Boron Determination

Nov 13, 2021 | SCIENCE

Boron Determination – Analytical Methods 

The element Boron is widely used for various industrial applications. It is critical to understand multiple Boron sampling, decomposition, and determination methods in the current times. 

Boron is a vital element for plants, animals, and varied industrial applications. It is present in lower concentrations in the animal tissues, about 1 mg B/L. However, it is an essential micronutrient for humans. Boron deficiency in plants leads to decreased growth, loss in yield, sometimes death, and it depends on the severity of the deficit. It is crucial to note that excess Boron is also toxic to living beings.

Concerning industrial applications, Boron acts as an essential element in steel, glass, borosilicate glass films, among others. It commonly takes the role of a thermalizing agent in nuclear reaction materials. You can also find it as a dopant for silicon in the semiconductor manufacturing industries

Introducing Boric acid

Why measure Boron?

Even a tiny change in the Boron content or concentration can impact the semiconductor-grade silicon properties. It also affects other properties such as hardenability, hot workability, and the creep resistance of steel and alloys. Boron also has vital applications in cancer treatments through Boron Neutron Capture Therapy (BNCT). There is a high requirement for precision while determining the Boron quantity to evaluate the tumor specificity and pharmacokinetics for the BNCT process. 

In other situations, we need to determine the Boron measure in microscopic samples, for example, biopsy-needle samples. The accuracy is crucial while measuring Boron in various applications. Therefore, the different processes of sample preparation, determination, purification of boron concentration, and isotopic composition are critical.  

Sample Preparation

The hot water extraction method is the popular one for determining the index of Boron in soil. But the amount of Boron extraction through this method depends on the extraction time and temperature. Additionally, the high chances of resorption of Boron during the cooling period also affect the process. To summarize, the hot water extraction procedure is difficult to standardize and is very time-consuming. It can also be tedious for regular use in case we need reproducible results.

Many reports recommend boron extraction of soils using dilute CaCl2 solutions. This process can remove the issues associated with hot water extraction. However, it is essential to remember that cold-dilute CaCl2 can extract a lesser Boron quantity than the hot water method. Studies that used 31 US soils and 100 Dutch soils confirm such findings. But they also showcased that the values of cold water extraction are correlated to the hot extraction procedure.  

We also have different methods like azomethine- H spectrophotometry and plasma optical emission spectrometry. Here, the problem lies with the Iron (Fe) extracted with the help of acid extractants. It interferes with the accurate determination of Boron values. Some alternative methods like the extraction using sorbitol in B-sorbitolchelate or microwave heating with BaCl2 or water. These offer similar results to the ones we obtain from the usual hot-water extraction method or the spectrophotometric determinations.

Decomposition of Boron

Dry ashing, wet ashing, and microwave dissolution are the standard methods used to decompose biological samples. Apart from these, we have other procedures for specific applications. It is possible to determine Boron concentration for interlaboratory applications using timber samples extraction with H2SO4 or HCl.

Dry ashing

Dry ashing is a procedure that is performed in a muffle furnace. You can use a suitable container such as quartz or platinum crucibles similarly to the methods described by Gaines and Mitchell for the spectroscopic azomethine-H method. It is also quite similar to inductively coupled plasma spectrometry by Brown et al. Furthermore, for determining Boron quantity, the samples from dry ashing are dissolved in dilute HNO3.

Wet ashing

Wet ashing uses digestion containers to place the samples and strong mineral acids such as HNO3, H2SO4, HClO4, or mixtures. The container is heated with a reflux system to decompose the sample. It is vital to use digestion containers devoid of Boron like the borosilicate glass because; it has higher chances of contaminating the sample and greater blank values. HNO3 is the preferable candidate for wet ashing. They are helpful for plasma source methods because they offer a much simpler matrix than the other mineral acids. If chemicals like HF or HCLO4 are used in wet ashing, the digest is evaporated to determine by drying it and redissolving it in HNO3. 

Microwave dissolution

Microwave dissolution is yet another procedure that many analytical methods use for Boron determination. These processes use high-pressure microwave dissolution of different biological materials in closed Teflon vessels. Various techniques like spectrophotometric, ICP-OES, and ICP-MS make use of this method. You can also use the same microwave dissolution method for decomposing non-biological materials like steel samples with the help of aqua regia. However, it would be best if you used HCl/HNO3/HF for coal ash samples.

It is faster than the other methods, and it uses a smaller quantity of acid for determination purposes. Additionally, high-pressure microwave digestion also prevents sample volatilization and cross-contamination. Overall, it results in low blank values.

The digests of the biological samples contain a more significant amount of dissolved carbon and other organic compounds. Hence, it can create problems in specific methods of Boron determination. However, we can conclude that the ICP-OES and the MS methods are the best for microwave digestion of biological materials. And the azomethine-H procedure was satisfactory only for samples that contained higher levels of Boron.

Boron Determination Methods

Before moving on to the Boron Determination Methods, following separation, pre-concentration, and solvent extraction is essential. There are different types of solvent extraction, such as exchange separation, chromatographic separation, and gas-phase separation.

Spectrophotometric Methods

Colorimetric methods are a type of spectrometric method which makes use of particular reagents for color development. It helps in the Boron determination process. Additionally, there are also flow injection methods to introduce the samples in the various spectrophotometric methods.

Boron has the property to form fluorescent compounds when it reacts with different reagents in concentrated acid and even under milder conditions. The Fluorimetric methods use chromotropic acid as a fluorimetric reagent for Boron determination. However, it suffers from different problems and interference during the process.

Ionometric Methods

Boron is generally separated from the sample matrix for the potentiometric determination process. It is then treated with HF, and the resulting BF4- is potentiometrically measured with a selected electrode. Some Ionometric methods don’t require the separation process. However, they are highly influenced by the sample matrix, which can shift the potential.

Atomic Spectrometric Methods

The Atomic Emission Spectrometry (AES) and Atomic Absorption Spectrometry (AAS) introduce the samples into a flame of acetylene or something similar. Here, this process atomizes the elements of the sample. The latter method uses the concept of free atoms to absorb photons of discrete energy values from a cathode lamp that holds the element, here, Boron.

Plasma Source Methods

In method uses plasmas as ionization sources. The growth of plasma sources and analytical instruments offers greater sensitivity and lowers the detection capability for Boron determination. It is better than the spectrophotometric, the AES/AAA, or the nuclear methods that are time-consuming.  

Conclusion

Apart from the aforementioned methods, we have other methods like Nuclear Reaction Analytical methods for Boron determination. We can see a significant evolution of Boron determination methods over the years from an analytical instrumentation perspective. Currently, the plasma source method is commonly used based on the available technologies.

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