The H atom of the COOH group may be replaced by metals, in exactly the same way as it is replaceable in inorganic acids, producing either neutral or acid salts, depending upon whether all or only a part of the acid H atoms are replaced by the basic element. Thus, with sulfuric acid: OH ONa / / SO_{2} (H_{2}SO_{4}) + NaOH = SO_{2} (NaH_{2}SO_{4}) + H_{2}O \ \ OH OH Sulfuric Acid Acid sodium sulfate OH ONa / / or, SO_{2} (H_{2}SO_{4}) + 2NaOH = SO_{2} (Na_{2}SO_{4}) + 2H_{2}O \ \ OH ONa Sulfuric Acid Neutral sodium sulfate Similarly, with oxalic acid: COOH COOK | (H_{2}C_{2}O_{4}) + KOH = | + H_{2}O COOH COOH Oxalic acid Acid potassium oxalate or, COOH COOK | (H_{2}C_{2}O_{4}) + 2KOH = | + 2H_{2}O COOH COOK Oxalic acid Neutral potassium oxalate Similarly, the acid H atom of either an organic or an inorganic acid may be replaced by the alkyl group of an alcohol, producing "ethereal salts," or "esters." Thus, with nitric acid; NO_{2}OH(HNO_{3})+C_{2}H_{5}OH = NO_{2}OC_{2}H_5(C_{2}H_{5}NO_{3})+H_{2}O Nitric acid Ethyl alcohol Ethyl nitrate And, with acetic acid; CH_{3}·COOH(H_{4}C_{2}O_{2})+C_{2}H_{5}OH = CH_{3}·COOC_{2}H_{5}+H_{2}O Acetic acid Ethyl acetate With dibasic or polybasic acids, either one or more of the carboxyl H atoms may be replaced with an alcohol radical, so that both acid and neutral esters of all such acids are possible. Examples of all of these different types of derivatives of organic acids are frequently found in plant tissues. The occurrence, properties, and functions of a particular type of glycerol, and other esters of organic acids, which are known as fats and waxes, are not taken into consideration in the following discussions, but reserved for a subsequent chapter dealing specially with them. SOME COMMON ORGANIC ACIDS Free organic acids, or their mineral salts or volatile esters, sometimes occur as separate and characteristic individual compounds in particular species of plants, or fruits; but much more commonly, two, three, or even more acids or their derivatives, are associated together.