Activity meter

Direct-reading meters suitable for use with ion-selective electrodes are available from a number of manufacturers they are sometimes referred to as ion activity meters. They are very similar in construction to pH meters, and most can in fact be used as a pH meter, but by virtue of the extended range of measurements for which they must be used (anions as well as cations, and doubly charged as well as singly charged ions), the circuitry is necessarily more complex and scale expansion facilities are included. They are commonly used in the millivolt mode. 

The use of a pH meter or an ion activity meter to measure the concentration of hydrogen ions or of some other ionic species in a solution is clearly an example of direct potentiometry. In view of the discussion in the preceding sections the procedure involved will be evident, and two examples will suffice to illustrate the experimental method. 

Procedure. Set up the ion activity meter in accordance with the manual supplied with the instrument. 

Fig. 2.15. Commercial potentiometers, (a) Metrohm ion activity meter 610 and pH-meter E 588 (b) Orion Model 611 pH meter and Model 811 microprocessor pH meter. 

The Basic Protocol describes the determination of water activity of a product using a chilled mirror dew-point water activity meter. Dew point is a primary measurement of vapor pressure that has been in use for decades (Harris, 1995). Dew-point instruments are accurate, fast, simple to use, and precise (Richard and Labuza, 1990 Snavely et al., 1990 Roa and Tapia de Daza, 1991). In a dew-point instrument, water activity is measured by equilibrating the liquid-phase water in the food sample with the vapor-phase water in the headspace, and then measuring the vapor pressure of the headspace. The basic principle involved in dew-point determinations of vapor pressure in air is that air may be cooled without change in water content until it saturates. The dew-point temperature is the temperature at which the air reaches saturation. It is determined in practice by measuring... 

Methods of assessing rebound and withdrawal in more clinically relevant terms have been available for many years (e.g., [8]). In the past decade, regulatory authorities have insisted on discontinuation studies after short-term use as well as long-term maintenance or relapse prevention. This applies to hypnotics in equal force despite most product licences now restricting hypnotic use to short-term (2 1 weeks) use. An adequate number of subjects, normal and insomniac, must be studied. Rebound needs assessment over more than one night, with a variety of techniques including PSG, activity meters, and subjective reports. Standard sleep questionnaires can be used for the subjective assessments. 

The present Section describes basic protocols satisfying ICH S7A recommendations for core battery CNS studies. Included are protocols for measuring general behavioral signs induced by test substances (Irwin Test), effects on spontaneous locomotion (Activity Meter Test), effects on neuromuscular coordination (Rotarod Test), effects on the convulsive threshold (Electroconvulsive Shock (ECS) Threshold and PTZ Seizure Tests), interaction with hypnotics (Barbital Interaction Test) and effects on the pain threshold (Hot Plate Test). 

Locomotor activity can be quantified in rodents by a variety of means. The method described below uses interruptions of photoelectric beams. The difference between Irwin estimations of drug effects on spontaneous activity and activity meter tests is mainly a question of quantification. Activity meter tests are generally carried out using automated apparatus with a larger number of animals, and are therefore less labor intensive but permit more precise statistical analyses. The quantitative data obtained enable the generation of dose-response curves and more precise estimations of the minimal effective dose (MED) or the dose which increases or decreases locomotion by 50 % (ED50). 

Another complication with activity meter tests is the phenomenon of habituation. All animals placed in an unknown environment will tend to explore it more at the beginning of the exposure with a decline in exploration with time. This can be clearly seen in most activity meters despite the apparent simplicity of the test environment. Thus, when locomotion is followed over sequential time slots, an apparent increase or decrease in drag effect over time cannot simply be interpreted as a change in its pharmacological activity. The substance may be interacting with the process of habituation. It is for this reason that activity meters are not the ideal means of measuring the duration of drag action. 

Because the activity meter evaluates spontaneous behavior, the behavioral baseline is intrinsically variable and subject to many kinds of influence, including lighting, apparatus cleanliness, ambient temperature, noise level, and even time of day. As a consequence, particular care has to be taken to ensure constant experimental conditions to obtain reproducible results. Furthermore, when comparing different drug treatments, it is important to take advantage of the fact that several animals can be evaluated simultaneously, by distributing the different treatments in a balanced fashion over the test period, and even in the positions within the experimental apparatus or of the observation chambers in the experimental room. 

More important is the time at which a test substance is administered in relation to placing the animal in the activity meter. Because of potential interactions of the test substance with the process of habituation, we think it is important to start testing when the drag effect has had time to reach its maximum, i.e. an appropriate interval after drag administration. If the animal is treated and immediately placed in the activity meter, the kind of drug effect observed may critically depend on the interaction between the onset of drag action and the habituation process. 

Fig. 1. Activity meter test. Effects of caffeine and chlorpromazine (p.o.) on locomotion in the rat.

The Aqualab CX2 water activity meter (Decagon Devices Inc., Pullman, WA) detects water condensation on a chilled mirror (dew point temperature). The instrument is sensitive to water activity units of <0.001. Readings take 5 min or less and are accurate to 0.003 water activity units. 

Water activity a ) determination a was measured at 25°C with ar Aqualab TE8255 water activity meter (Decagon Devices, Inc. Washington, USA). 

Reactions of considerable extent occur even in very dilute solutions of non-metals in alkali metals. Small concentrations of dissolved non-metals also influence physical properties of the molten metals. Very exact analyses are necessary to define the chemical potentials of non-metals in alkali metals. Oxygen can be removed from sodium, for instance, to such a degree, that only 0.1 to 0.01 wppm remain in solution. Electrochemical cells have the ability to estimate such extremely low concentrations. Carbon in liquid alkali metals, which are in contact with austenitic stainless steels, is in the same range of concentrations. Thus, only activity meters, based on gasanalytical devices or electrochemical cells, are able to measure such low carbon concentrations. There is still need for the development of analytical procedures to estimate nitrogen in alkali metals with the same sensitivity and accuracy. 

There is an old problem with activities - we carmot measure them directly (there is no lab instrument called activity meter) nor can we easily and correctly calculate them. Yet, with electrolytes and the increased number of particles in solution we have to figure out, even approximately, what the activities of the solutes are. So -as is often done in physical chemistry - the problem is split into an easy part and a difficult part. Activity is expressed as a product of concentration, either molar, M, or molal, m, and something called activity coefficient, y. The easy part is of course concentration, M or m, and the difficult part is the activity coefficient, y. 

Selective activity meter Hyperactivity More aggressive Excessive stereotyped self grooming Sauerhoff and Michaelson, 1973... 

Varimex activity meter activity Hyperactivity at 6-8 weeks, normal at 12 weeks Dubas and Hrdina, 1978... 

There was an activity meter fitted in the stack, or chimney, through which the air blown through the reactor was vented. Early on the Thursday morning, there was a sharp increase in its reading. This was noted by the physicist who was then on duty, but no special action was taken because he regarded it as the normal consequence of the first movement of the air through the pile and up the stack (chimney). This increase was followed by a steady drop in the curve for about two and a half hours after which time stack activity rose steadily. 

Fig.1. A Schematic diagram of a measuring set-up. B Ion activity meter with all utensils needed for a quantitative determination. (Depending on the calibration, the indicated ion content of the sample solution can be read off of the logarithmically divided scale directly in M, meq/1, g/1, ppm, etc.)...

---