104 PLANT PRODUCTS 



It is very easily oxidized in the laboratory, and becomes 

 oxidized in the soil by bacterial action. In the presence of 

 an excess of alkaline oxalates the heavy metals, like iron 

 and copper, produce double salts with the alkalies, which 

 are soluble. 



The homologues of oxalic acid are also important. A 

 member of the series, malonic acid, HOOC.CH 2 COOH, has 

 no great interest for present purposes, but succinic acid is 

 present in many plants, and is produced during fermentation, 

 whilst its oxidized products are met with in still larger 

 amounts. 



Malic Acid, or Hydroxy Succinic Acid, HOOC.CH 2 .- 

 CH(OH).COOH, occurs in apples, gooseberries, cider, and 

 many other fruit materials. 



Tartaric Acid, Dihydroxy Succinic Acid, HOOC.- 

 CH(OH).CH(OH).COOH, is found in considerable quantities 

 in grapes and wine. The deposits in wine casks, known as 

 argol, is one of the chief sources of tartaric acid. Argol, 

 purified by crystallization, is known as tartar, or cream of 

 tartar. The purified argol is treated with calcium carbonate 

 and calcium sulphate to obtain a precipitate of calcium 

 tartrate, which is subsequently decomposed by sulphuric 

 acid. The recovered calcium sulphate supplies all that is 

 necessary for the former part of the process. Tartaric 

 acid is non-volatile ; crystallizes well ; is easily decomposed 

 by heat, giving off a smell of burnt sugar ; forms soluble 

 salts with the alkalies ; insoluble salts with the alkaline 

 earths ; complex ions with iron and copper ; and produces 

 a potassium hydrogen tartrate which is insoluble in water, 

 though soluble with decomposition in either acids or alkalies. 

 Tartaric acid is used medicinally, for summer drinks, for 

 photography, for silvering mirrors, for bleaching, and for 

 dyeing. 



Citric Acid, HOOC.CH 2 .C(OH)(COOH).CH 2 .COOH+ 

 H 2 O, is a very common plant acid. L,emons can produce 

 as much as five or six per cent., and Dyer found that most 

 plant roots contained acids, largely citric acid, up to about 

 i per cent. I,emon juice is boiled with calcium carbonate 



