Mannitol-boric acid method

The method is based on the tendency of organic polyhydroxy-compounds with two or more hydroxyl groups in the cis-position to form complexes with boric add Gripenberg, 1966). When about 5g of mannitol or another hexose is added to lOOmL of neutralized (pH 7) seawater, the acidity increases to a pH near 3-4. The amount of alkali used to bring the sample back to neutrality is then equivalent to the boric add content. 

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). 

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For determination of the total boron content, the spectrophotometric curcumin method is described, which was first applied to seawater by Greenhalgh and Riley (1962) but was modified and simplified by Uppstrdm (1968). An outline of the mannitol-boric acid method is presented in Section 11.2.7.7. 

The method may be applied to commercial boric acid, but as this material may contain ammonium salts it is necessary to add a slight excess of sodium carbonate solution and then to boil down to half-bulk to expel ammonia. Any precipitate which separates is filtered off and washed thoroughly, then the filtrate is neutralised to methyl red, and after boiling, mannitol is added, and the solution titrated with standard 0.1M sodium hydroxide solution ... 

The complexes with boric acid and borates have been of more interest. The ability of D-mannitol to render boric acid more acidic is well known and forms the basis of an anal3rtical method allowing boric acid to be titrated as a monobasic acid. The boric acid complexes have been... 

Exactly the same methods can be used when equilibrium reactions for combination with the solvent or with added substances are involved. In this way it can be shown that when an added substance combines more completely with the ion than the acid, it increases the observed acid strength. Thus glycerol or mannitol combines more completely with B(0H)20 than with B(OH)3, and so increase the apparent acidity of boric acid. Whether dealing with a group... 

The effect of mannitol on the acidity of boric acid is sufficiently great that the latter behaves like a strong monobasic acid and can be titrated directly this observation is the basis of common methods for the determination of boric acid. 

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