Myth: “Online measurements are the same as laboratory measurements. And so are the measurement procedures”.
In laboratory colourimetric procedures the reagents are added one after the other to get the colour response. This is mainly because we are limited by two hands per person. The other laboratory practice limitation is the precision of the manual operation. Compared to the laboratory procedure, online measurement procedures use an online analyser, a robot, and therefor very precise in handling and operation and it can add many reagents at the same time or in a defined precise sequence because it can control multiple pumps.
Colourimetric online measurements procedures came in two flavours. There are flow through measurement systems and batch measurement systems. Batch measurement systems follow the laboratory procedures. While in flow through measurement systems all parts are filled with solutions and liquids all times and pumped by different pumps, in batch systems the liquids are pumped one after the other, in many cases via a tricky multi solution pumps where the liquids are selected by valves1. Complete reaction kinetics information can only be collected in flow through systems.
Kinetic information of colour complexation
The reaction kinetics contains information about the fight for equilibrium of measured ions against inferences and can be used for more precise and accurate concentration calculation.
The advantages of flow through systems:
- kinetic information may be used to calculate measurement results.
- bubbles or missing reagents and sample can be detected easily2.
- fouling is limited and can be eliminated easier.
- very high operational stability.
The conclusion to avoid batch measurement systems is easy and straight forward to make.
(1)Valves reduce the operational reliability dramatically and must be avoided in unobserved automated operations. See Seibold TCM (total costs of measurement).
(2)The surface area between liquids and air is called meniscus and can be detected by the photometer as an abrupt signal change (light is blocked).