“Think laterally - think outside the square - be creative!” How many times have we heard these admonishments as we struggle to find the solution to an analytical problem? When we do find the solution, it’s often stimulated by something outside our normal experience; when we recognize a new dimension to cut through the complexity and produce something simple and elegant. Thermometric titration can provide that extra dimension in solving difficult titration problems.

The following is an account of how a little lateral thinking was used to solve a prospective client’s problem using thermometric titration. In this case, the client is a custom galvanizer, who is building a new, state-of-the-art plant. The company saw that the current analytical methods didn’t fit with the concepts that they wanted expressed in the new plant; that is, one operated by a small, multi-skilled workforce, automated to the highest practicable degree, and tight (even on-line) process control to minimize quality variations and to drive down costs.

The analytical problems centered around the galvanizing pre-treatment tanks - principally the acid cleaning and a “pre-flux” solutions. In the acid cleaning solution, the analytes were free acidity, iron, zinc and chloride. In the pre-flux, the analytes were ammonium, iron, zinc, chloride and “acid capacity” - a measure of the remaining buffer capacity of the solution. Currently, free acid was determined by manual titration with caustic soda, “acid capacity” by manual titration with hydrochloric acid, chloride by manual titration with silver nitrate, zinc and iron by separate determinations with atomic absorption analysis, and ammonium by Kjeldahl distillation. The speed of analysis was incompatible with the process control they wanted to impose on the new plant. The brief was to come up with an integrated solution which could be managed by fast on-line titrimetric analysis, feeding results to a process controller.

Analysis by thermometric titration appeared to offer the best chance of a solution to the problem. Central to the simplicity and versatility of thermometric titrations is the sensor itself; a sensitive but robust thermistor. A thermistor is a non-specific sensor, it only measures temperature. If a chemical reaction is either exothermic or endothermic, it is a potential candidate for a thermometric titration. This is in contrast to pH, redox and ISE electrodes, which as their names suggest, are all restricted to specific application areas.

Because these electrodes are electrochemical sensors, and measure potential changes in the test solution, they need to have direct electrical contact with the solution. This is their Achilles’s heel, and delicate sensor membranes and reference junctions are subject to poisoning, fouling and clogging. In contrast to thermometric sensing, electrochemical sensors introduce complexity, reduce flexibility, and increase maintenance and potential system down-time.

Because the thermistor sensor is impervious to most chemical solutions, it is often unnecessary to make serial dilutions of concentrated plant liquors, thus saving time and reducing the possibilities for error. In many cases, it is possible to use very concentrated titrants. Typically, the time taken for thermometric titrations is only a few minutes.

Experience in the practice of thermotitration has shown that it is frequently possible to analyze a number of analytes in the one sample with a single titration. This concept of obtaining multiple endpoints in single titrations offered the best prospect for a simple, manageable system. This was the starting point to solving the client’s problems. Analyzing the acid cleaning solution was tackled first. By titrating synthetic mixtures of acid, Zn²⁺, Fe²⁺ and Fe³⁺ (Fe³⁺ is an occasional contaminant) with sodium hydroxide solution, it was found that a maximum of three endpoints could be obtained; the first was for the neutralization of the free acid, the second for the precipitation of the Fe³⁺ as a hydrated oxide, with the third representing the combined precipitation Fe²⁺ and Zn²⁺ contents of the solution as hydroxides. Examination of synthetic miuxtures showed that Fe²⁺ and Zn²⁺ hydroxides co-precipitate with sodium hydroxide as titrant. Figure 1 shows the thermotitration second derivative curve (used for endpoint location) for an acid cleaning solution contaminated with Fe³⁺.

It was decided to determine the Fe²⁺ content separately by thermometric redox titration, and subtract this from the combined Fe²⁺ and Zn²⁺ content analysed in sodium hydroxide titration. Using potassium dichromate as titrant, the resulting reaction was strongly exothermic, and endpoints of exceptional precision were obtained. Figure 2 shows the first and second derivative curves for this determination. From a process analysis point of view, there are obvious advantages in using the same sensor for both acid-base and redox titrations.

A challenge was posed by the combined presence of ammonium and zinc in the pre-flux solution. If sodium hydroxide was used as titrant, the problem of potential interference by formation of zinc ammine complex could arise. It was then recalled that a method commonly used in the fertilizer industry could be pressed into service. This involves the reaction of ammonium with formaldehyde to form hexamethylene tetramine (“hexamine”) and acid:

Since the ammonia is destroyed and the zinc liberated, (where the acid formed is chemically equivalent to the ammonia), it is now possible to titrate the ammonium (as acid) and the combined Fe²⁺ and Zn²⁺ content in the one titration with sodium hydroxide. As in analyzing the acid cleaning mixture, determination of the Fe²⁺ content is by separate dichromate titration. Figure 3 illustrates the first and second derivative titration curves for the determination of ammonium and combined Fe²⁺ and Zn²⁺ content. Determination of the acid capacity of the pre-flux solution proved to be an easy thermometric titration using hydrochloric acid as titrant.
 

At this stage, the client was offered a very simple solution; all analytes could be determined by thermometric titration (with the chloride estimated from the concentrations of the cationic analytes), using just one thermometric titration control unit and four piston burettes . If the client required an independent estimation of chloride, this would be done with a well-proven simple thermometric titration using silver nitrate as titrant. The client has the choice of using the thermotitrator as an independent titrator on the laboratory bench, or eventually integrating it into an on-line analysis system. In the case of the on-line system, a single titration vessel with a single sensor is used for all determinations, with a system of positive displacement dosing pumps regulating flows of all samples and titrants. The vessel is washed and drained automatically. Either as a laboratory analyzer, or as an on-line field analyzer, the titration software permits total automation of the analytical process.

Thermometric titration offers the innovative analyst new possibilities in producing simple answers to complex problems.

For further information contact:

Tom Smith
Antom Technologies Pty Ltd
15 Tarpon Street
The Gap, Qld 4061
Australia

Telephone & Facsimile: +617 3300 2614

E-mail:  antom@globec.com.au