Thermometric titration of aluminium adapted to analyse hydrofluoric acid in a mixed acid solution used in the etching of semi-conductor wafers for a titrimetric method for fluoride
Aluminium in solution isn't the easiest element to analyse. It causes problems in flame spectroscopy because of the highly refractory nature of its oxide.
Titrations can cause problems as well.
Aluminium salts are used in water treatment, deodorants and many other applications.
Slow reaction kinetics of aluminium with EDTA means that aluminium in solution must first be heated with an excess of EDTA, with the excess then back-titrated with copper or zinc.
Basic aluminium (aluminate) solutions are produced for water treatment and other purposes, and are also the life blood of the massive world alumina industry where tens of millions of tonnes of alumina are produced every year from bauxite.
The traditional approach has been to determine the aluminium in concert with the free hydroxyl and carbonate contents of the liquors.
While excellent results may be obtained, errors in the analysis of the hydroxyl content are inevitably added to those in the aluminium determination.
There is a need for a direct titrimetric method with traceability to a primary standard.
The thermometric titration of aluminium with fluoride has been known for over 30 years, and last year, Multitrator adapted this application to analyse hydrofluoric acid in 'polyetch', a mixed acid solution used in the etching of semi-conductor wafers.
In the presence of sodium and potassium, aluminium reacts with fluoride to form insoluble dipotassium sodium hexafluoride, and this reaction is sufficiently exothermic to form the basis of an excellent titrimetric method for fluoride.
With six moles of fluoride reacting with one mole of aluminium, it isn't hard to see the potential for a highly precise determination of aluminium, and that's the way it has turned out.
Multitrator developed methods for handling acidic, neutral and basic aluminium solutions which, it says, are very precise indeed.
In analysing a solution of ammonium aluminium sulfate, it recorded a coefficient of variance (%RSD) of just 0.03.
For a basic aluminate (Bayer) liquor, the CV was an 0.07.
In the alumina industry, the company says, a CV of 0.2 is considered very respectable.
To ensure traceability of the procedure, the sodium fluoride titrant was standardised against an aluminium solution prepared from 99.99% pure metal.
Multitrator has prepared a set of application notes describing this technique, available by email.
