Boron concentrations

Two colorimetric methods are recommended for boron analysis. One is the curcumin method, where the sample is acidified and evaporated after addition of curcumin reagent. A red product called rosocyanine remains it is dissolved in 95 wt % ethanol and measured photometrically. Nitrate concentrations >20 mg/L interfere with this method. Another colorimetric method is based upon the reaction between boron and carminic acid in concentrated sulfuric acid to form a bluish-red or blue product. Boron concentrations can also be deterrnined by atomic absorption spectroscopy with a nitrous oxide—acetjiene flame or graphite furnace. Atomic emission with an argon plasma source can also be used for boron measurement. 

Calculate the amount of boron present by reference to a calibration graph of absorbance against boron concentration (mg L 1). Multiply the result obtained by the appropriate volume correction factor arising from neutralisation of the sample. 

Calibration. Take 5, 10, 25, 50, 75 and lOOmL of the standard boric acid solution (2.5 x 10 4M) and make each up to lOOmL with distilled water this yields a boron concentration range up to 2.70mgL 1. Continue with each solution as described under procedure (b), i.e. one-hour reaction time, except that the initial neutralisation of the boron solution to pH 5.5 is not necessary. Construct a calibration graph of absorbance at 516 nm against boron concentration, mg L 1. For maximum accuracy, the calibration should be carried out immediately prior to the analysis of samples. 

Boron concentrations and isotopes are also useful geochemical tracers of contamination in MORB. Boron concentrations are low (<2 ppm) in unaltered ocean floor basalt but high in altered basalts (>8 ppm B) (Spivack and Edmond 1987 Ryan and Langmuir 1993). Goldstein et al. (1989) measured B concentrations in their samples and found them to be less than 1.6 ppm, inconsistent with contamination. More recently, B isotopes have been used to assess contamination since large differences in 5 B are known to exist between seawater, sediments, and unaltered MORB. Sims et al. (2002) reported that 6 B for their 9°N EPR samples were inconsistent with incorporation of any seawater or seawater-derived material. 

Figure 14.8 Effect of boron concentration on the hardnesses of FeB glasses.

For the purpose of ultrashallow sub-tenth-micron boron doping, energies of a few KeV may be used. The distribution of boron concentration decreases very rapidly with depth, typical values being -1020 to 1022 atoms.cm-3 at the surface falling almost exponentially to -1013 to 1014 atoms.cnr3 at a depth of 20 nm. If... 

Figure 2 shows that samples with boron concentrations ranging over four orders of magnitude can be effectively treated. The samples were hydrogenated at 122°C the upper three samples for one hour, and the most heavily doped sample was hydrogenated for four hours. Note that the spreading resistance increases toward the surface. The value of resistivity... 

Fig. 2. Spreading resistance profile of four B-doped samples of (100) Si hydrogenated at 122°C. The three higher resistivity samples were hydrogenated for one hour the lowest resistivity sample was treated for four hours. The resistivities were obtained from a calibration curve. Note the greater penetration depth of atomic hydrogen as the boron concentration decreases.

Fig. 8. SIMS profiles of 2H and nB in plasma-passivated B-implanted and annealed samples used in channeling studies of B—H complexes by Marwick et al. (1988). 1000 angstroms was etched off the surface of this sample to eliminate a layer containing a large excess of hydrogen. Nevertheless, some excess over the boron concentration remains at shallow depths. The histogram shows the deuterium profile used to analyze the data using calculated flux profiles.

Bech Nielsen et al. (1988) also made a channeling study of the 2H in B-2H complexes that was similar in principle to that just described but differed in details of technique and in some of the conclusions arrived at. Also, they did not investigate the position of the boron atoms. They used Si uniformly doped with a high (1 x 1019 B/cm3) boron concentration, rather than implanted boron. The surface was etched off after hydrogenation, but no SIMS data was presented to confirm the uniform hydrogen concentration assumed. The penetration depth of the H was given as —7000 A. The channeling measurements were performed at 30 K. For analysis of their data Bech Nielsen et al. used the same model, based on the assumption of statistical equilibrium (SE) of the channeled ions, already described in connection with the measurements of implanted deutrium... 

Fig. 8. Effect of an oxide film on the subsurface hydrogen concentration produced by a given exposure to plasma gases. The two curves refer to deuteration for 1 hr at 150°C of samples with a boron concentration of 1017 cm-3, one with and one without an oxide layer. 

