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The most straightforward definition of an Acid Gas is any Gaseous Compound which, when dissolved in Water, will form an Acidic Solution. The most common types of Acid Gas are Carbon Dioxide (CO₂) and Hydrogen Sulphide (H₂S), though there are many other varieties as well, including Hydrogen Chloride (HCI), Hydrogen Fluoride (HF), Sulphur Oxides (SO₂ and SO₃) and Nitrogen Oxides (NO_x).^[1] The pH scale ranges from 0 to 14, anything above 7 is basic while anything below 7 is acidic. Water has a neutral pH of 7 so once a Gas is mixed with Water, if the resulting mixture has a pH of less than 7 that means it is an Acidic Gas.^[2]

In the Oil and Gas sector, Acid Gas is a particular typology of Natural Gas or any other Gas Mixture containing significant quantities of Hydrogen Sulfide (H₂S), Carbon Dioxide (CO₂), or similar Acidic Gases.^[2]

The term/s Acid Gas and Sour Gas are often incorrectly treated as synonyms. Strictly speaking, a Sour Gas is any Gas that specifically contains Hydrogen Sulfide in significant amounts; an Acid Gas is any Gas that contains significant amounts of Acidic Gases such as Carbon Dioxide (CO₂) or Hydrogen Sulfide (H₂S). Thus, Carbon Dioxide by itself is an Acid Gas but not a Sour Gas.

1. ENVIROTECH Online, 28 May 2019, What Is an Acid Gas?

2. Wikipedia, Acid Gas