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Vol. 5(8), pp. 3The McAllen International Orchid Society JournalAugust 2004

A Short Review of The pH Scale

R J Ferry

Consider a container of water. For our purposes, we'll assume it's pure water; not the kind normal kind coming from the faucet that has all sorts of additives, intentionally put in or otherwise. When table salt (NaCl) is dissolved in water, it separates into Na+ and Cl-, and the Na+ ions are attracted to the OH- ones and the Cl- ions are attracted to the H+ ions. In short, each ionized hydrogen atom is actually an atom without its electron (which has a negative charge), which is why it has a + charge. A few (not many: about one in ten million, actually) water molecules ionize, that is they separate into their constituent parts of H+ and OH-, but all life processes depend on this tiny amount of ionization. Indeed, living cells must maintain their internal cellular levels of H+ and OH- ions within narrow limits because even small changes greatly influence important cellular reactions.

The percentage of H+ (or OH-) ions in solution is described by a range of numbers known as the pH scale. This scale runs from 0 to 14. A solution with the same number of H+ and OH- ions (e.g. pure water) has a pH of 7. A solution with more H+ ions (i.e., lacks electrons) is acidic and hence has a lower number, while a solution with more OH- ions (has a comparative overabundance of electrons) is basic (= alkaline).

Fig. 1. A horizontal pH scale using the exponential number-values for acidity and alkalinity.

Simply put, on the pH scale, 7 is neutral, and numbers lower than 7 (to 0) are increasingly acidic while numbers above 7 (to 14) are increasingly basic (alkaline) (Fig. 1).

Fig. 2. pH values of some common substances.

This is all well and good, but it doesn't explain (to the ordinary person) why the scale runs with seven being neutral and then down to zero for acidic or up to fourteen for basic. The pH value of a few common substances are shown in the figure (Fig. 2), but most explanations of pH in flower books just gloss over what pH is, and the would-be flower hobbyist ends up being impressed, but somewhat confused. The point is that we need to begin with how many grams of Hydrogen (by weight) is in so many liters of pure water, and it turns out that pure water is only slightly ionized; there are 1.008 grams of H+ in 10,000,000 liters of pure water, or--put another way--the ratio is pretty much 1:10,000,000, or--put still another way--1/10,000,000 of water is Hydrogen.

First off, a pH number is a logarithm, and a logarithm is an exponent. For example, 32 = 9. in this case the 2 is the exponent. In the case of 34 (which is 3 x 3 x 3 x 3), so 34 = 27 x 3 = 81, and the 4 is the exponent. Decreasing exponentially, instead of increasing, 3-1 would be one-third, 3-2 is one-ninth, and so forth.

Now, count the zeros! There are seven in the number 10,000,000, so that means the H-ion concentration in 10,000,000 liters of pure water is 10-7. Hence, we arrive at the pH value of pure water as 7 since 7 is the log of 10,000,000. Since these are logarithimic values, each number indicates an increase (or decrease) exponentially. In other words, a pH of 6 means the solution contains 10 times as many H-ions as one with a pH value of 7.

At this point, don't throw up your hands with all this and go to another page! What we need to know is that most orchids generally like substrate (for example the bark mixture, or the tree on which they're mounted) to be very slightly acidic. That's one reason why we mix in a smattering of Sphagnum moss with the bark as we're potting a plant. Sphagnum moss (particularly the fresh live stuff) is mildly acidic, and tends to be an anti-bacterial agent, (it was used to dress wounds during the American Civil War). Orchids may do well with certain types of fungi growing in and around their roots, but certain bacteria rot the roots, and the live Sphagnum helps prevent that bacterial action.

Despite all of the above, don't get the idea that "a little is good so a lot is better." Remember, using the pH scale, acidity or alkalinity increases ten times with every number higher or lower than 7, so if some book recommends a pH of 6.3 to 6.5 as optimum for a particular species, don't go off the deep end and get the idea that 5.3 to 5.5 is better! Finally, how can you tell what the pH of your potting mix is? Well, there are some good pH meters for sale, and there are some not-so-good ones on the market. Remember, buying a pH meter is like buying a light meter for photographic work: you're buying accuracy, and the cheaper one may give you a number, but not an accurate one. Shop around and ask! We'll talk more about pH, meters, and pH meter readings at future society meetings.

Copyright © 2004 R J Ferry