Closed loop systems, seasonal variations in the composition of the pulp, shear forces and interdependence between many of the chemical variables are some of the factors that contribute to the complexity of wet-end chemistry. Multivariate methods are therefore often required to obtain an overview of the relations between variables in the wet end and variables affected by wet-end chemistry.

In this project an electronic tongue based on voltammetry has been evaluated as a tool for wet-end monitoring. The electronic tongue is equipped with working electrodes of g-old, iridium, platinum and rhodium and signal analysis is performed using multivariate methods like principal component analysis (PCA) and partial least squares regression (PLS). In a measurement series for evaluating the sensitivity profile of the electronic tongue it was found that redox active species and global variables like pH and conductivity were best modelled, with correlation coefficients for validated samples higher than 0.85. Models for detection of specific cations were not satisfactory.

The sensitivity of the electronic tongue has also been evaluated with respect to classical wet-end va1iables like cationic demand, zeta potential and retention as well as concentrations of process chemicals. Correlation coefficients for these variables in the same range as for the best-modelled variables described above show that the sensitivity profile for the instrument makes it well suited for wet-end characterisation. In an on­ line measurement series the electronic tongue was mounted in a WIC Compact autosampler/analyser and measured continuously during two months without problems. The robustness of the measurement probe therefore seems to fulfil the demands for an on-line instrument used in the chemical environment of the white water system.

The large variation in the wet-end chemistry of the paper machine and drift in the signals from the electronic tongue make the construction of stable models difficult. Development of calibration and drift compensation methods for the instrument is therefore a key issue for continuous progress in this application project. If these problems can be resolved, the electronic tongue can be expected to meet the requirements as a fast, flexible and sensitive instrument for advanced wet-end characterisation.