Styphnate, barium, trihydrate

Barium styphnate (barium salt of 2,4,6-trinitrobenzene-l,3-diol BaS) forms an anhydride and two hydrates, monohydrate and trihydrate only the anhydride and the monohydrate are used in practical applications [20,75,76]. According to Bagal the monohydrate of barium styphnate exists in three polymorphic forms listed in Table 5.9 [33]. 

Barium styphnate can be prepared either as a monohydrate or a trihydrate and the kinetics of tiie dehydrations and the decompositions of both forms have been studied by Tompkins and Young [139]. The decomposition of the dehydrated monohydrate (542 to 581 K) exhibited a sigmoid curve which initially fitted the power law and the induction period was very short E = 153 kJ mol ). Reaction at a non-coherent interface proceeded along grain boundaries and into the sub-grains. 

Barium styphnate trihydrate forms yellow-orange needles. It is more soluble in water than the monohydrate. Trihydrate loses two crystal waters at 105 °C and red needles of monohydrate are formed [33]. The original work of Griess reports the trihydrate formed as rhombohedric prisms with low solubility in water [63]. 

Direct precipitation of BaS anhydride is not possible as one of three polymorphic forms of the monohydrate is produced depending on reaction conditions. The red form is created when freshly prepared styphnic acid is used for precipitation of barium styphnate. It is formed by direct reaction of equivalent amounts of TNR and barium carbonate. The yellow stable salt is formed when old TNR is used for BaS preparation, in an acidic environment or in excess precipitating agent. It is commonly prepared via the sodium salt of styphnic acid. The trihydrate of BaS forms in reaction mixture with low concentration of reagents and without stirring. The trihydrate also sometimes contaminates the monohydrate [33]. 

Preparation of barium styphnate monohydrate for practical applications is similar to preparation of LS. Magnesium styphnate is used as a starting material which reacts with barium nitrate, chloride, or acetate resulting in precipitation of barium styphnate. A reaction temperature of 60-85 °C is recommended. Product precipitates in form of the trihydrate, which dehydrates to the monohydrate by the slow addition of sufficiently dilute lutric acid [75]. It is difficult to re-crystallize in a suitable form and it is therefore recommended to use a crystal-modifying agent (e.g., carboxymethyl cellulose) [2, 26, 27]. 

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