Water Quality Mathematical Expressions & Relationships Reference Sheet (Page 2 of 3) in pdf

1023 atoms of calcium. Therefore, a millimole is 1/1000th of a mole. To calculate meq

Ca/L from the reported value in mg/L, we must know something about calcium.

Calcium has a molecular weight of 40.08 grams/mole

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Calcium has a valence of +2

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The equivalent weight = (40.08grams/mole)/(2 equivalents/mole) = 20.04

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grams/eq

To convert to mg/meq you simply multiply g/eq by 1000 mg/g and divide by 1000

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meq/eq, thus g/eq = mg/meq

If your sample contains 30 mg Ca/L, what is the concentration in meq/L?

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Meq Ca/L = (30 mg Ca/L)/(20.04 mg/meq) = 1.50 meq Ca/L

Electrical Conductivity (EC), Specific Conductance - simply defined this is a measure

of a solution's ability to conduct electricity. It is the reciprocal of a solution's electrical

resistance. The units of resistance are ohm-cm. The units for conductance is

Seimen/meter (S/m). Not too long ago EC was measured in units of mho/cm. It turns out

that Seimen/meter equals mhos/meter (mhos/m), microSeimen/centimeter (uS/cm) is

equivalent to umho/cm and milliSeimen/centimeter (mS/cm) is equivalent to

millimho/cm (mmho/cm). No matter what the term appearing on our reports (Electrical

Conductivity (EC), Specific Conductance, Conductance, Conductivity, etc,), we report

uS/cm, or umhos/cm corrected to 25°C.

Total Dissolved Solids (TDS) - this is also a measure of the salts dissolved in water. Not

surprisingly, there is a relationship between TDS and EC. Generally speaking the TDS in

mg/L is about 2/3-3/4 of the EC measured in uS/cm. So, now you have a way of

calculating TDS if you know the EC; TDS ~ 0.7 EC.

Hardness - hardness is defined as the sum of the calcium (Ca) and magnesium (Mg)

concentrations, both expressed as calcium carbonate in milligrams per liter (mg/L). To

calculate hardness from the calcium and magnesium concentrations (mg/L), we must first

convert these concentrations to milliequivalents/Liter (meq/L). This conversion in terms

of concentration allows the calcium and magnesium to be added together.

Often water treatment technicians express hardness with units of grains per gallon. 1grain

(gr) per gallon (gal) = 17.1 mg/L = 17.1 ppm. Conversely, 1 mg/L = 0.0585 gr/gal. So if

you know your water's hardness in terms of grains per gallon you can convert that value

to mg/L or ppm by multiplying by 17.1. In the reverse, if you have the value in terms of

mg/L or ppm multiply that value by 0.0585 to obtain that value in terms of gr/gal.

Water Quality Mathematical Expressions & Relationships Reference Sheet Page 2