Potassium tetraphenylborate, solubility product

I. Sodium tetraphenylborate Na+ [B(C6H5)4] . This is a useful reagent for potassium the solubility product of the potassium salt is 2.25 x 10 8. Precipitation is usually effected at pH 2 or at pH 6.5 in the presence of EDTA. Rubidium and caesium interfere ammonium ion forms a slightly soluble salt and can be removed by ignition mercury(II) interferes in acid solution but does not do so at pH 6.5 in the presence of EDTA. 

Sodium tetraphenylborate, Na B(C6H5)4, is an important ionic predpitant for potassium ion. The solubility product of the potassium salt is 2.25 x 10 . Introduced by Raff and Brotz in 1951, this reagent has many analytical uses. A review of the various applications has been given by Cluley, who also described procedures for the separation and determination of potassium in silicates. In add solution at pH 2 or pH 6.5 in the presence of EDTA, the separation is almost specific... 

Potassium, rubidium and caesium ions together with tetraphenylborate ions form a precipitate which does not dissolve readily in water. According to W. Geilmann and W. Gebauhr the solubility products are ... 

For concentration, the rubidium and caesium ions are precipitated in the form of tetraphenylborates together with potassium ions, which serve as carriers. The lower solubility of the rubidium and caesium tetraphenylborates (see the solubility products) compared with the potassium salt means that under the selected precipitation conditions the rubidium and caesium yields are virtually quantitative. 

The solubility of the precipitate is lowest between pH 4 and 6. Its solubility product is around 2.3-10 (at 20 °C). If a minor excess of the precipitating agent (Le., sodium tetraphenylborate) is available in solution, the solubility of the precipitate is negligible. Co-precipitation of calcium and magnesium ions may lead to serious errors when they are first precipitated. This interference is minimized, however, as carbonates which, in contrast to potassium tetraphenylborate, are soluble in acetic acid. Potassium tetraphenylborate is crystalline and starts to decompose when heated above 100 °C. Other cations which form stable precipitates with tetraphenylborate under the conditions applied are rubidium, cesium, ammonium, mercury, thallium(/) and silver (see Section 11.2.3.6). In natural seawaters, however, the concentrations of these constituents are so low as to be negligible. 

Carbanions can also function as electron-donors to organoboron compounds the products are complex ions. Thus reaction of trimethylboron with methyl-lithium in ethereal solution affords lithium tetramethylborate. The aromatic analog, sodium tetraphenylborate Na[B(C6H5)4], is an important reagent for gravimetric determination of potassium and a number of other ions, because lithium and sodium tetraphenylborate are soluble in water whereas the potassium, rubidium, cesium, and ammonium salts are almost completely insoluble therein. 

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