Potassium tetraphenylborate, solubility

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. 

The water-soluble material was filtered through a 1.4- xm frit. An aliquot of the solution was treated with excess sodium tetraphenylborate. The potassium tetraphenylborate that precipitated was collected and dried. This analysis indicates that 18.7 meq of potassium ion was formed in the reaction. 

Dilituric acid (5-nitrobarbituric acid) forms a sparingly soluble compound with potassium ions. The decrease in absorbance of the reagent gives a measure of the K content in the solution. Potassium ions can be precipitated by tetraphenylborate. The excess reagent forms an ion-pair with the cationic Cu(I)-neocuproine complex, and this ion-pair is extractable into methyl acetate. The absorbance of the extract is measured at 456 nm [83]. An indirect method involving the reaction of potassium tetraphenylborate with mercury(II) ehloranilate is very sensitive [84]. 

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. 

Ammonium may be determined by predpitation with sodium tetraphenylborate as the sparingly soluble ammonium tetraphenylborate NH4[B(C6H5)4], using a similar procedure to that described for potassium it is dried at 100°C, For further details of the reagent, including interferences, notably potassium, rubidium, and caesium, see Section 11.38,... 

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... 

The nickel complex, Ni ( 5115)2, has been made by the action of the Grignard reagent on nickel (II) acetylacetonate (217) or from potassium cyclopentadienyl and the ammine [Ni(NHs)el (S N)2 in liquid ammonia (58). It forms dark emerald-green crystals which sublime at 80-90° and which, when heated in nitrogen, melt, with decomposition and the formation of a nickel mirror, at 173-174°. It is only slowly oxidized in air, and cold water neither attacks nor dissolves it it is, however, readily soluble in organic liquids. Oxidation of the compound yields an orange-yellow solution containing the ion [Ni( 5H5)2]+, which is stable for a short period in weakly acidic media, and which may be precipitated as the reineckate or tetraphenylborate. 

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. 

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. 

It may prove advantageous to perform concentration by coprecipitation with potassium as a sparingly soluble tetraphenylborate compound. 

The gravimetric determination of potassium as its barely soluble tetraphenylborate is recommended and described here. This method has also been applied to seawater for an ocean-wide survey of the element (see Table 11-1). The procedure is preferred because of its high accuracy and precision and the minimum amount of equipment required to perform the analysis. Basically, the method outlined here is that developed by Wittig (1950) and Raff and Brotz (1951). A brief description of a potentiometric method is presented in Section 11.2.3.6. 

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