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13 <br />PART ONE <br />Production and effects <br />Hydrogen sulphide, H2S, is a gas that is widespread in nature, and well-known because of its <br />odour. It can arise from the decay of some kinds of organic matter, especially albumins. An <br />example is the white of an egg, an albumin that can release large amounts of H2S. The odour <br />of H2S is most commonly described as the odour of rotten eggs. <br />H2S also occurs in many ground waters. Its presence in such waters is due not so much to <br />breakdown of organic matter as to the bacterial reduction of sulfate. By this it is meant that <br />certain bacteria are able to split oxygen from the sulfate ion, SO4-, a common constituent of <br />natural waters, and use it to oxidize organic matter. The sulfur is then left in the form of the <br />sulfide ion, S -, which immediately changes by reaction with water to a mixture of H2S and <br />HS - (read H S ion). <br />Physical-chemical properties of hydrogen sulfide <br />H2S is a gas slightly heavier than air. It condenses to a liquid only at the low temperature of <br />- 62° C. It is fairly soluble in water. At 20° C, it can dissolve in pure water to the extent of <br />3850 milligrammes per litre (mg/1), or 2.7 litres of H2S gas per litre of water. The solubility <br />decreases about 2.5 % for each degree increase of temperature. The stated solubility is the <br />amount that will dissolve when the pure gas is brought into contact with pure water. From H2S <br />diluted with air, it will dissolve only in proportion to its concentration in the gas mixture. Thus, <br />for example, air in which the concentration of H2S is 0.1 % (1000 parts per million, ppm) by <br />volume of H2S will, if brought to equilibrium with pure water at 20° C, produce a solution <br />containing 3.85 mg/l. Stated differently, water containing 3.85 mg/I of H2S can produce a <br />concentration of 0.1 %, or 1000 ppm, in air brought into contact with it. One mg/I in solution <br />can produce a concentration of about 260 ppm by volume in the air if the temperature is 20° C, <br />or 330ppm by volume if the temperature is 30° C. <br />When dissolved in water, hydrogen sulfide is partially ionized, so that it exists as a mixture of <br />H2S and HS -. The proportions'depend principally upon the pH of the solution. In a typical <br />natural water at a temperature of 20° C, and at pH 7.0, it is just 50 % ionized; that is, half of it <br />is present as HS " and half as un-ionized H2S. ble f shows the proportions ionized at other <br />pH levels. Temperature and mineral content of the water affect the degree of ionization, but . <br />only, by a, small amount. =The sulfide ion, S -, also exists in water, but not in appreciable <br />amounts except in solutions in which the pH is above 12. The solubility data given in the <br />previous paragraph applies only to the equilibrium between the gas and the slightly acidic <br />(low-pH) solution produced when it dissolves in pure water, or between the gas and the un- <br />ionized H2S in waters where the pH is not low.