Boron potentiometric titration

The main group duster chemistry discussed in this book can be considered to originate from two important, but apparently unrelated developments in inorganic chemistry in the 1930s. The first was the identification of the neutral boron hydrides by Stock [1]. The second was the observation by Zintl and co-workers [2-5] of anionic clusters formed from potentiometric titrations of post-transition metals (i.e., heavy main group elements) with sodium in liquid ammonia. 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

The determination of boron in silicon may be achieved by separating the B by fusion with Na202-Na2C03, followed by conversion of the B into boric acid, which may be determined by potentiometric titration.2... 

We have performed a study of acidic properties of boron (III) and vanadium (V) oxo-compounds in molten Nal at 700 C (see Table 10.4.1). A comparative study of strength of Lux bases OH", COj, S04 was determined by potentiometric titration using sodium pyrophosphate as acid. Two moles of the first base may be neutralized by 1 mole of P407, while two other moles react with the acid in ratio 1 1. On the base of e.m.f. drop magnitude at the equivalence point, the bases have been arranged in sequence 0H >C03 >S04 of basicity decrease. The equilibrium constants were not estimated. ... 

With boron-free glassware and potentiometric titration, a c.v. of about 1 % may be obtained. A detailed procedure, but more time-consuming than the cuicumin method, has been presented by Noakes and Hood (1961). 

From these experiments, Lorand and Edwards concluded that the conjugate base of phenylboronic acid has a tetrahedral, rather than trigonal structure. The dissociation of a hydrogen ion from phenylboronic acid occurs from the interaction of the boron atom with a molecule of water. As the phenylboronic acid and water react, a hydrated proton is liberated, thereby defining the acidity constant K, see Hartley, Phillips and James. This is depicted in heme 5 by considering an explicit water molecule associated with the Lewis acidic boron. The reported pK s of phenylboronic acid fluctuate between 8.7 and 8.9, with a recent in-depth potentiometric titration study refining this value to 8.70 in water at 25°C. ... 

While speculative, this structural interpretation of the interaction between boronic acid and the proximal tertiary amine through a bound protic solvent molecule (solvent insertion into the N-B bond) corresponds well with contemporary computational and potentiometric titration data, in which the formation of intramolecular seven-membered rings should not be ignored. 

The values for the bond length (from the X-ray crystal structure) and for the bond strength (from the potentiometric titrations) are those that would be expected for a hydrogen-bonding interaction manifested through a bound solvent molecule at the boron centre. 

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