Alberta Special Oilwell Boiler Practice Test

What unit expresses sulfite residual in boiler feedwater?

ppm as SO3 mg/L as SO3 % as SO3 ppb as SO3

Correct Answer:

ppm as SO3

Explanation:

Sulfite residual in boiler feedwater is reported as ppm as SO3. In boiler water chemistry, concentrations are typically given in parts per million to reflect the amount of a chemical per million parts of water, and the “as SO3” qualifier specifies the chemical form used for the measurement. For dilute aqueous solutions, 1 mg/L is essentially the same as 1 ppm, but the standard practice is to express sulfite residual specifically as ppm as SO3 to indicate the oxide form of the sulfite. Percent and ppb aren’t used here because they’re not appropriate scales for this trace-level, chemistry-specific measurement, and mg/L would be technically equivalent but isn’t the conventional boiler-chemistry default.

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