A substance's pH is a measure of its acidity or alkalinity as expressed in the concentration of hydrogen ions it contains. The pH (short for "power of hydrogen") scale is created by multiplying the exponent for the concentration of hydrogen ions in solution (0.1 equal to 10 to the power of -1, 0.01 equal to 10 to the power of -2) by -1. The pH scale runs from 1 to 14; vinegar (acetic acid), has a pH of 2, while a strong alkali like caustic lye (sodium hydroxide) has a pH of 12, and pure water, considered neutral, has a pH of 7. Such things as blood, soil, battery acid, and swimming pools are pH tested, but pH testing is especially important in ensuring the safety and quality of foods ranging from coffee and cheese to beer and wine. Although litmus paper can be used to test acidity in foods with a pH of 4.0 or less, the best way to ensure correct pH is by using an electronic pH meter, which uses a thin, glass-tipped probe to measure voltage changes caused by the hydrogen ions in solution. Here's what to look for when shopping for a pH meter.
Look for a sufficient level of accuracy. Meters used to measure pH in scientific research are accurate to within 0.002 pH units. If you're buying a pH meter to test the acidity of food, you'll most likely want a unit with an accuracy of 0.01 to 0.02 units, although you can get by with a pH meter with an accuracy of 0.2 units if the pH of the food item you're making won't exceed 4.0.
For certain foods where the sugar-to-acid ratio is important, such as jellies, jams, and preserves, you'll want a more accurate meter, regardless of the pH of the food, as the pH will affect the temperature at which the food jells and how well it sets. For other flavor-sensitive foods, using a less accurate pH meter could cause you to add acid when you don't need to and so alter the food's flavor.
Using a less accurate pH meter with foods whose pH can exceed 4.0 runs the risk of allowing foods with an unsafely high pH (4.6 or higher) to go on the market.
Your pH meter should not only be accurate, but its results should also be consistent from reading to reading. If it gives widely different results a minute apart, it's not reliable.
Look for a pH meter with at least 2-point calibration. To ensure accuracy, the pH meter must be calibrated to ensure its readings are accurate. Calibration is done with standard solutions of known pH. There are 3 standard solutions: pH 4, pH 7, and pH 10. If you are buying a pH meter to test acidic foods, you need to have 2 points of calibration, pH 4 and pH 7. If you are buying a pH meter for scientific research, then you'll also need the pH 10 calibration standard solution.
Calibration solutions are color-coded for easy identification. The pH 4 solution is usually red, the pH 7 solution is usually yellow, and the pH 10 solution is usually blue.
You should calibrate your pH meter at the same temperature as the item you're testing to ensure accuracy.
Choose a unit with the appropriate style of electrode for your testing needs. If you're pH-testing foods with little or no oil, or foods where oil can be added later or separated out, you can get by with standard glass-tipped electrodes. For foods with a high-oil content where the oil cannot be separated out, such as salad dressings, you will need electrodes that won't be clogged by the solution they're immersed in.
Some pH meters come with several types of electrodes, while other meters require you to buy the electrode you need separately.
If you are going to use the pH meter in a number of places, choose a meter where the electrode isn't permanently attached. If the electrode breaks, your meter will be useless.
Look for a durable unit. Your pH meter should be rugged enough to stand up to the environment or environments you'll be using it in. A unit designed to be taken into the field should be resistant to dirt and possibly also to harsh chemicals. If you test for acidity on just-cooked foods, your pH meter should be able to withstand a temperature of at least 160 degrees Fahrenheit (71 degrees Celsius).
Look for automatic temperature compensation. Because pH readings are affected by temperature, buying a pH meter with automatic temperature compensation is helpful if you produce a large amount of food items that need to be tested and don't have the time to let the food cool to room temperature before testing. The temperature compensation feature should have a range that covers the temperature at which your sample is to be tested.
If you use a pH meter without temperature compensation, you can either test samples only at room temperature or use a temperature compensation chart to determine the correct pH.
Consider a pH meter that connects to a computer. This kind of meter automatically transfers a sample's pH and temperature readings to the computer, useful if you routinely test a large number of samples at a given time. This feature adds about $75 to $100 to the overall cost of the pH meter, however, so if you test only a small number of samples at a time, you may not be able to justify the additional cost.
Shop for the best price. You can buy a pH meter from a scientific supply company or directly from the manufacturer' website. Prices range from under $100 to over $1,000; to get the best deal, assess your needs and ask the company's representatives about the capabilities and features of their pH meters before you buy.