Turbidity measurements Turbidimetry

Turbidimetry measurements, using monochromatic light, yields data that can be used to determine particle size distributions. It requires simple optical technology, but complex computational software to handle the Mie theory conversion. The sensitive diameter range for latex-water suspensions was found to be 0.1 to 10 pm. Different types of sensors have been conceived and applied to various experimental situations. The method is particularly useful in crystallization experiments. Other applications include agglomeration, attrition and nucleation studies. Applications of the equipment and software to studies of emulsions, fumes and aerosols are also envisaged [18]. 

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In the early development of turbidimetry, scattering of light was not reproducible from one sample to another because it was difficult to control the processing of the sample. In 1927, this led Wells to voice his scepticism of the reliability of results obtained from instruments relying on the measurement of scattered radiation [62]. In view of this, turbidity measurements were not accepted as standard methods at that time. Once the limitations of such optical methods were understood, however, their real potential could be exploited. Nowadays, a reproducible suspension is easily attained, particularly in flow analysis. As a consequence, several turbidimetric procedures have been successfully implemented and Wells statement is no longer valid. 

In short, turbidimetry is the measurement of the degree of attenuation of a radiant beam incident on particles suspended in a medium, the measurement being made in the directly transmitted beam. Thus, turbidity (T) may be expressed as ... 

Turbidity decreases the intensity of the incident beam of light as it passes through a solution of particles. The measurement of this decrease in intensity is called turbidimetry. Analogous to absorption spectroscopy, the turbidity is defined as ... 

The photometric end point has also been adapted to precipitation titrations. The suspended solid product causes a decrease in the radiant power of the light source by scattering from the particles of the precipitate. The equivalence point occurs when the precipitate stops forming, and the amount of light reaching the detector becomes constant. This type of end point detection is called turbidimetry because the amount of light reaching the detector is a measure of the turbidity of the solution. 

Analogously to spectrophotometry, stray radiation can alter the measured turbidance (Eq. 4.6), the limitation becoming more severe at lower transmitted power. The presence of suspended matter in the processed sample is inherent to turbidimetry and leads to an amplification of stray light due to scattering effects. This effect has not been systematically investigated in flow analysis. 

Nephelometry is a variant of turbidimetry in which the power of the scattered radiation is measured at an angle of <180° to the source (normally 90°). Forward angles give the best potential sensitivity for larger scattering species but it is not easy to differentiate scattered light from the incident beam. Nephelometry is applied to lower concentrations than turbidity because the dark background is zero compared with full illumination in turbidimetry, a fact that was first reported in 1921 [63]. 

Turbidity is determined by comparison with a standard of formazine using turbidimetry [14], or by nephelometry if mainly colloids are measured [13, 14]. The results are expressed by the formazine unit of turbidity (FT). 

Nephelometry is the measurement of turbidity by the direct evaluation of the degree of light scattering taking place in the medium. It is much more appropriate to media of lower turbidity in which the suspended particles are small. Turbidimetry and nephelometry can offer considerable time-saving advantages over gravimetric methods for the determination of particle concentrations, and are nondestructive techniques. 

In summary, the simplicity of measurement makes turbidity spectra methods attractive for on-line monitoring of particle size distribution, particular / if the distribution is known to be monomodal. In this case average particle size, determined by turbidimetry may be sufficient for continuous monitoring and control. This has been shown by Hamielec and coworkers (10) who, by techniques of state estimation, have used turbidity data to estimate other states of an emulsion system, and applied closed-loop control accordingly. 

In a comparative study, we assessed the average values of PLA domain diameters in IPNs by turbidimetry using UV-visible spectroscopy. The turbidity r of an IPN sample was determined from the measurement of its transmittance Tr at 460 nm, and calculated according to the Beer-Lambert law as follows r = -In (Tr) / e, where e is the sample thickness (2 mm). Then, following Blundell s pioneering study on a system constituted of domains of PMMA embedded in a polyurethane matrix (19), we applied his theoretical expression to our IPN systems to estimate the average size of PLA microdomains dispersed within the PMMA matrix. Thus, the t value led to the calculation of the parameter Bfy) as shown in eq 1 ... 

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