Appendix IV A. Clarity of Solution

Using identical test-tubes of colourless, transparent, neutral glass with a flat base and an internal diameter of 15-25 mm, compare the liquid to be examined with a reference suspension freshly prepared as described below, the depth of the layer being 40 mm. Compare the solutions in diffused daylight 5 min after preparation of the reference suspension, viewing vertically against a black background. The diffusion of light must be such that reference suspension I can readily be distinguished from water R, and that reference suspension II can readily be distinguished from reference suspension I.

A liquid is considered clear if its clarity is the same as that of water R or of the solvent used when examined under the conditions described above, or if its opalescence is not more pronounced than that of reference suspension I.

Hydrazine sulfate solution Dissolve 1.0 g of hydrazine sulfate R in water R and dilute to 100.0 mL with the same solvent. Allow to stand for 4-6 h.

Hexamethylenetetramine solution In a 100 mL ground-glass-stoppered flask, dissolve 2.5 g of hexamethylenetetramine R in 25.0 mL of water R.

Primary opalescent suspension (formazin suspension) To the hexamethylenetetramine solution in the flask add 25.0 mL of the hydrazine sulfate solution. Mix and allow to stand for 24 h. This suspension is stable for 2 months, provided it is stored in a glass container free from surface defects. The suspension must not adhere to the glass and must be well mixed before use.

Standard of opalescence Dilute 15.0 mL of the primary opalescent suspension to 1000.0 mL with water R. This suspension is freshly prepared and may be stored for up to 24 h.

Reference suspensions Prepare the reference suspensions according to Table 2.2.1.-1. Mix and shake before use.

Turbidity standard The formazin suspension prepared by mixing equal volumes of the hydrazine sulfate solution and the hexamethylenetetramine solution is defined as a 4000 NTU (nephelometric turbidity units) primary reference standard. Reference suspensions I, II, III and IV have values of 3 NTU, 6 NTU, 18 NTU and 30 NTU respectively. Stabilised formazin suspensions that can be used to prepare stable, diluted turbidity standards are available commercially and may be used after comparison with the standards prepared as described.

Formazin has several desirable characteristics that make it an excellent turbidity standard. It can be reproducibly prepared from assayed raw materials. The physical characteristics make it a desirable light-scatter calibration standard. The formazin polymer consists of chains of different lengths, which fold into random configurations. This results in a wide assay of particle shapes and sizes, which analytically fits the possibility of different particle sizes and shapes that are found in the real samples. Due to formazin's reproducibility, scattering characteristics and traceability, instrument calibration algorithms and performance criteria are mostly based on this standard.

The degree of opalescence may also be determined by instrumental measurement of the light absorbed or scattered on account of submicroscopic optical density inhomogeneities of opalescent solutions and suspensions. 2 such techniques are nephelometry and turbidimetry. For turbidity measurement of coloured samples, ratio turbidimetry and nephelometry with ratio selection are used.

The light scattering effect of suspended particles can be measured by observation of either the transmitted light (turbidimetry) or the scattered light (nephelometry). Ratio turbidimetry combines the principles of both nephelometry and turbidimetry. Turbidimetry and nephelometry are useful for the measurement of slightly opalescent suspensions. Reference suspensions produced under well-defined conditions must be used. For quantitative measurements, the construction of calibration curves is essential, since the relationship between the optical properties of the suspension and the concentration of the dispersed phase is at best semi-empirical.

The determination of opalescence of coloured liquids is done with ratio turbidimeters or nephelometers with ratio selection, since colour provides a negative interference, attenuating both incident and scattered light and lowering the turbidity value. The effect is so great for even moderately coloured samples that conventional nephelometers cannot be used.

The instrumental assessment of clarity and opalescence provides a more discriminatory test that does not depend on the visual acuity of the analyst. Numerical results are more useful for quality monitoring and process control, especially in stability studies. For example, previous numerical data on stability can be projected to determine whether a given batch of dosage formulation or active pharmaceutical ingredient will exceed shelf-life limits prior to the expiry date.

When a suspension is viewed at right angles to the direction of the incident light, the system appears opalescent due to the reflection of light from the particles of the suspension (Tyndall effect). A certain portion of the light beam entering a turbid liquid is transmitted, another portion is absorbed and the remaining portion is scattered by the suspended particles. If measurement is made at 90° to the light beam, the light scattered by the suspended particles can be used for the determination of their concentration, provided the number and size of particles influencing the scattering remain constant. The reference suspension must maintain a constant degree of turbidity and the sample and reference suspensions must be prepared under identical conditions. The Tyndall effect depends upon both the number of particles and their size. Nephelometric measurements are more reliable in low turbidity ranges, where there is a linear relationship between nephelometric turbidity unit (NTU) values and relative detector signals. As the degree of turbidity increases, not all the particles are exposed to the incident light and the scattered radiation of other particles is hindered on its way to the detector. The maximum nephelometric values at which reliable measurements can be made lie in the range of 1750-2000 NTU. Linearity must be demonstrated by constructing a calibration curve using at least 4 concentrations.

