What Exactly Is pH?

Here’s a look at what acidity and alkalinity mean at the molecular level and how pH is measured, plus a few sample exam questions covering the topic.
What Exactly Is pH?
A schematic representation of the electron shell structure of hydrogen (not what the hydrogen atom actually looks like).

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When operators are asked to define pH, many struggle to describe what it is and how it is measured. Let’s look at those matters — and explore how questions about pH might be worded on a wastewater licensing exam.

The technical definition of pH is: the logarithm of the reciprocal of the hydrogen ion activity in a given solution. So, you’re probably asking: What the heck does that mean? If we look at what pH represents, we find that it is the measurement of how acidic or alkaline (basic) a solution is. If we grab a sample of treatment plant influent or effluent and measure the pH, what we’re really measuring is the balance of the amount of acid and base chemicals in that water.

pH basics

Some fundamental facts about pH:

  • pH is measured on a scale of 0 to 14.
  • pH is the measurement of the activity of free hydrogen (H+, acid) and hydroxyl (OH-, base) ions in a solution.
  • pH 7.0 is considered neutral, or balanced; it has the same amount of acid and base ions.
  • pH below 7.0 is considered acidic.
  • pH above 7.0 is considered alkaline (or basic).
  • pH is commonly used to describe the activity of the hydrogen ion. An ion is a charged atom or molecule. An atom of hydrogen is made of one proton and one electron (Figure 1), and donates (or shares) its electron easily. Because an atom of hydrogen can share its electron with other elements easily, hydrogen can bond with atoms of other elements, forming what is known as an ionic bond.

Understanding atoms

All atoms have positively charged protons and negatively charged electrons. Most atoms also have neutrons, which are not charged (neutral). Hydrogen in its purest form has one electron and one proton. However, if a neutron happens to be present in the atom’s nucleus, or center, we call that an isotope of hydrogen. The elements deuterium (one neutron) and tritium (two neutrons) are isotopes of hydrogen. Neutrons give an atom weight but do not alter its ionic charge.

One more thing about atoms before we discuss the formation of compounds, like water. Atoms have electrons whirling around their nucleus, which contains protons and usually neutrons. The electrons are arranged in shells, a term used to describe the orbits of electrons around the nucleus. Compare an atom to our solar system for a minute: The sun is analogous to the nucleus of an atom where protons and neutrons are found, the planets revolving around the sun are analogous to electrons.

Atomic theory tells us that the innermost shell nearest the nucleus can contain no more than two electrons. The next shell can hold up to eight electrons. The third shell from the center can hold up to 18, and the fourth and fifth 32 each. The sixth can hold up to eight or 18, and the seventh (and last) shell either two or 18. An atom of plutonium (Figure 2), which has 94 protons (thus an atomic number of 94), has electrons all the way out into the seventh shell.

Forming molecules

Now, about those ionic bonds. A good example is water, made of one oxygen atom and two hydrogen atoms (H2O). A hydrogen atom has one electron but has room in its shell for two. An atom of oxygen has eight electrons. Its innermost shell is satisfied with two, but the second contains only six, leaving room for two more.

Oxygen, therefore, is happy to share its outer electron shell with hydrogen, and when two hydrogen atoms are present, they are satisfied because the vacant spaces are now full with electrons. The compound is said to be stable. If a compound has one more electron than protons, the compound is negatively charged. If there is one more proton present than electrons, the compound is positively charged.

Many operators are familiar with polymer, a coagulant used in biosolids processing and as an aid to settling. Polymers are charged chemical compounds: a cationic polymer is positively charged, an anionic polymer is negatively charged and a nonionic polymer is neutral.

The free hydrogen ion is positively charged (H+). The compound known as hydroxyl, made up of one hydrogen and one oxygen atom, is negatively charged, since it has one extra electron. When H+ is joined to a water molecule (H2O), the resulting compound is hydronium (H3O+). It is the hydronium ion that gives acids their lower pH values and imparts sour taste to acidic liquids.

A base (or alkali) substance is one that will accept protons, thereby neutralizing the acid. An example of an alkali is hydroxyl (OH-), which when combined with the acidic hydronium ion (H3O+) neutralizes, forming two molecules of water: OH- + H3O+ → 2H2O.

