Determination of low boron concentrations in nutrient solution
To study boron (B) deficiency and toxicity in plants in flowing solution culture, it is necessary to establish a wide range of solution B concentrations. The ability of inductively coupled plasma atomic emisson spectrophotometry (ICPAES) to determine solution B concentrations ranging from 0.15 μM to 925 μM was investigated. The reliablity of B concentration determination in nutrient solutions containing <10 μM B is poor. A technique, involving sorption by a B-specific resin (Amberlite-743), was established to concentrate the B present in low B nutrient solutions and to enable reliable measurement using ICPAES following elution from the resin. Acceptable, reproducible recoveries of B from low B solutions, containing known concentrations of B, were obtained using this technique. The technique enables the imposition, monitoring and maintenance of solution B concentrations well below the direct detection limits of ICPAES and colorimetric procedures. ei]L V Kochian
Soils as boron source for plants
Boron (B) is one essential micronutrient required by plants for their normal development and growth. However, throughout the history of this element research, it has been established that concentration ranges in the soil solution causing either deficiency or toxicity symptoms in plants are smaller than for any other element; in addition to this situation, those ranges change according to the plant specie; that is, a concentration interval of B can be normal for an specific type of plant, while for others can be either toxic or deficient, which has had a profound influence for a better knowledge of B behavior in soils. Soil factors affecting availability of B to plants are pH, texture, moisture, temperature, content and quality of organic matter, and content and kind of clay. The B concentration of soil solution is generally controlled by reactions with B adsorbing surfaces involving aluminium and iron oxides, magnesium hydroxide, clay minerals, calcium carbonate, and organic matter. These B adsorption reactions can be described using either empirical models (the Langmuir adsorption isotherm equation, the Freundlich adsorption isotherm equation, and the phenomenological Keren model) or chemical models (the constant capacitance model, the triple layer model, and the quadruple layer model) that can use changing conditions of solution pH and B concentration. These models can also be used to describe B desorption reactions for non-hysteretic soils. A number of analytical techniques have been used for determining B concentrations in different kind of materials being the spectrophotometric methods, that use azomethine-H, the most extensively applied to analysis of soil samples. The purpose of this article is to examine all advances that have contributed to understand the fundamental behavior of B in soils and the direct impact on its management in agricultural systems for the sustainable production.
MAS-NMR spectroscopy studies in the setting reaction of glass ionomer cements.
OBJECTIVES
The main objective is the characterisation of the setting reaction in glass ionomer cements based on experimental ionomer glasses with different fluorine content and a commercial glass ionomer cement liquid by using 13C CP/MAS-NMR, 29Si, 27Al and 31P MAS-NMR spectroscopy in order to receive information specifically about the cross-linking process.


METHODS
Different fluorine containing glass compositions based on 4.5SiO2-3Al2O3-1.5P2O5-(5-z)CaO-zCaF(2) where z=0-3, were mixed with a commercially available polymer liquid to form glass ionomer cements. The cements were subjected to 27Al, 13C CP/MAS, 29Si, and 31P MAS-NMR analysis.


RESULTS
The 27Al spectra showed clearly the formation of six-fold coordinate Al(VI), that may crosslink the carboxyl groups in the poly-acid molecules. A shift towards to more positive values of the carboxyl peak in the 13C CP/MAS-NMR spectra showed clearly the proton dissociation of the carboxyl groups. A shift towards more negative values was observed in the 29Si MAS-NMR spectra, suggesting formation of hydrated silica gel and consequently formation of additional Si-O-Si bonds. 31P MAS-NMR spectra also reflected changes in the coordination state around a PO4(3-) tetrahedron. Increasing the fluorine content of the glasses resulted generally in increased reactivity during setting, due to promoting cross-linking and repolymerisation of the silicate phase, followed by clear changes in the MAS-NMR spectra.


CONCLUSIONS
The cross-linking process during the setting reaction of glass ionomer cements can be followed by MAS-NMR spectroscopy observing the conversion of Al(IV) to Al(VI). The acid base setting reaction is completed in 1 day and no further significant changes in the MAS-NMR spectra can be observed. Further study is required in order to understand the role of phosphorus.