pH meters are a better choice over the pocket tester. The majority of portable meters are bigger than pocket pH testers. They are long-lasting and ideal for accurate pH readings in the field. They use an array of replaceable pH electrodes for individual applications and conditions.
Bench top pH meters are mainly used in testing or research labs. In general, they are more expensive but provide precise accuracy and more testing options. Also, bench top meters use a wide array of replaceable pH electrodes.
Types of Electrodes
pH electrodes are comprised of a general purpose glass electrode (measuring electrode) and a reference electrode. They are combined into one electrode and referred to as a combination pH electrode. Combination pH electrodes are filled with a salt solution made up of Potassium Chloride (kCl) soaked with a silver/silver chloride (Ag/AgCl) reference solution. The combination electrode is best for general purpose applications.
Since standard electrodes contain silver in the reference solution within the electrode, there are numerous applications where this kind of electrode can't be used. The subsequent solutions cannot be measured with general purpose electrodes.
- Heavy metals including silver, iron and lead
- Proteins
- Organics such as acetone
- Low ion solutions such as distilled water
- High sodium concentrations such as solutions containing large amounts of salts
- Sulfides
If a sample of the solution contains any of these contaminants, the pH electrode may possibly work for a short period of time before it eventually fails.
If you want to test the pH of the solutions mentioned, the following electrodes can be used:
Colomel Reference electrodes are intended to work in solutions containing proteins, organics, low ion activity and heavy metals. They are filled with Mercury/Mercury Chloride (Hg/Hg2Cl2) reference solution.
Double-Junction Reference electrodes are intended to work in the same applications as the colomel electrode as well as in highly concentrated solutions. This is because of the presence of two reference junctions to filter out any possible contamination of the reference electrode.
Teflon Junction Reference electrodes are designed for applications where the solutions to be measured can obstruct the reference of a standard electrode. Teflon Junction Reference electrodes are recommended when working with solutions like paints, gels and pastes.
Electrode Care-Preparing the pH Electrode to Measure Solutions
The pH electrodes are delivered moist with storage solution. In some instances, after an electrode has been sitting idle in storage for a while, white crystals may become visible near the bulb of the electrode or around the electrode cap. These crystals are the result of the potassium chloride solution (kCl) which has evaporated and crystallized. Before initially using the electrode, rinse it with some distilled water, and soak it in a buffer solution for 10-20 minutes.
Electrode Conditioning
1. Remove the protective cap from the bottom of the sensor and wash the electrode with distilled or deionized water.
2. Soak the electrode in a beaker of one of the subsequent liquids for about an hour so the electrode can be hydrated again.
**Potassium Chloride (kCl), 3.8 or 4.0 molar concentration.
**4.01 Buffer Solution.
**7.00 Buffer Solution.
**Tap water.
3. After one hour, wash the electrode with distilled water. You are now ready to calibrate the meter and record measurements.
Calibration
How do you calibrate? In order to correctly calibrate a pH meter, you must choose at least two buffers for a two point calibration (except pH meters that have only a 1-point calibration feature), one must always have a pH of 7, and a second point which covers the predicted pH of your sample mixture. For instance, if you measured an acidic solution (pH below 7), you would choose the pH 4.01 and 7.00 buffers to calibrate the meter. Likewise, if you were measuring mostly basic samples (pH above 7), you would choose pH 7.00 and pH 10.00 buffers for calibration.
Calibration is also referred to as standardized and slope in a few pH meters. To standardize a pH meter means to calibrate at a pH of 7.00. After this, you slope the meter to either the pH 4.01 or 10.00 buffers.
Electrode Storage
After measurements have been completed, the electrode should be stored in a way which will keep the bulb of the electrode constantly moist or hydrated.
All electrodes are always shipped with a protective cap, or "rubber boot". This cap can be used to store a tiny amount of fluid. For long and short term storage, add a few drops of potassium chloride (kCl) or 4.01 buffer solution to the cap and place the cap back on the electrode.
The electrode can be stored in a beaker or an electrode saver bottle filled with kCl solution. Buffers with a pH of 4.01 or 7.00 are also ideal for storage. For short term storage, tap water must be used. Do not use distilled or deionized water.
Commonly Asked Questions
Q.What is ATC?
A.ATC (Automatic Temperature Compensation) is an option found on pocket pH testers and handheld and bench top meters.
Q. Why is it used?
A. ATC allows the meter to compensate for temperature disparity that may affect the accuracy of pH measurements.
Q. How is this done?
A. AMeters with ATC recognize one of several types of temperature probes or have a temperature sensor built into the electrode. These probes or electrodes calculate the temperature of the sample and establish the pH value at that temperature.
Q. Do pH meters need calibration?
A. Yes. Before taking the pH measurement of your solution, you must calibrate the pH meter, especially if it has been stored or used to test a pH range that greatly differs from the one you currently have to test. When you calibrate a meter, you are basically inspecting and fixing the pH meter and probe ensuring that it reads pH correctly.
Please Note: The information contained in this publication is intended for general information purposes only. This publication is not a substitute for review of the applicable government regulations and standards, and should not be construed as legal advice or opinion. Readers with specific questions should refer to the cited regulation or consult with an attorney.
