| Summary: | 碩士 === 國立臺灣大學 === 農業化學研究所 === 91 === The molybdenum-blue method has been widely used for the colorimetric determination of dissolved phosphate in laboratory. The single solution reagent proposed by Murphy and Riley（1962）consists of an acidified ammonium molybdate containing ascorbic acid and a small amount of antimony. Its basic theory is that orthophosphate react in an acid and reduced medium with molybdate to form intensely colored molybdenum-blue complex. However, molybdate could also react with organic acids, especially for which have carboxyl and hydroxyl groups, to form stable complexes. Thus there are severe interferences with the protocol of molybdenum blue method in the presence of organic acids, such like citrate, oxalate, and tartrate.
CBD（citrate-bicarbonate-dithionite） is a common extractant for P occluded within Fe（and Al）oxides and hydrous oxides. Ammonium oxalate extractable P represents a potentially useful measure of the P associated with the amorphous Fe and Al oxides in soils. In this study, a method was developed whereby excessive molybdate was added to those extracts to eliminate the interferences of citrate and oxalate with the phosphomolybdate complexes necessary for color development. While those extracts can be digested and the organic acids destroyed to allow for P determination. The development and recommended procedures of excessive molybdate addition method and digestion method for colorimetrically measuring P in CBD and ammonium oxalate soil extracts are described in detail.
Although the ascorbic acid procedure of Murphy and Riley (1962) is the most versatile method for soil test P determinations, its dependability for the various soil extract analysis should be carefully investigated. The second objective of this study consequently is to demonstrate the possible errors caused by interfering ions most commonly encountered in soil test. Olsen, Bray P1, Mehlich Ⅰ, Mehlich Ⅲ, AB-DTPA, Morgan and Morgan-Wolf soil extraction were studied. The chelate DTPA in AB-DTPA soil extract was found to interfere the formation of phosphomolybdate blue complex. Ammonium nitrate in Mehlich Ⅲ soil extract causes fading of the blue color within one hour. The development of the molybdenum blue is delayed in the presence of fluoride, and adding a small amount of boric acid can easily reduce these interferences. In addition, the brown color of soil extract itself could probably make some mild errors in determining P by the Molybdenum blue method as well.
|
|---|
