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Ups and Downs of the pH Scale

Studying the pH of solutions is a basic skill to help understand the behavior of chemicals, but sometimes it is not very well explained. pH is simply a measure of the number of hydrogen ions in a solution (H+). [1] Hydrogen is a very common component of organic molecules and it has the distinction of being the most abundant element in the universe. [2] Hydrogen consists of only one proton and electron. When it has a positive charge (H+) there is only a proton. This means that it can accept an electron or share an electron with other atoms. [3]

To understand what pH really means, we must go to the atomic level. Imagine a bottle of water. If we assume that the water is 100% pure then it will consist of mostly H2O and very few charged particles: H+ and OH-. In pure water the concentration of H+ will always equal the concentration of OH-, thus their charges cancel each other out. In this case, the solution is called neutral, meaning it has no electrical charge. If we increased number of H+ ions in our bottle then it would become more basic. If we removed all of the H+ ions, it would become very acidic.

pH is not the only measurement of whether a solution tends to be more acidic or basic. pOH is the measure of the number of hydroxyl groups in a solution (OH-). [4] It corresponds directly to alkalinity, which is the ability of a base to neutralize acids. At room temperature pOH = 14 – pH. If we wish to increase the acidity of the water, we would increase the number of OH- ions. To nullify the effect of the added OH- ions, we could add more H+ ions. In practice, the ratio of H+ and OH- to H2O is less than 1 part per billion but this example is convenient to explain the concepts of acidic and basic solutions. Additionally, for the number of OH- ions to increase, the number of H+ ions must decrease because they are both formed when two water molecules react with each other.

During this reaction hydrogen from one molecule jumps to another, creating H+ and OH-. [3] The formula that describes this reaction is shown below:

H2O + H2O = H3O+ + OH-

Note: H3O+ is usually written as H+ because the proton will jump between adjacent water molecules freely. [3]

The pH scale only goes to 14 is because the strongest naturally occurring bases and acids will not generally exceed a logarithmic range of 15 values. It is possible to have solutions with pH higher than 14 or lower than 0 with superbases and superacids. [5] They do not naturally occur. The pOH scale is the opposite of the pH scale. The gradations of the pH scale are logarithmic. [4] This means there is a 10x difference between 7 and 8 and there is a 100x difference between 7 and 9. To give you an idea of the magnitude of the logarithmic scale, the difference between pH 1 and pH 14 expressed as mass would be a paperclip at pH 14, a steam roller at pH 7, and the Great Pyramid at pH 1.

Calculating pH

Since pH is defined by a negative logarithm, we can find it if we know the concentration of the H+ ions. For example, given an H+ concentration of 1x10-10 moles, the pH would be:

pH = -log[H+]

pH = -log(1x10-10) = -(-10) = 10

Conversely, if we know the pH is 12, we can calculate the number of moles of H+ by solving for H+, remembering that log is the inverse operation of base 10 exponents.

pH = -log[H+]

H+ = 10 - pH

H+ = 1x10-12

Calculating pOH

The formula for pOH is very simple. If we know the pH of a solution, we can find the pOH by subtracting pH from 14. For example, a solution with a pH of 8 would have a pOH of 14-8 = 6.  

Similarly, if we knew the concentration of H+ to be 1x10-5 and wanted to find the pOH we would use the definitions of pH and pOH:

pH = -log[H+]

pH = -log[1x10-5] = -(-5) = 5

pOH = 14 – pH

pOH = 14 – 5 = 9

Measuring pH

The most common way to measure pH for high school students and the general populace is by using an indicator. An unknown pH solution reacts with the indicator to determine whether the solution is acidic or basic. pH strips are common measurement tools that have multiple indicators on each strip which can be matched to a chart of color values to determine pH. These are not very accurate due to temperature, salt concentrations, and other substances in the unknown solution. [6]

The glass electrode method is used in most labs. In the same way that a battery is formed when two electrodes are immersed in an electrolyte, this meter consists of a pH reference solution stored inside a glass bulb. [6] The reference solution is in contact with a reference electrode and an exterior electrode is in contact with the unknown solution. The glass prevents Hfrom passing through it but allows to passage of Na+ to maintain the electrical circuit. The H+ activity difference between the solution inside the glass bulb and outside of it are measured as an electrical potential and converted into pH through the Nernst equation.

At 25˚C, the Nernst equation is [7]:

E = 0.0592 * log [ H+reference / H+solution]

Since the pH is defined as pH = -log[H+], we can substitute it into the equation and re-write the equation as:

E = 0.0592 * pHreference – pHsolution

Therefore:

pHsolution = 0.0592 * E - pHreference

Applications of pH

Electrochemistry – pH determines the reduction potential (voltage) of electrochemical cells (batteries and fuel cells).

Seawater – the pH of seawater has been measured for many years and there is a correlation between increased carbon dioxide emissions and higher acidity levels in all of the Earth’s oceans. This has affected sea life in a negative manner. [8]

The pH of bodily fluids and organs is very important. Increased acidity of the blood causes irreversible cell damage. Causes of metabolic acidosis are kidney disease, poisoning, and severe dehydration. [9]

References

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