The pH Phenomenon

Jan. 1, 2006
Monitoring water pH

About the author: Michael Neiheiser is technical support accounts manager for National Testing Laboratories, Ltd. He can be reached at 800.458.3330, ext. 215, or by e-mail at [email protected].

Many people determine the quality of water they consume by how it smells, tastes or looks. Although these criteria are important to the consumer, they are primarily aesthetic properties of the water. A glass of water may look, smell and taste good, but that is just part of the story. As a bottler, one must start with an even more basic approach to properly address these aesthetic conditions. The measure of water’s pH is one such fundamental provision.

What Is pH?

The pH scale is a measurement of the acid or base strength of a substance and is calculated by determining the concentration of free hydrogen ions (H+) in the solution. In English, pH can stand for “hydrogen power,” “power of hydrogen,” or “potential of hydrogen”—all synonymous terms. We use the symbol [H+] with square brackets to signify the concentration of free hydrogen ions.

High [H+] indicates an acidic solution; low [H+] indicates a basic or alkaline solution. The pH scale runs from 0 to 14, with 0 being the most acidic, 7 neutral and 14 being the most basic. It is a logarithmic scale based on powers of 10, so 1 pH unit change equals a tenfold change in H+ ion concentration. For example, a pH of 5 is 10 times more acidic than a pH of 6. A difference of two units, from 6 to 4, would mean that the acidity is 100 times greater, and so on:

pH = -log[H+]

In a sample of pure water, the [H+] is equal to 1 x 1010-7 moles per liter (0.0000001 moles per liter). In pure water, the number of hydrogen ions in the solution equals the number of hydroxide ions (OH-), so the concentration of hydroxide ions [OH-] must also equal 1 x 10-7 moles per liter.

The excess hydrogen ions in acids give them interesting properties. Acids readily react with metals and other materials. Strong hydrochloric acid is produced in the human stomach to help digest food at a pH around 2. In dilute concentrations, acids are responsible for the sour taste of lemons, limes, vinegar and other substances. Bases are also very reactive. The strong base NaOH is used in many household cleaning agents such as oven cleaner and drain clog remover.

Water pH

So, what does pH mean for water? The pH of pure water is 7. As discussed earlier, at pH 7, the concentration of H+ ions is equal to the concentration of OH- ions in solution: [H+] = [OH-]. The normal range for pH in surface water systems is 6.5 to 8.5 and in groundwater systems is 6 to 8.5.

In general, acidic water with a low pH (<6.5) is considered soft and corrosive. Therefore, the water could easily absorb metal ions such as iron, manganese, copper, lead, zinc and other potentially toxic metals. This can cause premature damage to any non-treated metal piping in contact with the process water and contribute a metallic taste to your product. One way to treat the problem of low pH water is with the use of a neutralizer. A typical neutralizing chemical is soda ash. Another method is to pass the water through a sacrificial media. For instance, sacrificing calcium carbonate into the water, will reduce corrosiveness and raise pH.

Basic water with a pH greater than 8 indicates that the water is hard. Hard water does not pose a major health risk, but can cause aesthetic problems. These problems include a specific alkali taste to the water. A high pH can also cause scale to form. This is because at a pH around 8, the calcium in the water combines with carbonates in the water to form calcium carbonate. Calcium carbonate can form scale deposits on exposed surfaces such as filler nozzles or plumbing. These tiny particles float around in product water, giving it a cloudy, turbid appearance. At even higher pH (>10), the water can actually make your eyes sting and possibly give you a sore throat.

But how do we get the acidic or basic water in the first place? To start, we must keep in mind that water molecules are constantly in motion and colliding with other neighboring atoms. Also, each water molecule carries a dipole, or net charge, across the molecule. This dipole causes each molecule to behave like a tiny magnet with both a positive and negative end. This dipole causes water molecules to be attracted to each other; the positive hydrogen is attracted to the negative oxygen of a nearby molecule. This is called hydrogen bonding.

Because the oxygen atom in water tends to monopolize the electrons in the molecule, the hydrogen protons are only loosely held to the molecule. The attraction between adjacent water molecules allows them to swap hydrogen protons. In fact, many molecules that contain hydrogen can swap protons with water molecules. When the water contains other positively charged ions such as magnesium and calcium, the negative oxygen ions will form bonds with them. This leaves more free hydrogen ions in the solution, raising the [H+]. When more negatively charged ions, such as chloride or bromide ions, are in the water, they will attract the free hydrogen ions, lowering the [H+] (and subsequently raising the [OH-]).

Chlorine. If you use chlorine as a sanitizing agent (in plants), or to disinfect source water in transit, then pH should be an important aspect of your process. The sanitizing ability of chlorine is achieved by it turning into hypoclorous acid. pH affects the efficiency of chlorine by determining the amount of hypoclorous acid (available free chlorine) that is formed:

  • At pH 6.5, 90% of the chlorine will be hypochlorous acid;
  • At pH 7.5, 50% of the chlorine will be hypochlorous acid; and
  • At pH 8, 20% of the chlorine will be hypochlorous acid.

As mentioned previously, a pH of 6.5 or lower would be unacceptable because of the corrosion it could cause your system. The compromise is a pH of 7.2 to 7.6, preferably the midpoint of 7.4. Remember, if your pH drifts too high, it will require more chlorine for adequate disinfection.

Reverse osmosis. Reverse osmosis (RO) systems are very pH sensitive. The oldest technology in RO membranes utilizes cellulose acetate membranes, which generally operate at the lower range of the pH scale. The newer, thin film composite membranes have operational pH ranges that are usually on the more basic end of the pH scale. Some RO membranes utilize a wide range on both sides of the scale. Whatever your equipment choice, it is important to monitor closely the pH of the influent water to avoid damage to your RO membranes.

In addition, the ability of the RO membrane to filter specific contaminants can also be affected by pH. One such contaminant is hydrogen sulfide. The form that the hydrogen sulfide takes in the water is dependent on pH. If the pH is less than 6, the hydrogen sulfide will be a gas and cannot be removed by an RO unit. If the pH is greater than 8 in water that is being treated (feedwater), the hydrogen sulfide will occur as sulfide ions that can be removed.

Determining pH

pH can be determined in several ways. pH meters electronically measure [H+], giving pH as a digital readout. pH indicators are also used. Indicators are substances obtained from plant material that change color depending on the degree of acid or base of the substance they are mixed with. A few drops of the indicator are added to a solution of the substance to be measured. The color change of the indicator is matched to a chart, which corresponds to a specific pH. Some indicators have a wide range of color changes from acid to base. Others are exclusive to a small range within the pH scale, for example, a pH of 2.5 to 5.

No matter what type of equipment you use, the careful and consistent monitoring of your water’s pH should be examined at several steps in your process, as part of your routine quality-monitoring program, to ensure the highest quality product for the consumer.


About the Author

Michael Neiheiser

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