Determination of Extracted Boron

Several techniques are available to determine boron in soil extracts. Titrimetric, fluorometric, and bioassay methods were used earlier but are not commonly used now. In general, they are timeconsuming, and some interferences are encountered. Colorimetric and spectrometric methods, which are more common, reliable, and accurate, will be discussed here.

Colorimetric Methods

Colorimetric methods for B determination are relatively inexpensive to perform and are somewhat free of interferences. The turmeric test (170,171) showed some promise earlier when it was discovered that dilute solutions of boric acid will change the color of turmeric paper from yellow to red. The procedure however, was long and required the precise control of temperature-regulated water baths. Berger and Truog (152) reported that the use of the turmeric paper test led to great difficulty because of its insensitivity due to its inability to differentiate between small amounts of boron.

The quinalizarine method is less laborious and more expeditious, whereas the curcumin method has the advantage of using easily prepared and easier to handle reagents (172). According to Berger (173), the mixing of 98% sulfuric acid-quinalizarin solution with the unknown solution generates a considerable amount of heat, and it was found that the higher the temperature, the redder is the color of the test solution.

It was suggested that the solution be cooled to room temperature regardless of the temperature reached when the solutions were mixed. So it was possible and convenient to read unknown solutions in a colorimeter at a uniform temperature.

Porter et al. (174) saw the introduction of azomethine-H method as an answer to the handling difficulty involved in working with sulfuric acid for the carmine method. They added that the problem of having to concentrate boron in the solution of low boron concentration was also avoided. They concluded that an automated scheme improved the azomethine-H reagent method by overcoming the effect of sample color by dialysis.

Wolf (175) concluded that the results of boron determination using the azomethine-H method were in agreement with those of the curcumin method, and probably more reliable for soils high in nitrate. Also, the azomethine-H results (values) for plant boron agreed more closely with spectrographic analysis than the curcumin. Gestring and Soltanpour (176) found that the azomethine-H colorimetric method and inductively coupled plasma-atomic emission spectrophotometer (ICP) analysis were highly correlated. Both methods of analysis gave boron values comparable to National Bureau of Standards (NBS) values for dry-ashed plant samples; however, wet digestion using concentrated nitric acid resulted in interferences for the azomethine-H method but not for the ICP analysis.

Spectrometric Methods

The suitability (177) of the ICP spectrometer system for analysis of complex matrices was demonstrated by the high analytical precision and reproducibility of boron in alfalfa and in white bean (Phaseolus coccineus cv. Albus) (NBS samples). There was no interference from soluble organics observed in the complex soil solution matrices examined, although their presence would confound any colorimetric technique. It was possible to quantify boron in soil solutions to levels of 5 to 15 ng mL^-1, with extended integration periods utilizing the 249.773 nm emission line.

Parker and Gardner (178) employed ICP emission spectroscopic analysis of boron in distilled water and 0.02 M CaCl₂ solution, and indicated that the extractable boron level was not affected by the presence of CaCl₂. According to John et al. (179) the ICP method has the following advantages over the present colorimetric techniques: (a) carbon black is not needed since the color of the solution does not affect the analysis; (b) nitric acid digestion of samples can be utilized since ICP is not affected by the presence of nitrates; (c) other elements can be determined simultaneously; and (d) analysis by ICP is simple and rapid.

The use of Mehlich 3 extractant has been found to be simple, rapid, and practical in determining the availability of boron and a number of other nutrients in soils (180) with the ICP spectrophotometer. Using the ICP method, the Mehlich 3-extracted boron is well correlated with hot-water-soluble boron. The clear filtered extract (after shaking soil, Mehlich 3 reagent in 1:10 ratio for 5 min at 80 oscillations/min) is transferred into ICP tubes and analyzed by ICP at 249.678 nm (181). The ICP atomic emission spectrometry has also been used successfully in the determination of total soil B (182).