Environmental Engineering: Fundamentals and Design - Chapter 2

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TOC > Ch. 2 > 2.1 Mass concentration units

# Chapter 2 Environmental Measurements

James R Mihelcic, Richard E. Honrath, Jr. and
Noel R. Urban

## Mass concentration units

### Concentrationā€¦it's a matter of taste.

One packet of Kool-Aid contains about 5 g of drink mix. According to the directions, mixing this with 2 L of water and some sugar will make the perfect batch of drink. But how would the taste change if this same amount of Kool-Aid was dumped into Lake Superior which holds just over 12x106 L of water? What if it was dumped in a Dixie cup instead? Different taste.

Clearly, the amount of water used matters when mixing Kool-Aid . The changes in flavor are the direct result of changes in the concentration of the mix. Much in the same way that concentration affects the taste of the Kool-Aid, it has effects of environmental importance as well.

### Why does concentration matter?

Concentration is the driving force for:

Diffusion: a higher concentration gradient means more rapid diffusive mass transport.

Chemical reactions: in some cases, a higher concentration results in a higher reaction rate.

Biotic effects:

- Nutrients: a higher concentration means a faster growth rate.
- Toxicity: A higher concentration means more severe toxic effects.

The approach to expressing concentration and the attendant units varies with the medium of interest. Some work best for air while others are more appropriate for soil, and others still for water.

### Mass/Mass Units

An industry located in Syracuse, New York used the chlor-alkali process to produce chlorine gas (Cl2) and sodium hydroxide (NaOH) from salt (NaCl). Mercury used in the process and was lost to the waste stream through leakage and disposal. An estimated 75,000 kg of mercury was discharged to Onondaga Lake between 1946 and 1970.

Mercury discharged to the lake has become associated with soil particles (sediment) and has accumulated in the food chain. The mass/mass unit of expression is an appropriate way to report the concentration of contaminants in the sediment and biota. The two examples that follow explore this approach.

Mass/mass expression may be based on units of mass themselves, as calculated in the mass fraction,

| ; e.g. gram/gram
or, often more appropriately, on a "parts perā€¦" basis,
| ; e.g. parts per million
As you examine the two example problems below, consider what is meant in the statement above by "more appropriately."

### Mercury in Sediments

A one kilogram sample of lake sediment (dry weight) was found to contain 0.02 g of Hg. What is the mercury concentration expressed in ppmm?

### Mercury in Fish Flesh

Now, let's look at the mercury concentration in fish.

The U.S. Food and Drug Administration issues consumption advisories for mercury in fish when concentrations equal or exceed 0.5 ppmm (wet fish flesh).

If a channel catfish from Onondaga Lake weighing 6 kilograms (wet weight) contains 32 milligrams of mercury, what is the Hg concentration in ppmm? Would it be advisable to eat this fish?

### Mass/Volume Units

Mass/volume units are typically used when expressing the concentration of a chemical in water. Common units include g/L, mg/L and g/m3.

These concentrations can also be expressed on a ppmm basis:

We know the mass of chemical, but what is the mass of water? Given that water has a density of approximately 1000 kg/L, it is possible to convert a given volume of water into a mass of water.

### Mercury in the water column of Onondaga Lake

The mercury concentration in the water column of Onondaga Lake is 11x10-6 mg/L. Is there a more convenient expression than mg/L? What is the appropriate "parts per" representation for mercury in the water column of Onondaga Lake?

Parts per million doesn't seem to be the appropriate "parts per" expression. Let's try parts per trillion. That seems "more appropriate"!

### Visualizing "parts per"

Even though ppm, ppb and ppt are frequently used terms, it can be difficult to visualize them.

### How much is too much?

The EPA sets National Primary Drinking Water Regulations and National Secondary Drinking Water Regulations. The concentration levels that can be considered harmful vary from chemical to chemical. The following graph illustrates the EPA's maximum safe limit for arsenic, TCE, toluene and PCB and how different concentration levels can have equally as adverse effects on human health.

### Molar Concentration

Molarity (M), or moles per liter, is another way to express concentration. This approach is most often used to report concentrations of dissolved chemicals in water and is particularly useful in making calculations relating to chemical reactions where the stoichiometry is expressed in molar units.

Remembering the definition of a mole (the formula weight of a substance, expressed in grams) permits conversion between mole/volume and mass/volume units.

Eastern mole says "No you fool! A chemical mole! Click me to see."

### What exactly is a mole?

When you go shopping do you buy twelve eggs or a dozen eggs? It's often easier to use counting units that represent larger quantities. Same with molecules. One mole of a substance contains 6.022x1023 molecules of that material. And while the number of molecules in a mole is the same for all elements (Ca), ions (Ca2+) and compounds (CaCO3), their molecular weights differ.
Atomic weight = 12 g/mole | Atomic weight = 40 g/mole | Atomic weight = 16 g/mole

A mole is one gram molecular weight of a substance.

The gram molecular weight of a molecule is the sum of its component atomic weights. Thus Ca2+ has a molecular weight of 40 g (the atomic weight of Ca) and CaCO3 has a molecular weight of 100 g (the sum of the atomic weights of its component atoms).

Another advantage of expressing concentration as molarity is that the units are consistent with the stoichiometry of the reaction.

Reactions proceed on a molecule by molecule and therefore on a mole by mole basis, since 1 mole equals 6.022x1023 molecules . A number of moles of one substance reacts with a number of moles of another substance to yield a number of moles of product. For example,

Thus, while the masses of chemical differ greatly between reactions, the molar requirements are identical.

### Copper toxicity in Ceriodaphnia

In toxicity testing, the LC50 is the concentration of a chemical that will kill 50% of the test organisms.

The LC50 for Ceriodaphnia exposed to copper in water is 200 µgCu/L. Express the LC50 as a molar concentration.

or, "more appropriately" as:

### Mercury in fish flesh, moles/L

For a little more practice, let's take one more look at the Onondaga Lake catfish.

In the last example, it was determined that the concentration of mercury in a channel catfish was 11 ng/L. The atomic weight of mercury is 200.6 g/L, what is the molar concentration?

or, "more appropriately":

#### Credits

Liquid Concentration illustration: Sue Hill (Michigan Tech).
Aerial photo: Cornell University Library, New York Aerial Photographs Collection,
Historical Aerial Photographs of New York.
Scooped Sediment photo: Marty Auer.
Catfish photo: U.S. Fish and Wildlife Service National Digital Library.
Mercury photo: Public domain image from Wikimedia Commons.
Eastern Mole photo: National Park Service, Tallgrass Prairie National Preserve, Kenneth C. Catania.
Catfish Pair photo: Ryan Somma from Wikimedia Commons.

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