Measurement pipette method

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 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). 

Table 2.12. Measurement of the particle size distribution of a sample of precipitated calcium carbonate by the Andreasen pipette method...

Instrumental techniques for measurement of particle size distribution of powders have had a tremendous advancement in recent times. Numerous methods and procedures have been developed at a steady pace over the years, and there is the possibility of covering the wide spectrum from nanosystems, to ultrafine powders, and to coarse particulate assemblies. Many instruments offer nowadays quick, reliable results for a wide variety of powders and particulate systems, and for a number of applications. There is still, however, the need to understand the basic principles under which sophisticated instruments operate, as well as to resource to direct measurements under some circumstances. Some of the most modern instrumental techniques are based on an indirect measurement and carry out transformations among the different ways of expressing particles size distributions, that is, by number, surface, or mass. Sometimes it is advisable to avoid transformations because instruments assume a constant shape coefficient on such transformation, which is not necessarily the case, and overestimation or underestimations of size of certain particles may arise. Also, in very specific applications, or in cases of basic or applied research, is better to measure directly the most relevant particle size and particle size distribution. For example, if research is carried out in modeling of solid-liquid separations, a direct measurement of the Stokes equivalent diameter would be most appropriate. The aim of the exercise is to measure the particle size distribution of a sample of medium-sized dolomite, and compare the results with those of the Andreasen Pipette method. 

In some cases, we apply an observation method that in a similar way leads to the introduction of a random error in the final outcome. For example, we always read 0.1 mL if we use a 10 mL measuring pipette, calibrated in tenths of a millilitre. Acmally we round off the observation to one decimal place without mentioning or realising that. 

Sedimentation balances and (3 back-scattering techniques have been used for the cumulative measurements while sampling (Andreasen pipette method) or the absorption of radiation (photosedimentation or use of X-rays) are most frequently used for the incremental measurements. It seems that the incremental techniques, because of their advantages of easy operation and evaluation, and relatively simple instrumentation, have somewhat wider application. 

An improvement of the Andreasen pipette method is to use a pan attached to a sensitive balance which records the changes in weight of the pan as an increasing amount of suspending particles settle on it. Later, sedimentation techniques using light extinction by changes in turbidity of the suspension and x-ray were introduced for more sensitive and rapid measurements. 

Berg, S. Determination of particle size distribution by examining gravitational and centrifugal sedimentation to the pipett method and with divers. Symp. PSA, June 1958, Boston, ASTM STP 234 (1959), p. 143 - 171 /4/ Chung, H. S. Hogg, R. The effect of Brownian motion on particle size analysis by sedimentation. Powder Techn. 41 (1985) 3, p. 211 - 216 /5/ Allen, T. Sedimentation techniques of particle size measurement. Conf. PSA Sept. 1985, Bradford, Proceed, p. 24 - 45... 

This method relies on the simple principle that the flow of ions into an electrolyte-filled micropipette as it nears a surface is dependent on the distance between the sample and the mouth of the pipette [211] (figure B 1.19.40). The probe height can then be used to maintain a constant current flow (of ions) into the micropipette, and the technique fiinctions as a non-contact imaging method. Alternatively, the height can be held constant and the measured ion current used to generate the image. This latter approach has, for example, been used to probe ion flows tlirough chaimels in membranes. The lateral resolution obtainable by this method depends on the diameter of the micropipette. Values of 200 nm have been reported. 

Electrophysiological Experiments. Guinea pig myocardial cells prepared as described previously 24) were superfused at 37 C with a Tyrode solution. Electrical properties of the myocytes were examined by the patch-clamp methods (25) using fire-polished pipettes. The current was measured by means of a patch-clamp amplifier, stored on the tape through a digital PCM data recording system, and analyzed with a computer. 

It was shown later that a mass transfer rate sufficiently high to measure the rate constant of potassium transfer [reaction (10a)] under steady-state conditions can be obtained using nanometer-sized pipettes (r < 250 nm) [8a]. Assuming uniform accessibility of the ITIES, the standard rate constant (k°) and transfer coefficient (a) were found by fitting the experimental data to Eq. (7) (Fig. 8). (Alternatively, the kinetic parameters of the interfacial reaction can be evaluated by the three-point method, i.e., the half-wave potential, iii/2, and two quartile potentials, and ii3/4 [8a,27].) A number of voltam-mograms obtained at 5-250 nm pipettes yielded similar values of kinetic parameters, = 1.3 0.6 cm/s, and a = 0.4 0.1. Importantly, no apparent correlation was found between the measured rate constant and the pipette size. The mass transfer coefficient for a 10 nm-radius pipette is > 10 cm/s (assuming D = 10 cm /s). Thus the upper limit for the determinable heterogeneous rate constant is at least 50 cm/s. 

