If it turns pink, it's acid I think—you probably learned that useful phrase once upon a time, along with the second half of the same rhyme: "and if it turns blue, it's an alkali true." Measuring acids and alkalis (bases) with litmus paper is something pretty much everyone learns how to do in school. It's relatively easy to compare your little strip of wet paper with the colors on a chart and figure out how acidic or alkaline something is on what's called the pH scale. But sometimes that's too crude a measurement. If you keep tropical fish, for example, or you're a gardener with specimens that like soil of a certain acidity or alkalinity, getting things wrong with the litmus risks killing off your prized pets or your plants. That's why many people invest in a meter that can measure pH directly. What are pH meters and how do they work? Let's take a closer look!
Photo: US naval hospital technicians test a water sample for acidity, alkalinity, and chlorine levels. This sophisticated two-probe, digital meter is made by Hach. It can be hooked up to a computer with a USB cable to download data from its internal memory, which can store 500 measurements. Photo by Nick De La Cruz courtesy of Defense Imagery.
If you're interested in measuring acidity, it helps if you know what it is before you start! Most of us have only the faintest idea what an acid or an alkali really is. We know it's a substance that can "burn" our skin (though it's a chemical burn, not a heat burn), but that's about it. What's even more confusing is that we can safely eat some acidic things (lemons, for example, contain citric acid) but not others (drinking a chemical like sulfuric acid would be extremely dangerous).
Photo: Some acids, such as lemon juice, are perfectly safe to handle; others will burn your skin and can do painful, permanent damage.
Acids and alkalis are simply chemicals that dissolve in water to form ions (atoms with too many or too few electrons). An acid dissolves in water to form positively charged hydrogen ions (H+), with a strong acid forming more hydrogen ions than a weak one. An alkali (or base) dissolves in water to form negatively charged hydroxide ions (OH−). Again, stronger alkalis (which can burn you as much as strong acids) form more of those ions than weaker ones.
The pH (always written little p, big H) of a substance is an indication of how many hydrogen ions it forms in a certain volume of water. There's no absolute agreement on what "pH" actually stands for, but most people define it as something like "power of hydrogen" or "potential of hydrogen." Now this is where it gets confusing for those of you who don't like math. The proper definition of pH is that it's minus the logarithm of the hydrogen ion activity in a solution (or, if you prefer, the logarithm of the reciprocal of the hydrogen ion activity in a solution). Gulp. What does that mean?
It's simpler than it sounds. Let's unpick it a bit at a time. Suppose you have some liquid sloshing about in your aquarium and you want to know if it's safe for those angelfish you want to keep. You get your pH meter and stick it into the "water" (which in reality is a mixture of water with other things dissolved in it). If the water is very acidic, there will be lots of active hydrogen ions and hardly any hydroxide ions. If the water is very alkaline, the opposite will be true. Now if you have a thimble-full of the water and it has a pH of 1 (it's unbelievably, instantly, fish-killingly acidic), there will be one million times (10 to the power of 6, written 106) more hydrogen ions than there would be if the water were neutral (neither acidic nor alkaline), with a pH of 7. That's because a pH of 1 means 101 (which is just 10), and a pH of 7 means 107 (10 million), so dividing the two gives us 106 (one million). There will be 10 million million (1013) more hydrogen ions than if the water were extremely alkaline, with a pH of 14. Maybe you can start to see now where those mysterious pH numbers come from?
Photo: The pH scale relates directly to the concentration of hydrogen ions in a solution, but not in a simple linear way. The relationship is what we call a "negative exponential": the higher the pH (lower the acidity), the fewer the hydrogen ions—but there are vastly fewer ions at high pH than at low pH.
Suppose we decide to invent a scale of acidity and start it off at very acidic and call that 1. Then something neutral will have far fewer (one millionth or 10−6 times as many hydrogen ions) and something alkaline will have fewer still (that's one 10 trillionth, or one 10 million millionth, or 10−13 times as many). Dealing with all these millions and billions and trillions is confusing and daft so we just take a logarithm of the number of hydrogen ions and refer to the power of ten we get in each case. In other words, the pH means simply looking at the (probably gigantic) number of hydrogen ions, taking the power of 10, and removing the minus sign. That gives us a pH of 1 for extremely acidic, pH 7 for neutral, and pH 14 for extremely alkaline. "Extremely alkaline" is another way of saying incredibly weakly acidic.