Micromeritics PDF

UNIT III MICROMERITICS MICROMERITICS Definition: "The science and technology of small particles is known as micrometirics. It deals with the study of fundamental and derived properties of both individual as well as collection of particles"...

UNIT III MICROMERITICS MICROMERITICS Definition: "The science and technology of small particles is known as micrometirics. It deals with the study of fundamental and derived properties of both individual as well as collection of particles" The term micromeritics was given by J.M DallaValle. The unit of particles size used most frequently is the micrometer, also called the micron and is equal to 10-6 m or 10-4 cm or 10-³mm. TYPE OF PARTICLES Depending upon their size particles are divided in to three groups. 1. Colloidal particles Such types of particles are not seen by ordinary microscope and are only observed by ultra-microscope. 2. Emulsion and suspension Particles of suspension and emulsion can be observed by light microscope 3. Coarse particles These are observed on the basis of sieves. They are of three types,  Particles of coarse powder, larger than sieve number 20.  Particles of intermediate size, in range of sieve number 20-200. FARDAH FAROOQ 1 UNIT III MICROMERITICS  Particles of fine powder, smaller than sieve number 200. PARTICLE SIZE Particle size is related to the shape and surface area of individual particles. According to their shape, the particles are divided in to two groups. 1. Symmetrical particles The particles having specific crystal shape and can be expressed in term of their diameter (for example spherical) are known as symmetrical particles. Surface area = πd² Volume = π d ³/ 6 2. Asymmetrical particles The particles which have no specific crystal shape are termed as asymmetrical particles. 1. Surface diameter 2. Volume diameter 3. Projected diameter 4. Stokes diameter  Surface diameter (ds) The diameter of a sphere having same surface area as that of asymmetric particles. FARDAH FAROOQ 2 UNIT III MICROMERITICS  Volume diameter. (dv) The diameter of sphere having same volume as that of asymmetric particles.  Projected diameter (dp) The diameter of sphere having same observed area as that of asymmetric particles when viewed normal to its most stable plain. This diameter is usually determined by microscopic technique.  Stokes diameter (dst) This is the diameter of the sphere having same sedimentation rate as that of asymmetric particles. AVERAGE PARTICLE SIZE Any collection of particles is usually poly disperse (particles of more than one size) so if a sample of powder have particles of different sizes then the average particle size is determined by Edmondson equation 1/𝑝 (Σndp+f ) D mean = = Σndf Where, d = diameter of the particle in a given size range. n = number of the particles in the sample. p= index related to the size of an individual particle. f= frequency index.  The use of Edmondson equation depends upon the value of p & f. FARDAH FAROOQ 3 UNIT III MICROMERITICS For example if, p= 1, it becomes the expression of the particle length. p=2. It becomes the expression of surface area. p=3, it becomes the expression of volume of particles.  The value of p also decides the type of mean obtained. If, P= +ve, the mean is arithmetic. P = -ve, then mean is harmonic. P=0, then mean is geometric. If, F = 0, it indicate the expression for the number. F= 1; it indicate the expression for length. F= 2, it indicates expression is for surface area. F= 3, it indicate that the expression is for volume. PARTICLE SIZE DISTRIBUTION In a sample of powder, particles of various sizes are present. This distribution of particles of various sizes in a sample is known as particle size distribution. Types of particles size distribution: There are two types of particles size distribution i.e. FARDAH FAROOQ 4 UNIT III MICROMERITICS i. Number particle size distribution; In this technique, we count the number of particles in specific size range. The numbers of particles are usually calculated by microscopic technique. ii. Weight particle size distribution; In this technique we weigh the particles in specific size range. The weight of the particles is usually calculated by sedimentation or sieving method. iii. Frequency distribution curve; If the number of weight fractions is plotted against their size range, a frequency curve is obtained. This curve is known as frequency distribution curve. Such plot gives a visible representation of the distribution that an average diameter cannot achieve. This is important b/c it is possible that two sample may have the same diameter but different distribution. However normal distribution is not commonly found in pharmaceutical powders. Which are frequently processed by milling or precipitation? More commonly asymmetric or skewed distribution is obtained. FARDAH FAROOQ 5 UNIT III MICROMERITICS GRINDING / TRITURATION It is the process in which substance is reduce to fine powder on small scale crystalline substance are reduce to fine powder in pestle or morter, is known as trituration. Size reduction is divided into two categories; 1. Manual size reduction 2. Mechanical size reduction 1. Manual size reduction On small scale size reduction may be carried out by; i. Trituration ii. Levigation iii. Pulverization by intervention i. Trituration FARDAH FAROOQ 6 UNIT III MICROMERITICS By trituration grinding of solid substance is done by continuous mixing or reducing particle size I pestle and mortar. In this process two or more powders are intermittently mixed with each other by using pestle and mortar. For this purpose we use Wedgewood ware or glassware pestle and mortar ii. Levigation This process is also known as wet grinding. In this process particle size is reduced to fine powder by triturating substance with few drops of liquid, in which it is insoluble. This can be carried out by using pestle and mortar or spatula or muller. The most commonly used liquids are  Oils  Alcohol  Glycerine  Water iii. Pulverization FARDAH FAROOQ 7 UNIT III MICROMERITICS This is process by which solid substance are reduce to fine powder by means of triturating the solid with suitable solvent, which can be easily removed at the end of grinding. Examples: camphor is difficult to powder in mortar because particles are stick to each other as they are grinded. But when we add few drops of alcohol, ether or chloroform in camphor, it can be easily powdered. The solvent added evaporate quickly leaving behind fine state of camphor. Similarly, iodine crystals may be powdered by addition of ether. 