Calculation of a Metals Oxide Compound using a Metals Analysis

Ask the Expert Question:
Can an Oxide of a metal (i.e. Barium Oxide) be determined by running a metals analysis for barium and then using a calculation to get barium oxide?

Experts Response:
The calculation is pretty straightforward, but it does involve some assumptions.

We will start with the example you cited with barium. Barium has an atomic weight of 137.34, and it normally has an oxidation state of +2. It is one of the alkaline earth metals in the second column of the periodic table, and all of these metals tend to have an oxidation state of +2. In minerals oxygen always has an oxidation state of -2. As a result, one barium atom will combine with one oxygen atom to create barium oxide, BaO. Barium oxide has a molecular weight of 137.34 + 16 (for the oxygen) which equals 153.34. If we assume that all the barium is in the oxide form, then the barium oxide concentration is as follows.

Ba concentration x (153.34/137.34) = BaO concentration

This case is pretty straightforward. With iron, however, you also end up making an assumption about the oxidation state of the metal. You could have Fe in either the +2 or +3 state. The resulting oxides would either be FeO (is Fe is in the +2 state) or Fe2O3 (if Fe is in the +3 state.). If you are calculating these as oxides, I think it is reasonable to assume that the metals are fully oxidized, so we will use the +3 state.

For iron, we will calculate the ratio of the molecular weight of Fe2O3 over the amount of iron in each molecule (2 atoms). Iron has an atomic weight of 55.847.

Molecular weight of Fe2O3 = (2 x 55.847) + (3 x 16) = 150.694

Weight of iron = 2 x 55.847 = 111.694

So, like the barium example above, here is the calculation to determine the amount of Fe2O3 based on the measured Fe concentration.

Fe concentration x (150.694 / 111.694) = Fe2O3 concentration

NOTE: These calculations would present the maximum possible concentration of barium oxide – for example if the sample contained barium sulfate or barium carbonate (or any other form of barium) then the concentration of barium oxide would be overstated.

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