Fig. 30. SIMS plots of total deuterium density for silicon specimens with various boron concentrations, deuterated by plasma gases at 300°C. Full curves are the most recent measurements (Johnson, 1989), with deuteration time of one hour in all cases. Dashed curves are typical older data (Johnson, 1987), for which the plasma parameters may have slightly different the deuteration time for each of these was two hours. Each curve is labeled with its boron concentration in atoms/cm3. All sample surfaces were prepared in the same manner as those of Fig. 29.

Boron concentrations in field collections of selected species of plants and animals... 

Boron concentrations in soil water associated with optimal growth and plant injury... 

An interesting example of the contamination risks which may be caused by a laboratory vessel is that of boron. Determination of very low boron concentrations, involves a prior separation by distillation and subsequent analysis by spectrometry, with a suitable reagent such a curcumin or carminic acid. The use of laboratory vessels made of borosilicate glass (such as Duran or Pyrex) could lead to very large errors in the boron content found. Such errors are caused by sample contamination from the boron present in the glassware. 

The amount of boron required for BNCT can be estimated using the neutron capture cross sections, which are atomic properties, and thus pertain to the number, and not the mass, of the atoms present. Conservative estimates for successful therapy result in boron concentrations of around 20 ppm in tumor tissue, to at least match the dose liberated by neutron capture reactions in the other elements of biological tissue. This would correspond to around 109 boron-10 atoms per cell, assuming that one cell corresponds to 10-9 g. 

The first studies of Li isotopes in subduction zones concentrated on young convergent margin lavas. Moriguti and Nakamura (1998b) reported correlated Li isotope and fluid-mobile element (notably boron) concentration variations in the Izu arc, southeastern Japan (8 Li = +1.1 to +7.6), consistent with significant incorporation of Li from altered oceanic crust into arc lava sources (Fig. 6). A similar trend has been reported in samples of basalts and basaltic andesites from Mt. Shasta, California (5 Li = +2.5 to +6.5 Magna et al. 2003). 

Since boron concentrations in mantle minerals are exceedingly low, boron isotope analysis of mantle minerals are very restricted. On the basis of a boron budget between mantle and crust, Chaussidon and Marty (1995) conclnded that the primitive mantle had a 5 B-value of-10 2%c.ForMORB Spivack and Edmond (1987) and Chaussidon and Marty (1995) reported a 5 B-value of aronnd -4%c. Higher and lower 5 B-values observed in some ocean island basalts shonld be due to crustal assimilation (Tanaka and Nakamura 2005). 

Also, boron can be analyzed by colorimetry techniques (APHA, AWWA, and WEF. 1999. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington DC American Public Health Association). Boron in acidified aqueous phase reacts with curcumin to form a red-colored product, roso-cyanine. Presence of calcium and magnesium at high concentrations can interfere in the test. Another colorimetric measurement involves the reaction of boron with carmine or carminic acid in concentrated sulfuric acid. The color of the solution changes from bright red to bluish red or blue, depending on boron concentration. 

The most important polyborate species observed in solution are the triborate anions [B303(0H)4] and [B303(0H)5] , the pentaborate anion [B506(0H)4] , and the tetraborate anion [B405(0H)4] . The population distributions of these species found at 25 °C in 0.4 molar boric acid equivalent solution as a function of pH is shown in Fig. 2 [8]. Concentrations of these species may vary considerably with temperature and overall boron concentration, and other minor polyborate species not represented are also likely to exist in solution. Fig. 2 should be regarded as a snapshot of a complex system taken under one set of conditions. 

Table 29.2 Boron Concentrations in Selected Nonbiological Materials (Concentrations are in mg B/kg fresh weight [FW], dry weight [DW], or ash weight [AW], except where noted.)... 

Terrestrial plants, especially nuts and some fruits and vegetables, are rich sources of boron (Table 29.3). Honey is another good source of boron, and concentrations up to 7.2 mg/kg dry weight have been reported (Nielsen 1986). Boron concentrations are also elevated in marine plants, zooplankton, and corals, but are low in hsh and certain marine invertebrates (Table 29.3). No data were found on boron levels in terrestrial mammalian wildhfe. The average daily intake of boron in humans ranges between 1 and 25 mg however, populations residing in areas of the western... 

---