Pipette method

Figure 24 Viscosity of ice cream mix with different hydrocolloid types determined by the pipette method (flow time in seconds). Relation to interfacial hydration of fat globules (%H20).

When the volume of solvent required for the recrystallisation is large, compared to the quantity of solid material, initial removal of mother-liquor from the centrifuge tube by this capillary pipette method may be tedious. In such a case, centrifugation should be more prolonged to get more effective packing of the... 

Pipette Method—A widely used method for determining size-distribution consists in drawing samples of the suspension at various times from a fixed depth or from different depths at a fixed time.f Such samples may be drawn with a calibrated pipette. The samples are then evaporated to dryness and the residue weighed. Of the two possible methods for using the pipette, the one based on a fixed depth is used. The basic equations involved in the final computations of the data collected are given below. 

We present here two procedures for applying the pipette method The first used by the U. S. Department of Agriculture (Olmstead et al, 1930), and the other described by Keen (1931). Both of these methods pertain to the determination of silt and colloids in soil suspensions, rather than to a size-distribution. However, the descriptions given indicate the manner of using the pipette. 

Let us review briefly the meaning of the two procedures. The pipette method furnishes a means for measuring the diameter of particles settling at the same rate in accordance with Stokes law. The diameter is calcu-... 

A time chart for mechanical analyses by the pipette method. J. Sediment. Petrol., 5 93-95. 

A pipette method of mechanical analysis of soils based on improved dispersion procedure." U. S. Dept. Agr. Tech. Bull. No. 170. 

Modification of the pipette method of mechanical analysis. Proc. Soil Sci. Soc. America, 4 368-369. 

In the pipette method (Figure 7.7), concentration changes occurring within a settling suspension are followed by drawing off definite volumes, at predetermined times and known depths, by means of a pipette. The method was first described in 1922 by Robinson [14] who used a normal laboratory pipette. Various modifications were later suggested which complicated either the operating procedure or the apparatus [15]. Andreasen was the... 

Collecting and washing the crystals. Collect the crystals using the Pasteur pipette method or by vacuum filtration on a Hirsch funnel or a Buchner funnel. If the latter technique is employed, wet the filter paper with solvent, apply vacuum, break vacuum, add crystals and liquid, apply vacuum until solvent just disappears, break vacuum, add cold wash solvent, apply vacuum, and repeat until crystals are clean and filtrate comes through clear. 

A variant is the micro-pipette method, which is also similar to the maximum bubble pressure technique. A drop of the liquid to be studied is drawn by suction into the tip of a micropipette. The inner diameter of the pipette must be smaller than the radius of the drop the minimum suction pressure needed to force the droplet into the capillary can be related to the surface tension of the liquid, using the Young-Laplace equation [1.1.212). This technique can also be used to obtain interfacial tensions, say of individual emulsion droplets. Experimental problems include accounting for the extent of wetting of the inner lumen of the capillary, rate problems because of the time-dependence of surfactant (if any) adsorption on the capillary and, for narrow capillaries accounting for the work needed to bend the interface. Indeed, this method has also been used to measure bending moduli (sec. 1.15). 

Figs. 12-14. Methods for determining explosion limits. Fig. 12. Pipette method. Fig. 13. Compression method. Fig. 14. Capillary method. From ref. 22c. 

FIG. 21-18 Equipment used in the pipette method of size analysis. 

The Donnan pipette method can be very useful for comparing a number of detergents or for plotting the surface tension in relation to the concentration curve, but it fails to give all the information required because other factors such as redeposition must be taken into account in assessing the efficiency of a scouring agent. 

A simple sedimentation technique, which readily lends itself to the determination of crystal size distribution in the range 1-50 pm, is the Andreasen pipette method. Although it is generally better to prepare a fresh suspension of the crystals under test in a suitable inert liquid, it is possible to classify crystals suspended in their own mother liquor. If the difference in density between the particles and suspending liquid is <0.5gcm special care must be taken to avoid convection currents. The method, briefly, is as follows (BS 3406/2, 1986). 

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