Acids and bases are polar covalent molecules that dissociate into ions when introduced to water. According to the Arrhenius concept all substances which give H+ ions when dissolved in water are called acids while those which ionize in water to give OH- ions are called bases.
Acids, Bases, and pH
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HA → H+ + A- (Acid)
BOH → B+ + OH- (Base)
The common acids that are almost one hundred percent ionized are:
HNO3 - nitric acid H2SO4 - sulfuric acid HClO4 - perchloric acid
HCl - hydrochloric acid HBr - hydrobromic acid HI- hydroiodic acid
The acids above are called strong acids, because they completely dissociate into their component ions when placed in water. Other acids are incompletely ionized, existing mostly as the unionized form. Incompletely ionized acids are called weak acids, because there is a smaller concentration of ionized hydrogens (H+) available in the solution.
In the list of strong acids, sulfuric acid is the only one that is diprotic, because it has two ionizable hydrogens per molecule. (Sulfuric acid deprotonates in two steps: The first time a proton dissociates, it acts like a strong acid. The second time it acts like a weak acid.) The other acids in the list are monoprotic, having only one ionizable hydrogen per molecule.
There is a similar list of strong bases, ones that completely ionize into hydroxide ions and a conjugate acid. All of the bases of Group I and Group II metals except for beryllium are strong bases. Again, like the strong acids, the strong bases are completely ionized in water solution.
LiOH - lithium hydroxide NaOH - sodium hydroxide
KOH - potassium hydroxide RbOH - rubidium hydroxide
CsOH - cesium hydroxide Mg(OH)2 - magnesium hydroxide
Ca(OH)2 - calcium hydroxide Sr(OH)2 - strontium hydroxide
Ba(OH)2 - barium hydroxide
The bases of Group I metals are all monobasic. The bases of Group II metals are all dibasic. Aluminum hydroxide, Al(OH)3 is tribasic. Any material with two or more ionizable hydroxyl groups would be called polybasic.
The concentrations of acids and bases are almost always given in molarity (M) and vary from dilute (very low concentration) to concentrated (meaning completely made of acid or base with no water present).
pH and pOH
Another way the concentration of an acid or base solution can be indicated is through pH or pOH. The small p in front of the H or OH is an indicator for a mathematical process called a logarithm. The p can be translated as “take the negative log of” whatever value follows it. In the case of pH and pOH this translates mathematically to:
pH = -log[H+] and pOH = -log[OH-]
In this way, if we know the pH of a solution, we know the concentration of H+ in that solution and conversely, if we know the proton concentration we can predict the pH of the solution.
[H+] = 10-pH and [OH-] = 10-pOH
Example
If the concentration of H+ is 1.50 x 10-6 M, the pH = -log(1.5 x 10-6) = 5.8
If the pH of a solution is 8.45, the [H+] = 10-8.45 = 4.00 x 10-9 M
In the second example you might think that the [H+] is very small, but this makes sense because the pH indicates that the solution is basic and not acidic meaning there are very few protons in the solution.
The logarithmic scale allows the small concentrations of protons in the solution to be displayed in easy to manage values. The pH scale indicates which compounds can be defined as acids and which compounds can be defined as bases.
pH values < 7.00 are considered acidic
pH values > 7.00 are considered basic
pH value = 7.00 is considered neutral
This scale is based on the dissociation of H+ and OH- ions in water.
In pure water the concentration of H+ ions from the dissociation of the water molecules themselves is 1.00 x 10-7 M. This means that the pH value of pure water is 7.00 and has come to mean the point of neutrality in the scale. This is because if the concentration of H+ in pure water is 1.00 x 10-7 M then the concentration of OH- must also be 1.00 x 10-7 M as well:
H2O ↔ H+ + OH-
[H+] = [OH-] Neutral
Some other information we can derive from this statement for pure water:
pH + pOH = -log(1.00 x 10-7 M) + -log(1.00 x 10-7 M) = 14
Thus if we know the pH of a solution, we can calculate the pOH by subtracting the pH value from 14 and vice versa.
For example, what is the pH of a solution that has a [OH-] = 2.34 x 10-7M?
First we need to calculate pOH:
pOH = -log(2.34 x 10-7M) = 6.6
Second, we subtract the pOH from 14 to get the pH:
pH = 14 – 6.6 = 7.4
Polyprotic Acids and Bases
Polyprotic acids and bases are those that release more than one proton or hydroxide ion respectively when dissolved in water. This feature is very important when you are trying to calculate the pH of the solution. For instance, the strong acid H2SO4 (sulfuric acid) is diprotic. This means that when it is dissolved in water it releases 2 protons into the solution. To calculate the pH of this solution, you would need to take the overall concentration of the sulfuric acid and multiply its concentration by 2 prior to calculating its pH.
If you have a 0.250M concentration of H2SO4, then the H+ concentration would be twice the concentration of the overall acid or 2 x 0.250M = 0.500M H+
The pH of the solution would then be calculated as pH = -log(0.50M) = 0.301
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