The degree of dispersibility in emulsion ointments and creams is determined under the condition of coloration of the dispersed phase. The diameter of 1000 droplets is determined and then the percentage of droplets of different sizes is calculated. The method is easy to perform, but quality standards for emulsion creams and ointments are not yet specified in any pharmacopoeia. Determination of pH of ointments is necessary to monitor the behavior of drug substance and base during storage. A shift in pH indicates a change in their physicochemical properties. To determine the pH of ointments and creams suspended product poured 50 ml of distilled water with a temperature of 50-60 «C and shaken on a vibrator for 30 minutes. The resulting extract is filtered and conducted potentiometric titration according to the method of GF. All ointments produced by pharmaceutical companies, have a guaranteed shelf life, during which, if properly stored, they should remain stable. The stability of ointments is determined by the unchanged content of drug substances (within the established tolerance), structural properties and the rate of release of drugs. Stability testing of ointments is particularly essential if they are emulsion systems. One acceptable technique for testing the stability of such ointments is the colloidal stability methodology. Assessment of colloidal stability of a cream or ointment is carried out on a centrifuge at a speed of 6000 rpm for 5 min. Under the influence of centrifugal force, the emulsion breaks down the faster; the less stable it is. The absence of stratification of the product sample indicates the stability of the composition. Thermal stability of the product is determined by heating the ointment or cream in a desiccator at 60 ° C for 1 h. If the product is qualitative, the sample should remain homogeneous without stratification. Sometimes in ointments and creams it is necessary to determine the structural and mechanical properties (ultimate shear stress, which characterizes the strength of the structure and consistency of ointments, and plastic viscosity, which characterizes the flow of the system with a destroyed structure), the degree of release of drug substances from the preparation and the stability of the product under different storage conditions. These determinations are usually made when developing new ointments and creams or improving existing ones. The consistency of ointments and ointment bases requires objective evaluation, as it affects the processes of their preparation and packaging, the ease of application of ointments to the skin and the release of drug substances from them. One of the main factors on which the consistency of ointments depends is the ultimate shear stress. It characterizes the ability of ointments to provide some resistance to smearing, the ability to squeeze out of tubes, dispensers, etc. The ultimate (or critical) shear stress is the force in dynes acting on an area of 1 cm2 tangentially to the plane of displacement and causing irreversible deformation of the system.

The ultimate shear stress is also called the flow point, meaning the stress required for the system to start flowing (for irreversible deformation). The higher the value of this value, the more difficult the ointment is to spread. This is an important characteristic of ointments and ointment bases because it determines the ease of application of ointments. Usually a Volarovich rotary viscometer or a conical plastometer is used to determine the ultimate shear stress. Plastic viscosity, another important rheological characteristic of ointments and ointment bases, is determined on a Volarovich rotary viscometer RV-8. The degree of release of drug substances is a criterion for assessing the quality of ointments, which should become the main criterion for standardization and rejection (verification of compliance of the product quality with the requirements of the standard) of ointments. Methods for determining the degree of drug release in vitro and in vivo have been developed. In vitro methods. The technical performance of experiments by in vitro method can be different and is mainly determined by the properties of the included drugs. Direct diffusion method. In this case, the ointment sample must be in direct contact with the medium into which the drug substance diffuses. Method of diffusion through the membrane. The essence of the method is that the ointment under study is separated from the aqueous medium by a semipermeable membrane. This can be cellophane or lipoid membranes of animal origin, such as egg shells, a section of intestine or animal skin. The dialysis media are aqueous solutions or water. The apparatus design of these studies can vary. In recent years, there have been many installations that maximally approximate the conditions of experience to the conditions of a living organism. Most often these are two-chamber installations separated by membranes or membrane systems. One of the chambers contains the ointment and the other contains the dialysis medium. Despite the design differences, the setups obey the same principle and reflect the same dependencies. In vivo methods. In contrast to in vitro methods, these methods allow two processes to be assessed at once: the ability of the ointment base to release the active ingredients and the degree of resorption of the active ingredients through the skin. In vivo methods include the following studies: Determination of the resorbed amount of drugs by the difference between the applied ointment sample and the non-absorbed portion of the ointment. Such evaluation is acceptable on both animal and human skin. A certain amount of ointment is applied and rubbed evenly on a strictly limited area of skin using a template. A pressure of 100 mmHg is applied to this area using a cuff.