2. Mechanical size reduction In this method we can use  Ball mill  Hammer mill  Colloid mill  Fluid energy mill DETERMINATION OF PARTICLE SIZE There are different methods for the determination of particle size. These methods are mainly divided in to two groups' i.e. direct method and indirect method. In direct method, the dimension (i.e. length & width) of particles are directly measured with the help of a scale e.g. microscopy and sieving methods whereas during indirect method, some characteristics of particles are determined which FARDAH FAROOQ 8 UNIT III MICROMERITICS are then correlated to the particle size e.g. sedimentation following are the three main methods for the determination of particles size distribution 1. Microscopy 2. Sieving method 3. Sedimentation. Microscopy This is the accurate and direct method for the determination of particle size. At least 300 - 500 particles must be counted in order to obtain good size distribution analysis of data according to this method particles whose sizes are to be determined is prepared in liquid vehicle in which it is insoluble, when it is slightly soluble a saturated solution of the powder can be used for the preparation of a suspension. A drop of suspension is mounted on a slide and placed on mechanical stage. The microscope eye piece is fitted with a micrometer by which the size of particle may be estimated. Generally three types of microscopes are used depending on the sizeof the particles i.e.  Simple microscope (for 0.2-100 μm)  Ultramicroscope (for 0.01 -0.2 μm)  Electron microscope (for less than 0.01 μm) Advantages: The presence of particle of more than one component can be detected by this method. FARDAH FAROOQ 9 UNIT III MICROMERITICS Disadvantages: The number of the particles to be counted (300 - 500).to obtain a good estimation makes this method slow and tedious. SIEVING METHOD This method is applicable for particles in the size range of 50 - 5000μ. Sieving is the most widely used method for measuring the particle size distribution. It is based on "go" no go "principle. During sieving process the powder is passed over a perforated screen, with the result small particles will pass through sieve, while the oversized particles will be retained on the sieve. For detailed analysis of powder sample, a set of sieves are arranged in such a way that the finest sieve is on the bottom and the coarser is on the top. The weighed amount of the powder is placed on the top of the sieve and the sieve set is shaken for specific period of time. The standard sieve used is calibrated by the national bureau of standards. During the vibration of the sieves a fraction of powder will be retained on each sieve while the other fractions will pass through the sieve. After specific time, the shaker is stopped and the weight of each fraction is determined and the % age is calculated. Then these results are presented in the form of a cumulative oversize curve or as a histogram. ADVANTAGES: FARDAH FAROOQ 10 UNIT III MICROMERITICS This method is generally useful for coarser particles b/c measurement of sizes smaller than 50μm is difficult. DISADVANTAGES: i. A particle tends to aggregate during the process due to electrostatic charges. ii. Moisture can also lead to aggregation of powder and the actual particle size may not be obtained. iii. Attrition of particles during sieving lead to size reduction. iv. Sieve loading and duration of shaking can influence the results. SEDIMENTATION This technique is applicable for particle lying in the size range of 0.5-100μ. This technique is based on the sedimentation rate of particles. Several methods based on sedimentation are used. Most commonly used method is pipette method using Andresen apparatus. It consists of a cylindrical vessel having a nominal capacity of about 550ml. It has a vertical scale graduated from 0-20 cm. It also contain 10 ml pipette. For analysis 1 or 2 % suspension of powder in a medium containing a suitable deflocculating agent is introduced into the vessel. The vessel is shaken to distribute the particles uniformly throughout the suspension and the apparatus. At various time intervals, 10ml samples are withdrawn which are then evaporated and weighed or analyzed by appropriate means, correcting for deflocculating agent that has been added. FARDAH FAROOQ 11 UNIT III MICROMERITICS The sedimentation of the particles is governed by following stokes law equation. V V = rate of sedimentation, h= distance of fall in time "t" ρ s = density of the particle ρ f = density of the medium (fluid) g = acceleration due to gravity η = viscosity of the medium. The weight of each sample residue is called weight under size and the sum of successive weights is known as the cumulative weight under size. This is then plotted on the log scale against the particle diameter using a log probability graph paper. Various statistical diameters are then obtained from the plot. ADVANTAGES: The apparatus is inexpensive and technique is simple. The results obtained are precise provided the technique is adequately standardized. DISADVANTAGES: FARDAH FAROOQ 12 UNIT III MICROMERITICS This method is time consuming, since separate analysis is required for each experimental point on the distribution curve. IMPORTANCE OF MICROMETRICS IN PHARMACY The knowledge and control of the size and the size range of particles is of profound importance in pharmacy like.  The physical properties of powders such as bulk density and compressibility are dependent on particle size and size distribution for example the bulk density of light and heavy magnesium carbonate differs because of the difference in their particle size.  The rate of dissolution of poorly soluble drug is directly related to the size of particles. Generally decrease in size of particle increases the dissolution rate.  The chemical properties of particles such as the surface oxidation also depend on particle size.  The rate of absorption of drug and hence pharmacological activity depends on particle size of the drug material.  Elegance of the pharmaceutical - preparation such as emulsions, suspension, ointments, often depends upon particle size of the dispersed phase.  The drug release properties are also particle size dependent for example, cream, ointment, suppositories.  The stability of system such as colloids suspensions and emulsion depends on the particle size. Increase in particle size decreases the stability of these systems. FARDAH FAROOQ 13 UNIT III MICROMERITICS  Texture and color of certain drugs depends on the particle size for example, the difference in color of yellow and red mercuric oxide is due to difference in their particle size.  Pharmaceutical processes like extraction and drying are accelerated following the reduction in particle size of the material.  The adsorption capacity of a material increases by decreasing the particle size. FARDAH FAROOQ 14