Raw wastewater generally has a pH near neutral (7.0), although it may vary between 6 and 8. If significant hydrogen sulfide (H2S) is present in the collection system and the wastewater is odorous, then the pH may be lower than 6 because the amount of hydrogen dissolved in the water (as H2S) has increased, causing a shift in the balance of hydronium and hydroxyl ions.

Measuring pH

pH can be measured three ways: the electrode method, the colorimetric method and the hydrion paper method. The electrode, the most common and probably the most accurate, uses a probe and meter. The meter measures the slight voltage differences between a reference electrode and a measuring electrode. This voltage, in millivolts (mV), is converted to a pH reading.

The colorimetric method includes indicator reagents like bromthymol blue and phenol red to produce color in the solution — red for acid and blue for base. The liquid’s color and intensity are then compared against a set of color standards. The hydrion method uses a special test paper (litmus paper) dipped into the solution. The color produced on the paper is then compared against color standards. Typically, acids turn litmus paper red and bases turn it blue.

The electrode method is accurate if the meter is calibrated properly and the sample is fresh. Most meters can be standardized with three calibration standard pH buffers. Common pH buffers are pH 4, 7 and 10. When standardized with these buffers, the meter is considered accurate across a wide range of pH values.

The pH 7 buffer gives the meter a reference point to “know” what a balanced solution is; the pH 4 and 10 buffers give the meter “target” acid and alkaline values to hit. The meter’s ability to hit the target buffer value is called accuracy and is referred to as the meter’s slope.

A new pH electrode in fresh pH buffers is normally very accurate across this slope and can hit the target pH value very closely, whereas an old probe might miss the mark, giving the pH slope a low percentage slope reading. A new pH meter and probe assembly that is very accurate might have a slope percentage of 98 percent, while an old probe might have a slope value of less than 60 percent.

For instance, when calibrating a pH meter, an operator may find that the meter and probe unit accept the pH 7.0 buffer calibration value, but that when the probe is placed into a pH 4 or pH 10 buffer, the meter reads significantly lower or higher than the buffer’s stated value, or the meter gives an error code or the pH calibration is not accepted. Most pH probes will last about a year to 18 months, after which they tend to lose accuracy and must be replaced.

If performing a two-point calibration, users should be sure to bracket the expected pH reading with the proper buffers. For example, if the normal reading in plant effluent is 7.5, the meter should be calibrated with at least a pH 7 and a pH 10 buffer. If the reading turns out to be pH 6.8, the meter should be recalibrated with pH 7 and pH 4 buffers.

Other pH buffers can be purchased to use with your meter. Some folks use an additional standard buffer (like pH 6.76) to check the meter for accuracy along with the calibration buffers. Using an additional pH buffer, other than the calibration buffers, gives the user confidence in the meter’s accuracy.

Controlling quality

Quality assurance/quality control (QA/QC) is important when measuring pH (or any reportable value). QA/QC is all about accuracy and repeatability. Imagine you have an archery target and a quiver of 10 arrows. If all 10 arrows hit the bullseye, you are very accurate and repeatable. If most of the arrows miss the bullseye and are all over the map, you are neither accurate nor repeatable. If the majority of the arrows miss the bullseye, but are grouped in one area of the target, you are not accurate, but you are repeatable.

The difference between each pH unit is a tenfold value. That is, a pH 9 is ten times higher in hydroxyl ions (more alkaline) than at a pH 8. By extension, a pH of 13 is 100,000 times more alkaline than pH 8. Conversely, a pH reading of 4 would be 100 times more acidic than pH 6.

Operators who ask why they can’t just calibrate a pH meter with the pH 4 and pH 10 buffers don’t realize how large the difference between these readings really is. This is why it is important to use a pH 7 buffer in the calibration procedure. The meter needs a reference to know where the balance point is.

When faced with questions on an exam, many operators forget what they know about pH. The accompanying sidebar lists some questions about pH that are similar to actual questions I have seen on state licensing exams.

About the author

Ron Trygar is senior training specialist in water and wastewater at the University of Florida TREEO Center and a certified environmental trainer (CET). He can be reached at rtrygar@treeo.ufl.edu.

References

  • www.webelements.com
  • Basic Chemistry for Water and Wastewater Operators; D. Singh Sarai, PhD. AWWA, 2005.


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