AOAC Official Method 942.09 Monochloroacetic Acid in Liquids and Preservatives
Qualitative TestsFirst Action 1942
Final Action
A. Optical-Crystallographic Properties of Barium Salt (Applicable to commercial preservatives.)Dilute 4–5 mL test portion to 100 mL, add 6 mL H2SO4 (1 + 1),and extract with equal volume ether in separator. If emulsions form,extract in continuous extractor 1 h. Transfer ether extract to separator,add few drops phenolphthalein and 5 mL 0.05M Ba(OH)2, and shake 30 s. If aqueous layer takes on pink typical of phenolphthalein,
filter through paper into small beaker. Add ca 0.05MCH3COOHuntil colorless and evaporate to 1–2 mL on steam bath. Let remaining liquid evaporate spontaneously in air and finally in desiccator. If 5 mL 0.05M Ba(OH)2 does not give pink aqueous layer, add 5 mL more before separating. Repeat extraction with Ba(OH)2 solution several times or until pink solution is obtained, evaporating each barium solution in separate beaker. Examine crystals under polarizing microscope.
Barium monochloroacetate monohydrate crystallizes from H2O in plates, many of which are hexagonal in habit and frequently form in overlapping layers. Even in material that has been finely powdered for microscopic examination, pointed terminations of the plates, often in pairs, can be observed. In parallel polarized light (crossed nicols) extinction is parallel and sign of elongation is negative on more elongated plates. Plates invariably extinguish sharply with crossed nicols and therefore interference figures are not observed in convergent polarized light (crossed nicols). Since plates persistently lie in one orientation, significant refractive indices are determined by statistical method, lowest and highest indices respectively being measured on plates showing maximum double refraction.
These 2 indices are therefore arbitrarily designated as na (minimum value) and ng (maximum value). Two significant refractive indices are: na = 1.582 and ng = 1.611, both ±0.002, frequently shown on the platey fragments.