Capsules were equilibrated with a tracer solution overnight. A capsule pellet (0.2-0.5 ml) was then placed in 5 ml test buffer (PBS or RPMI-1640 medium, Gib-co/BRL, New York, NY) on a shaker and a 0.2-ml aliquot was immediately sampled by a screen-protected pipette with further samples being taken over the next 700 s. The tracer quantity was assayed using the methods described below. A final sample was taken after the capsules has been in contact with the buffer for several hours (equilibrated tracer quantity) and the increment to the tracer concentration at each time was calculated. From the progress of tracer to equilibrium on a semilog plot a slope denoted as the zero -order rate flux constant was obtained and has been used as a measure of capsule permeability. [3H] -Glucose (580 daltons),insulin (6.2 kDa), and ovalbumin (45 kDa) have been used as tracers. Radioactivity was measured by means of a Packard 2000CA Liquid Scintillation Counter (Packard Instruments,... 

Using measurements of different half-cell combinations, a set of standard reduction potentials may be constructed. This set will be similar to a table of standard reduction potentials. The solutions used in the half-cells must be of known concentration. These solutions are produced by weighing reagents and diluting to volume. The measurements will require a balance and a volumetric flask. It is also possible to produce known concentrations by diluting solutions. This method requires a pipette and a volumetric flask. Review the Solutions and Colligative Properties chapter for solution techniques. 

In a similar manner, the calibration of glassware, such as volumetric flasks, pipettes, burettes, measuring cylindres are duly carried out by specific methods recommended by Indian Standards Institution (ISI), British Standards Institution (BSI), National Physical Laboratory (NPL), United States Pharmacopoeia (USP) at specified temperatures (See Chapter 2). 

Information on particle size may be obtained from the sedimentation of particles in dilute suspensions. The use of pipette techniques can be rather tedious and care is required to ensure that measurements are sufficiently precise. Instruments such as X-ray or photo-sedimentometers serve to automate this method in a non-intrusive manner. The attenuation of a narrow collimated beam of radiation passing horizontally through a sample of suspension is related to the mass of solid material in the path of the beam. This attenuation can be monitored at a fixed height in the suspension, or can be monitored as the beam is raised at a known rate. This latter procedure serves to reduce the time required to obtain sufficient data from which the particle size distribution may be calculated. This technique is limited to the analysis of particles whose settling behaviour follows Stokes law, as discussed in Section 3.3.4, and to conditions where any diffusive motion of particles is negligible. 

Procedure (sample analysis). Dilute an aliquot of the extract from Method 6.2a. to give (s= 25 ml) a solution containing 220-248 pg P2O3 mh (96-108 pg P mh ) at 20°C. Thus for super triplephosphate by the AOAC extraction, dilute 13 ml extract to 100 ml and for the SI 1996 No. 1342 method, dilute 5.2 ml extract to 100 ml. Pipette 25 ml of this solution into a 100-ml volumetric flask, add 25 ml vanadium molybdate (20°C), make up to the mark with water (20°C), mix and stand for 10 min. Simultaneously prepare a fresh 50 pg PjOj mh (21.8 pg P mh ) reference standard against which the absorbance of the samples are measured. 

The method was checked by analysis according to Kjeldahl (reduction of the fixed nitrogen to ammonia) and by analysis of measured amounts of nitric oxide let into the cylinder from a pipette. 

Some techniques are known to provide higher variability than others. The choice of an appropriate method at the outset can improve precision. For example, a volume of less than 20 mL can be measured more accurately and precisely with a syringe than with a pipette. Large volumes are amenable to precise handling but result in dilution that lowers sensitivity. The goal should be to choose a combination of sample preparation and analytical instrumentation that reduces both the number of sample preparative steps and the RSD. Automated techniques with less manual handling tend to have higher precision. 

Frequently used single-point viscosity tests in the starch plant are orifice pipettes,56 orifice funnels,57 the Hot Scott viscometer, and various methods to determine alkaline fluidity.58 For absolute measurements of the rheological properties, rotating viscometers with coaxial cylinders are used.59 The paper industry uses mainly the Brookfield viscometer and the Hercules viscometer for determining shear-dependent viscosity, pseudoplasticity, and thixotropy. Oscillatory and capillary viscometers are used for more detailed viscosity characterization, such as yield value, elastic properties, and viscoelasticity.60... 

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