Described as a breakthrough in titrimetric analysis - thermometric titration of metal ions in the parts per million (mg/l) concentration range
tUp until now, Multitrator has promoted thermometric titrimetry as being ideally suited for industrial process and quality control; using a robust, simple, long-life thermometric probe to perform acid/base, redox, EDTA, precipitation and water titrations in what could be called the 'grams per litre, percent' concentration range.
An exception had been the analysis of chloride in natural waters, where it has demonstrated accurate results down to 20ppm.
Recently, they unearthed a technique developed over 30 years ago, where the authors claimed to have determined a wide range of metals at very low concentration levels.
This rather academic procedure has now been modified so that it can be used in any industrial or environmental laboratory.
The analyst now has a clear choice between potentiometric titrations using expensive, delicate and frequently short lived ion-selective electrodes of doubtful reliability; and thermometric titrations using a simple, robust, zero-maintenance probe.
The basic chemistry is unremarkable: a back-titration using an excess of EDTA to complex the metal ions in solution, and back-titration with a suitable metal ion which reacts rapidly with EDTA.
Multitrator has previously developed a highly accurate and precise EDTA method for process and quality control in the g/l range.
However, this procedure uses 1M EDTA as the titrant, so that there is a sufficiently large change in the rate of temperature to give sharp endpoints.
However, titration in the ppm or mg/l range demands titrants approximately two orders of magnitude lower, so that titres of a sensible magnitude may be obtained.
With titrant and analyte concentrations so low, the titration temperature curve is essentially featureless.
The solution to the problem is to use an external reaction catalysed by a trace of the titrant ion.
In the experiments, they used 0.02M EDTA solution to chelate metals present in the sample solutions and 0.02M manganese sulphate as the titrant.
At the endpoint, the first trace of Mn(II) catalyses a strongly exothermic reaction between hydrogen peroxide and a polyhydric phenol.
The original workers employed resorcinol, but there was none available.
Instead, gallic acid (3,4,5 trihydroxy benzoic acid) was tried.
This proved to work very well indeed.
They analysed copper in the range 60-120 ppm, and total hardness (Ca+Mg) in tap water at 30ppm.
The method is said to be remarkable for its accuracy and precision.
The use of EDTA in the determination has both advantages and disadvantages.
EDTA is able to complex a wide range of metallic cations, and this gives extraordinary versatility and applicability to the method.
The disadvantage is that this appetite for metals means that EDTA lacks specificity.
However, the analyst has at his or her disposal a variety of masking techniques which can improve specificity.
Multitrator considers that there is work to be done in exploring the lower analytical limits of this exciting technique, and already has ideas as to how to expand the range of analytes which can be measured.
