FYI,
No, I did not write this:
pH is a Measure. The symbol pH is a contraction standing for pondus Hydrogenii (=the "weight of hydrogen", the simplest element); pH is the relative measure of hydrogen ion concentration. Put another way, pH is the relative presence of H+ (Hydrogen ions) versus OH- (= Hydroxyl ions). Note when these two types of charged species get together they form an uncharged water (H2O) molecule.
You finally will understand this. In a "pure" solution of nothing but water there is a concentration of @ 10-7, that's ten to the minus seventh (0.0000001), or one in ten million molecules of water that on average have fallen apart/& are getting back together. The fancy mathematical expression for pH is that it's the negative (or one over, the reciprocal) of the logarithm (base 10) of hydrogen ion concentration, or
-log10 [H+] or 1/log10 [H+]
In our example of "pure" water this is log10 1/[H+]*. -log10 [10-7] = log10 [1/10-7] = log 10 [107] = 7
-log10 [10-7]
or the log of one over 10 to the minus seven which is a pH of 7.000. Ahhhh. Yay!
All that pH is is a measure of other "stuff" dissolved in the solution that affects (net increases/decreases) the concentration of Hydrogen (&/or Hydroxyl) ions. Making the average number of hydrogen ions greater (e.g. 10-6, 10-5, less 10 to the minus negative) means the pH is lower (!) See? Or more acidic (by definition). Going the other direction, having less concentration of H+, meaning the 10 to the minus power is more negative, denotes an alkaline, or basic condition. As an example, consider a hydrogen ion concentration of 10-8 = a concentration of 0.00000001 H+ ions, denotes a pH of 8.000. A 7.000 situation is termed a neutral pH. Makes sense to me.
Maybe one last example (or two). Let's say the hydrogen ion concentration is 10-7.8 or 0.00000078. What is this solution's pH. That's right, 7.8, like much of our beloved southwestern "liquid-rock" tap water. Note that in the above expressions the change between a pH point, let's say seven and eight represents a change of an order of magnitude, or ten times. Much like the logarithmic scale (Richter) used to describe earthquakes, a small difference in number represents a large change in hydrogen ion concentration. Going from a pH of 6.5 to 4.5 is a difference of 100 times less concentration! This is a big difference, and you should be aware and wary of the logarithmic nature of the pH scale.
I'll leave it up to the Big Editor, Don Dewey, to leave in/out mention of the upper/lower limits of dissociation of acids and bases, pH's of 0 to 14 under conditions of complete/full dissociation.
Reserve/Resistance to pH Change:
The capacity of a system to resist an upward (example 7.0 +) or downward (less than 7.0) pH is termed Alkalinity and Acidity respectively. The same quantity is in turn referred to as alkaline and acidic reserve. Read those last two sentences again, carefully. This understanding seems easily lost amongst aquarists.
In actual practice what happens is there are materials, chemicals suspended and dissolved and capable (like gravel, coral...) of dissolving (or going into solution), one's added intentionally as foods, treatments, source/tap water constituents that "fight" or resist change in pH. This is what we mean by buffering. Buffers aid/thwart efforts in changing chemistry/physics by resisting shifts (in pH in this case). This is the reason Alkalinity/Acidity tests must be applied in conjunction with simple pH, if much adjustment is needed/desired and/or you're dealing with expensive or sensitive livestock. There may be so much buffering capacity at different levels or points in the system's pH make-up that you may accidentally overshoot and drastically/tragically raise or lower the pH too much too soon. I cannot but begin to tell you how many times I've seen this happen. READ ON!
Biological Importance:
So all this being said and done, or at least written... so what? Allow me to present this pictograph:
Graph showing the relationship between pH and optimum efficiency:
100
Efficiency
(%)
0
7.0 7.2 7.4 7.6 7.8. 8.0
pH
In this graph I am trying to describe a hypothetical (but real and practical) relationship between a physiological function (like an enzyme system) efficiency and a range of pH. In the example you can see that peak efficiency is achieved only within a narrow limit, with function dropping off quickly at a slightly higher and lower pH.
This is the Real World! Fishes (& us!) are made up of these transient collections of enzyme systems; fundamentally that's what life is (at least one definition). These systems are affected (pro & con) by suitable 1) points, 2) limits & 3) slow changes in pH.
Maybe illustrating human blood pH phenomena is a good idea. Normal, acceptable range is something like 7.35 to 7.50 (pretty narrow, eh?); slightly basic. When/if you slow/stop ventilating your lungs, blood pH dips in sync. with build-up of carbon dioxide (CO2) in solution as carbonic acid. Alternatively, hyperventilating will blow off CO2, driving-up pH. Both conditions have their practical limits. Your blood has only so much buffering capacity and moderating mechanisms. At some juncture in moving your blood pH too much too fast you will stop, hopefully by ultimately only stopping doing whatever is causing the emergency shift or passing-out at which point your body will unconsciously shunt blood only to vital functions and allow restabilization.
Fish and other aquatic life are not as fortunate sometimes. Their metabolisms are often closely tied with the chemistry and physics of their surrounding medium (water). Too much, too soon shift or deviation in pH is dangerous! Fishes do have buffering mechanisms to resist these changes, especially sudden shifts in pH, but be warned! Healthy, conditioned livestock can weather such changes, but it will/does weaken them; & less-than-prepared organisms may be pushed over the edge. The challenge of ph shock may not/usually is not obvious at the time if the individuals survive, but other negative influences can/may cause their demise "mysteriously" within hours to days to weeks.
Certain events are particularly important to be on guard for pH shock; from system differences, pH adjustments, and "drift". Permit me to elaborate; I'm going to anyway.