Quantification of low concentrations of sodium borohydride in solution

Sodium borohydride (NaBH₄) has been identified as a potential solid-state hydrogen storage material for energy export due to its high gravimetric hydrogen content of ≈ 10.8 wt%. There is a strong need to accurately detect and quantify NaBH₄ in solution during synthesis, hydrolysis, and extraction processes, particularly at low concentrations (≤50 mM). Several studies report the formation of low concentrations of NaBH₄ in aqueous systems, however other researchers were unable to replicate these results. Unfortunately, some studies use indirect methods to confirm the presence of NaBH₄. This is the first study to evaluate all major techniques in-house for sub-millimolar sodium borohydride quantification, assessing impurity effects and emphasizing the need for multiple analytical methods, including direct detection such as nuclear magnetic resonance (NMR). We investigate various analytical techniques to determine the limits of detection for NaBH₄ in aqueous solutions and assess the advantages and disadvantages of each method. Titration and gas evolution have traditionally been used, but confirmation with complimentary direct techniques can prevent misidentification. Here Fourier transform infrared (FTIR), Raman and NMR spectroscopies were utilized, detecting concentrations as low as 256, 50 and 0.21 mM NaBH₄, respectively. Furthermore, electrochemical sensing showed detection limits of 0.42 mM NaBH₄ for cyclic voltammetry (CV) and 0.18 mM NaBH₄ for square wave voltammetry (SWV).