The optical property expressed as turbidity is the interaction between light and suspended particles in liquid. This is an expression of the optical property that causes light to be scattered and absorbed rather than transmitted in a straight line through the sample. The quantity of solid material in suspension can be determined by the measurement of the transmitted light. A linear relationship between turbidity and concentration is obtained when the particle sizes are uniform and homogeneous in the suspension. This is true only in very dilute suspensions containing small particles. Linearity between turbidity and concentration must be established by constructing a calibration curve using at least 4 concentrations.

In ratio turbidimetry the relationship of the transmission measurement to the 90° scattered light measurement is determined. This procedure compensates for the light that is diminished by the colour of the sample. The influence of the colour of the sample may also be eliminated by using an infrared light-emitting diode (IR LED) at 860 nm as the light source of the instrument. The instrument's photodiode detectors receive and measure scattered light at a 90° angle from the sample as well as measuring the forward scatter (light reflected) in front of the sample along with the measurement of light transmitted directly through the sample. The measuring results are given in NTU(ratio) and are obtained by calculating the ratio of the 90° angle scattered light measured to the sum of the components of forward scattered and transmitted light values. In ratio turbidimetry the influence of stray light becomes negligible. Nephelometers are used for measurements of the degree of opalescence of colourless liquids.

Measurements of reference suspensions I-IV with a ratio turbidimeter show a linear relationship between the concentrations and measured NTU values (see Table 2.2.1.-2). Reference suspensions I-IV (Ph. Eur.) may be used as calibrators for the instrument.

Requirements in monographs are expressed in terms of the visual examination method with the defined reference suspensions. Instrumental methods may also be used for determining compliance with monograph requirements once the suitability of the instrument as described below has been established and calibration with reference suspensions I-IV and with water R or the solvent used has been performed.

Apparatus Ratio turbidimeters or nephelometers with selectable ratio application use as light source a tungsten lamp with spectral sensitivity at about 550 nm operating at a filament colour temperature of 2700 K, or IR LED having an emission maximum at 860 nm with a 60 nm spectral bandwidth. Other suitable light sources may also be used. Silicon photodiodes and photomultipliers are commonly used as detectors and record changes in light scattered or transmitted by the sample. The light scattered at 90 ± 2.5° is detected by the primary detector. Other detectors are those to detect back and forward scatter as well as transmitted light. The instruments used are calibrated against standards of known turbidity and are capable of automatic determination of turbidity. The test results expressed in NTU units are obtained directly from the instrument and compared to the specifications in the individual monographs.

Instruments complying with the following specifications are suitable.

• — Measuring units: NTU. NTU is based on the turbidity of a primary reference standard of formazin. FTU (Formazin Turbidity Units) or FNU (Formazin Nephelometry Units) are also used, and are equivalent to NTU in low regions (up to 40 NTU). These units are used in all 3 instrumental methods (nephelometry, turbidimetry and ratio turbidimetry).

• — Measuring range: 0.01-1100 NTU.

• — Resolution: 0.01 NTU within the range of 0-10 NTU, 0.1 NTU within the range of 10-100 NTU, and 1 NTU for the range > 100 NTU. The instrument is calibrated and controlled with reference standards of formazin.

• — Accuracy: 0-10 NTU: ± (2 per cent of reading + 0.01) NTU. 10-1000 NTU: ± 5 per cent.

• — Repeatability: 0-10 NTU: ± 0.01 NTU. 10-1000 NTU: ± 2 per cent of the measured value.

• — Calibration: with 4 reference suspensions of formazin in the range of interest. Reference suspensions described in this chapter or suitable reference standards calibrated against the primary reference suspensions may be used.

• — Stray light: this is a significant source of error in low level turbidimetric measurement; stray light reaches the detector of an optical system, but does not come from the sample; < 0.15 NTU for the range 0-10 NTU, < 0.5 NTU for the range 10-1000 NTU.

Instruments complying with the above characteristics and verified using the reference suspensions described under Visual method may be used instead of visual examination for determination of compliance with monograph requirements.

Instruments with range or resolution, accuracy and repeatability capabilities other than those mentioned above may be used provided they are sufficiently validated and are capable for the intended use. The test methodology for the specific substance/product to be analysed must also be validated to demonstrate its analytical capability. The instrument and methodology should be consistent with the attributes of the product to be tested.