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How does the IRmadillo differ to a refractometer?

The IRmadillo is based on FTIR spectroscopy, using multiple wavelengths of infrared light to create a spectrum, which can be interpreted to measure the concentration of many different chemicals at once, in complicated and dynamic processes. It can also be used for qualitative analysis by comparing the spectrum to spectra obtained during known states (for example “in specification”, “out of specification” states, or “early stage”, “mid stage” and “end stage” for batch reactions).

Refractometers work by measuring the refractive index of a solution using a single wavelength of light, and comparing the refractive index with a known calibration curve for that mixture to understand the concentration of a single chemical in the process. Refractometers are often low-cost instruments, and can be a cost-effective solution for certain processes. However there are many cases where a refractometer would not be suitable. Some things that affect the refractive index of a material are:

  • Temperature changes: fluctuating temperatures (in some even cases even by a couple of °C) can affect the refractive index of a liquid more than compositional changes, making refractometers useless.
  • Complex mixtures: simple mixtures of one chemical dissolved in another can easily be analysed using refractometers, but with three or more chemicals it can be impossible to obtain a meaningful measurement as the same refractive index can be produced by different mixtures of the different chemicals.
  • pH changes: whilst the absolute pH of a mixture is unlikely to affect the refractive index, where organic acids and basis are present it is possible that a pH change will change the reading – for example, the refractive index of acetic acid changes dramatically whether it is in the acid or sodium salt form.

These conditions mean that refractometers should only be considered for very simple mixtures and processes, that are already very well controlled.

How do you know you can trust a refractometer measurement?

The simple answer is: you cannot. Although the fundamental physics does not change and refractometers do not tend to drift in calibration, your process may drift outside of the very narrow and strict calibration range the instrument operates inside of. For example, if the instrument is calibrated to 25°C but the process drifts to 28°C you may start receiving erroneous readings and not know. Likewise, if an unknown contaminant appears in the process stream this could result in erroneous readings, again with no warning. The IRmadillo reports a statistical confidence with every measurement meaning you can trust the measurement. If it identifies that the process is outside of calibration you will be notified, and can take the required action.

What would we use refractometers for?

Refractometers do have a place in industrial chemistry and manufacturing, and are a cost-effective option for:

  • Measuring water in a single other chemical, with no contaminants and very stable temperature control: in this somewhat idealised case, refractometers can give accurate readings that can be used for additional process optimisation and control.

What would we recommend choosing the IRmadillo over refractometers for?

Most industrial applications that are seeking optimisation and control are complex with challenging requirements. These are the sorts of conditions that broad wavelength spectroscopy excels in, such as:

  • Where multiple chemicals need to be measured: the IRmadillo measures every chemical in the mixture, with every scan. Refractometers can only measure a single chemical, and then only in simple or very stable mixtures.
  • Where the temperature, pressure, pH and other properties of the process can drift or fluctuate: as long as the required conditions are varied during the calibration phase (and this can happen in the laboratory before an on-line installation) then the IRmadillo will continue giving meaningful measurements regardless of the actual temperature, pH etc.
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