Apparent volume

Figure 15.10. The dependence of apparent volume resistivity on time of polarisation of acrylic polymer (Perspex). (Reproduced by permission of ICI)...

The apparent volume resistivity is dependent on the polarisation time (Figure 15.10). The initial polarisation current is effective for some time and if only a short time is allowed before taking measurements low values for volume resistivity will be obtained. 

Porosity constitutes a important criterion in a description based on straining. Porosity is determined by the formula V /Vc, in which V c is the total or apparent volume limitated by the filter wall and is the free volume between the particles. The porosity of a filter layer changes as a function of the operation time of the filters. The grains become thicker because of the adherence of material removed from the water, whether by straining or by some other fixative mechanism of particles on the filtering sand. Simultaneously the interstices between the grains diminish in size. This effect assists the filtration process, in particular for slow sand filters, where a deposit is formed as a skin or layer of slime that has settled on the... 

We have pointed out above that the volume of the solid crystal is not a useful basis for comparing different salts in the same solvent. It is, however, quite satisfactory, when we are comparing the apparent volumes of the same salt in different solvents, as is being done in Fig. 59. 

The apparent clearance (Cl/F) and the apparent volume (Vd/F) estimates are related to the bioavailability after oral dosing. 

The apparent volume of distribution (Vd) slightly increases depending on plasma volume (Fp), tissue volume (Ft), and free tissue fraction (ft) whereas the half-life slightly decreases with significantly increasing free plasma fraction. 

Where excessive foaming occurs, the apparent volume of water present in the boiler drum increases (light water). The persistence of foam increases with increases in ... 

Calculate the apparent volume (in pnt1) and radius (in pm) of a helium atom as determined from the van der Waals parameters,... 

Having prepared the cements, a number of physical techniques were employed to study them. Porosity was determined by measuring the apparent volume and the solid volume, the former by straightforward linear measurement, the latter using a helium pycnometer. This technique was used to avoid the problems of dissolution that arise when water is used... 

Figure 4. Schematic description of the swelling process. The molecules of the swelling liquid start to penetrate inside the polymer framework from its surface (a) and to solvate the polymer chains. The polymer chain start to stretch out and to move away from one another the apparent volume of the polymer increases and the first nanopores are formed (b). Swelling stops when increasing elastic forces set up by the unfolding of the polymer chains counterbalance the forces which drive the molecules of the swelling agent into the polymer framework (c).

FIGURE 1. Change in the apparent volume of distribution of PCP as a function of time following administration of an intravenous bolus dose of 3H-PCP (6.4 pg) in a male dog (19.5 kg)... 

Gabapentin Modulate calcium channels and enhance GABA activity Loading dose Not recommended due to short half-life Maintenance dose 900-3600 mg/day in 3-4 divided doses (doses up to 1 0,000 mg/day have been tolerated) Half-life Not established 5-7 hours (proportional to creatinine clearance) Apparent volume of distribution 0.6-0.8 L/kg Protein binding less than 10% Primary elimination route Renal Drowsiness, sedation Peripheral edema, weight gain... 

Lamotrigine Modulate sodium channels Loading dose Not recommended due to increased risk of rash Maintenance dose 1 50-800 mg/day in 2-3 divided doses. Doses should be initiated and titrated according to the manufacturer s recommendations to reduce the risk of rash Half-life Not established Monotherapy 24 hours Concurrent enzyme inducers 12-15 hours Concurrent enzyme inhibitors 55-60 hours Apparent volume of distribution 1.1 L/kg Protein binding 55% Primary elimination route Hepatic Ataxia, drowsiness, headache, insomnia, sedation Rash... 

Levetiracetam Unknown Loading dose Not recommended due to excessive adverse effects Maintenance dose 1 000-3000 mg/day. Start at 1 000 mg/day and titrate upward as indicated by response Half-life Not established 6-8 hours Apparent volume of distribution 0.5-0.7 L/kg Protein binding less than 10% Primary elimination route 70% renal 30% hepatic Somnolence, dizziness Depression... 

Half-life Monotherapy 7-9 hours Concurrent enzyme inducers 2.5-4.5 hours Apparent volume of distribution 0.6-0.8 L/kg Protein binding 96% Primary elimination Not established Dizziness, somnolence, irritability, slowed thinking ... 

An important parameter of the one-compartment model is the apparent volume of the body compartment, because it directly determines the relationship between the plasma concentration and the amount of... 

A discussion of all the reasons for this phenomenon is beyond the scope of this chapter, but a simple example will illustrate the concept. Highly lipid-soluble drugs, such as pentobarbital, are preferentially distributed into adipose tissue. The result is that plasma concentrations are extremely low after distribution is complete. When the apparent volumes of distribution are calculated, they are frequently found to exceed total body volume, occasionally by a factor of 2 or more. This would be impossible if the concentration in the entire body compartment were equal to the plasma concentration. Thus, Vd is an empirically fabricated number relating the... 

A is a function of the two rate constants (ka and kei), the apparent volume of distribution (Vd), and the amount of drug absorbed (Dg). After ka and kei have been evaluated and A has been determined by extrapolation, a value for Vd can be calculated if it is assumed that Dg is equal to the dose administered, i.e., absorption is 100% complete. 

The extent of distribution to an organ is usually expressed as the apparent volume of distribution of the organ, k). Because the blood concentration is used as a reference, the apparent volume of an organ, fj, can usually be expressed in the following way ... 

Similarly, concepts of solvation must be employed in the measurement of equilibrium quantities to explain some anomalies, primarily the salting-out effect. Addition of an electrolyte to an aqueous solution of a non-electrolyte results in transfer of part of the water to the hydration sheath of the ion, decreasing the amount of free solvent, and the solubility of the nonelectrolyte decreases. This effect depends, however, on the electrolyte selected. In addition, the activity coefficient values (obtained, for example, by measuring the freezing point) can indicate the magnitude of hydration numbers. Exchange of the open structure of pure water for the more compact structure of the hydration sheath is the cause of lower compressibility of the electrolyte solution compared to pure water and of lower apparent volumes of the ions in solution in comparison with their effective volumes in the crystals. Again, this method yields the overall hydration number. 

Studies interested in the determination of macro pharmacokinetic parameters, such as total body clearance or the apparent volume of distribution, can be readily calculated from polyexponential equations such as Eq. (9) without assignment of a specific model structure. Parameters (i.e., Ah Xt) associated with such an equation are initially estimated by the method of residuals followed by nonlinear least squares regression analyses [30],... 

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