Rules of Thumb—Chemistry

Updated: May 28

This post provides tips and insights I've come across focused on chemistry topics for wastewater treatment operators. A search on Google for “Rule of Thumb” leads to the following definition:

“A broadly accurate guide or principle, based on experience or practice rather than theory.”

Yes, this is me at one of my favorite municipal wastewater treatment plants in Oswego, IL. (January 2021)

PPM vs. mg/L

I was reading a chapter in a "wonderfully-friendly" textbook for water and wastewater treatment plant operators, from the chapter titled Chemistry 7: Chemical Dosage Problems and realized I have been too relaxed in often using the units of meassure ppm (parts per million "parts") and mg/L (milligrams per liter) interchangeably without giving any thought to the difference between them. The sentence quoted below is now going to make me more careful in how I use these units. Here's the rule-of-thumb from this chapter:

In the range of 0−2,000 mg/L, milligrams per liter are approximately equal to part per million.

Source: American Water Works Association. "Basic Science Concepts and Applications." Principles and Practices of Water Supply Operations. Third Edition. Denver, Colorado: American Water Works Association, 2003.

For many treatment plant operators, using either unit is going to be just fine under most conditions. But part of my job involves treating complex industrial waste streams using Fenton's reagent, requiring dose concentrations of iron (e.g., ferric chloride, ferrous sulfate) and hydrogen peroxide that can be much higher than 2,000 mg/L and I have been using both units seemingly as my mood, rather than science, motivates me. As of today I am going to stop doing that. I'm going to keep this "rule" in mind and use ppm whenever I'm working in a higher concentration range.



Most of us are concerned with those things (solutes) added to water (the most common solvent) that are either soluble or insoluble. But you will also see the phrase "sparingly soluble" used and I've recently come across the best interpretation of these terms, taken from an excellent little book (shown below) I highly recommend for anyone who spends time in a laboratory. I quote the following from Mitchell's Laboratory Solutions book.

Solubility refers to the maximum amount of solute that dissolves in a given amount of solvent at a given temperature. Solubility is usually expressed by the number of grams of solute per 100 milliliters (mL) of solvent. Solubility, however, is also expressed in terms such as "soluble," sparingly soluble (or slightly soluble)," and "insoluble." These qualitative and somewhat subjective terms. As a guideline to these terms, not a strict definition, consider that a substance is:

(1) "soluble", if more than 1.0 gram of the substance dissolves in 100 mL of solvent;

(2) "sparingly soluble", if 0.1 to 1.0 gram of the substance dissolves in 100 mL of solvent;

(3) "insoluble", if less than 0.1 gram of the substance dissolves in 100 mL of solvent.

When using solubility tables, a fourth term may be found: “decomposes.” A few substances will decompose when added to a solvent; the mixture will break down into two or more simpler substances; thus undergoing a chemical change. When a substance dissolves it is going through a physical change, but when it decomposes it is going through a chemical change. When a substance has decomposed, it no longer has the same chemical properties it did before decomposition because it no longer is the same chemical. Hydrogen peroxide, for example, decomposes to water and oxygen as shown in the chemical formula below.

A dilute solution contains only a small amount of solute, whereas a concentrated solution contains a larger amount of solute for a given amount of solvent. The terms dilute and concentrated are relative, and therefore their use is somewhat ambiguous.

For a small but important set of chemicals, however, the term concentrated does have a definite meaning. These chemicals are all common acids and bases and usually exist in aqueous solution or as nearly pure liquids. For these chemicals, the term concentrated refers only to the typical values shown in the following table.

Source: Mitchell, Sharon Grobe. "Laboratory Solutions for the science classroom ." Batavia, Illinois: Flinn Scientific, 1995.

In addition to the limited categorization of solubility provided above, here's a more detailed breakdown reproduced from Sigma-Aldrich's website.

Here is one more source describing solubility, quoted from the (excellent) Nalco Water Handbook.

For those materials that are very slightly soluble, which in this text arbitrarily defines as being less than 2000 mg/L (the approximate solubility of CaSO4), the solubility product is a useful tool.

S: Soluble, over 5000 mg/L

SS: Slightly soluble, 2000 to 5000 mg/L

VSS: Very slightly soluble, 20 to 2000 mg/L

I: Insoluble, less than 20 mg/L

Source: The Nalco Water Handbook. Second Edition. New York: McGraw-Hill Book Company, 1988. (pgs. (3.12 -3.13)

Solubility of Oxygen in Water

Solubility of Carbon Dioxide


TDS and Conductivity

The relationship between TDS (total dissolved solids) and conductivity depends on the water chemistry. For example, 1,000 mg/L of NaCl will give a different conductivity than 1,000 mg/L of MgSO4. The very rough rule of thumb is: TDS, mg/L × 1.6 = Conductivity (µS/cm). The factor of 1.6 used in the equation has a typical range of 1.4 to 1.8, though wider variations are certainly possible.

When possible, the best correlation is developed from the analysis of a specific water or waste stream for both conductivity and TDS from which a specific correlation factor is produced. Then, if the water chemistry remains fairly constant, conductivity can serve as a good indication of TDS. If the water chemistry changes significantly, the rule of thumb will not work.


Relative Oxidation Power

I do a lot of work using Fenton's reagent to oxidize complex compounds in waste streams that would have a negative impact on biological treatment units. The goal with Fenton's is to break the complex compounds into simpler compounds that can then be used by bacteria as a source of food. The Fenton's reagent approach requires using iron as a catalyst to boost or increase the oxidative power of hydrogen peroxide combined with lowering the pH of the waste stream being treated to a range of 2 to 5. Of course, after Fenton's treatment, the pH needs to be readjusted to the neutral range before the stream is introduced into the bioreactor.

The reactive species I choose depends on the waste stream being treated and I often do several iterations of treatments comparing just hydrogen peroxide vs sodium permanganate vs chlorine. How each of these reactive species compare to one another is shown in the table below.


Water Hardness

Does the water in your home feel soft or even slimy to the touch? If you travel, you will likely notice differences in how water feels compared to what you are familiar with at home. For me, you can really feel the difference in the shower where, when the water is soft, it seems more difficult to rinse off. Sodium ions dominate in soft water, in contrast to calcium and magnesium being more prevalent in water that is hard.


The Sulfur Cycle


The Meaning of a Chemical Equation

The chemical equation for a reaction gives two important types of information: the nature of the reactants and products and the relative number of each. The reactants and products in a specific reaction must be identified by experiment. Besides specifying the compounds involved in the reaction, the equation often gives the physical states of the reactants and products:

For example, when hydrochloric acid in aqueous solution is added to solid sodium hydrogen carbonate, the products carbon dioxide gas, liquid water, and sodium chloride (which dissolves in water) are formed:


Source: Zumdahl, S. S., Zumdahl S. A., & DeCoste, D. J. (2021). Chemistry: An atoms first approach. Boston, MA: Cengage Learning.


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