Method chaining feature involving two separate titrations solves the problem of analysis of a mixture of nitric, acetic, and phosphoric acids
A thermometric titrimetric method has been developed for the determination of the individual components in a mixture comprising phosphoric, nitric, and acetic acids.
The method involves two separate titrations.
The first determination involves the titration of the mixed acids with standard sodium hydroxide solution.
Two sharp endpoints are obtained; the first resulting from the combined protons from the nitric acid and the first proton of the phosphoric acid.
The second endpoint represents the combined proton from the acetic acid and the second proton of the phosphoric acid.
There is also a broader, third endpoint for the third proton of phosphoric acid, but this is non-stoichiometric.
In the second titration the acid mixture is strongly basified with aqueous ammonia solution, and titrated with magnesium nitrate solution.
The reaction enthalpy of the formation of insoluble magnesium ammonium phosphate provides the basis for a very sharp endpoint.
To calculate the amount of nitric and acetic acids, the amount of phosphoric acid is expressed as a volume of standard sodium hydroxide solution, and subtracted from the titres for the first and second endpoints respectively in the acid/base titration.
Calculations may be performed in a dedicated spreadsheet linked to the titration software.
This method may be fully automated to run the two titrations as a single contiguous sequence, and is suitable for routine operation in a process control laboratory.
Precision (one standard deviation) for the determination of phosphoric acid is 0.04%w/v, while that of nitric and acetic acids is 0.2% and 0.1%w/v respectively.
