What are Equivalent Weights?

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What are Equivalent Weights?

Most geochemistry plots (e.g., Stiff, Piper, and Schoeller diagrams) display data in millequivalents (i.e., equivalent weight) rather than in the absolute concentration values. The use of equivalent weight provides an understanding of "how much" of certain compounds exists relative to other compounds even though each compound has a different atomic weight. The equivalent weight of a particular ion is calculated by dividing the the atomic weight by the ion valance.

 

The Equivalent Weight field in the Constituents table is used to convert between the measured mass concentration and the concentration in equivalents or milliequivalents per liter. The equivalent weight field should be entered as positive for cations and negative for anions. If the concentration values are entered as milligrams per liter or parts per million then the values in the equivalent weight field will convert the concentrations to meq per liter. The following is a list of ions commonly found in groundwater.

 

Cation / Anion

Equivalent Weight

Na

22.9898

K

39.098

Ca

20.04

Mg

12.1525

Fe (II)

27.97

Fe (III)

18.65

Mn

27.469

CI

-35.453

HCO3

-61.016

SO4

-48.03

CO3

-30.004

NO3

-62.04

NH4

-18.039

PO4

-31.66

 

A source of equivalent weights is http://www.gewater.com/handbook/control_water_analyses/fig40-2.jsp.