Eric Trompier looks at a unique continuous reagent addition technique which has proved its efficiency in many highly specific applications.
Certain industries with complex applications regularly encounter difficulty in achieving accurate enough results using inflection point titration with the conventional incremental addition method.
Continuous reagent addition is the preferred method where:
* Close inflection points need to be separated, eg complexometric titration
(Ca2+ and Mg2+ by EDTA).
* There are non-symmetrical peaks, e.g. citric acid (triacid) with NaOH.
* There is non symmetrical electrode response before and after the inflection point, e.g. titration of HF with La(NO3)3 using F- ISE (LaF3 crystal).
* Trueness and repeatability of inflection points is essential, eg low silver or chloride concentration (<30 mg/l).
* The electrode follows continuous variation of the medium, eg TAN/TBN in oil.
To summarise, in any application where inflection points or pKas are very close or slope curves weak, continuous reagent addition should be adopted.
How the technique works
In continuous reagent addition, an addition technique patented by Radiometer Analytical, the addition speed is permanently controlled by the slope of the titration curve with acquisition of the measurement points (potential (or pH) and delivered volume couples) dependent on the slope of the curve.
Titration speed is controlled by the Titration Manager according to the curve slope (similar to PID regulation) taking into account the data entered by the operator:
* A maximum titration speed (used when the derivative of the curve is close to zero).
* A minimum titration speed, which the titration approaches at the equivalence point of the titrated species/titrant, corresponding to the point where the titration curve derivative passes through a maximum.
The speed change during the titration is shown in Fig. 1.
In incremental addition mode, the acquisition of measurement points required to determine the inflection points follows a simple rule: one increment equals one point. In continuous addition mode, Radiometer Analytical has its own algorithm for storing the points of the curve based on the:
* High sampling speed of the Titration Manager.
* Titration speed.
* Value of the derivative of the titration curve (dE/dVol).
* Maximum number of measurement points which can be stored for a particular titration
(1000 points for a Titration Manager).
* Volume of titrant required to plot the curve.
This makes it possible to increase the number of measurement points acquired during the titration in the areas corresponding to the inflections.
In the parts of the curve with a low slope, the addition speed of the reagent is greater and the variation in the measured potential (or pH) small; this means that the algorithm will collect less measurement points.
As soon as the titration shows an increase in the calculated derivative:
* The titrant addition speed decreases and, as the sampling rate remains constant, the algorithm increases the number of points stored,
* The predominance given to the measured potential against the titrant volume further emphasises this. Fig. 2 explains this process.
Conclusion
For complex titrations, it is always advisable to choose a titration system integrating continuous addition mode.
The Titration Managers in Radiometer Analytical's TitraLab range offer both incremental and continuous addition techniques guaranteeing they will meet your needs regardless of the type of potentiometric titrations to be performed.
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Eric Trompier is a TitraLab Product Specialist with Radiometer Analytical SAS, Villeurbanne, France. www.radiometer-analytical.com
