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Introduction

Turbidity is a widely used technique in which monitoring particles in a liquid can provide valuable insight into the system. In the case of crystallization, for example, it can accurately define the metastable zone width (MSZW), while for suspended solid mixtures, it can quantify the solid content over a wide concentration range.

However, turbidity probes cover a range of designs which can result in different performance depending on the specific application and the chemicals involved.

HEL produces turbidity probes that can work in one of two ways:

• Back scattering method, where light reflected by the solids is measured in comparison with `clean’ solvent.
• Transmission method, where a reflection `mirror’ is placed close to the end of the probe and we measure the decrease in reflected light picked up by the probe.

This report will  look at two very different applications and compare the performance of the above HEL probe (and the two methods) with a Mettler Toledo turbidity probe (model Trb 8300), which works only in back-scatter mode

Measurements Of Suspended Solids

The response of the back-scatter probes to the addition of flour to 500 ml of water, in a stirred (~250 rpm) vessel, was compared.  Ideally, the turbidity should increase linearly with solids loading.

The results are presented in Figure 1.  This shows that the HEL and Mettler probes respond in an identical manner, with linear response to solids content initially.

However, at ~ 120 g/litre, the Mettler probe starts to curve noticeably.  At loadings in excess of 200 g/litre, the probe did not respond to further addition of powder.  This is in line with Mettler’s own documentation.

Crystallization Monitoring

Objective

A common application of turbidity is to measure the appearance and disappearance of crystals, as a solution is heated and cooled, allowing the so-called metastable zone width (MSZW) to be automatically estimated.

The sensitivity of the probes is important in this application so that the effect is detected as soon as the first crystals appear and as the last crystal dissolves.

Back-scattering Data

The HEL scattering probe and the Mettler probe are compared in Figure 2, where 4-acetyl biphenyl (ABP) in toluene is recrystallized.

The crystallization point is evident from the temperature kink, and this corresponds closely to the response of both probes.  Thus, the probes are equally sensitive.

Transmission vs Back-Scattering

In Figure 3, the HEL back-scattering probe was replaced by the transmission type with a reflectance window, and compared against the Mettler probe.

The HEL probe picks up the event considerably sooner — a difference of around 4 minutes, equivalent to roughly 1 °C in temperature.

AstraZeneca have reported data on a different Mettler turbidity meter (FSC 402). Even with the gain set to an optimum level, the operator detected the appearance of crystals 12 minutes before the meter did (Ref. 1). That differs from our experience with the HEL detector, where the probe is often marginally faster than the operator.

Conclusions

The two turbidity techniques supported by HEL address the two main applications well. Back-scattering gives a linear response to suspended solids across a wider concentration range, while transmission detects the onset of crystallization earlier.

Miniature probes are also available, allowing turbidity studies at the ~2 mL scale, and a compact multi-probe turbidity unit is available for screening work.

Turbidity measurement of this kind underpins our crystallization and particle studies systems, including CrystalEYES and CrystalSCAN.

Reference 1: Parsons et al., Trans. IChemE, Vol 81, Part A, July 2003, pp. 700–704.
Data in this note is extracted from HEL internal report RO13.001.2004-02-12.