Stat instrument

Recently a new pH-stat instrument (PHM290 with autoburet ABU901) was introduced by Radiometer International A/S. This instrument can be connected directly to a personal computer, so that titration data and pH can be recorded in a Windows-based program. A recording of corrosion potential must then be made by another instrument, but both titration data and potential measurements can be recorded simultaneously using two inputs. 

Fig. 6-1. Equipment for lepidocrocite synthesis 1. Inlet for base addition, 2, Glass electrode both 1 and 2 are connected to a pH stat instrument, 3, Reaction beaker, 4. Magnetic stirrer, 5. Magnet, 6, Porous ceramic block for air entry, 7, Thermometer,...

Point-of-care testing has now become feasible with the introduction of the handheld i-STAT instrument and similar portable devices, such as the optically based Reflotron. The i-STAT (Fig. 2) contains both potentiometric and amperometric sensors integrated into sensor arrays that are included within disposable cassettes [87-89]. Eight tests are possible with the most advanced cassette, the ECg+, and these are sodium, potassium, chloride. 

Cyclic voltammetry (CV), a widely used potential-dynamic electrochemical technique, can be employed to obtain qualitative and quantitative data about surface and solution electrochemical reactions including electrochemical kinetics, reaction reversibility, reaction mechanisms, electrocatalytical processes, and effects of electrode structures on these parameters. A potentio-stat instrument such as the Solatron 1287 is normally used to control the electrode potential. The CV measurement is normally conducted in a three-electrode configuration or electrochemical cell containing a working electrode, counter electrode, and reference electrode, as illustrated in Figure 7.1. However, with alternative configurations, CV measurements can also be performed using a two-electrode test cell. The electrolyte in the three-electrode cell is normally an aqueous or non-aqueous liquid solution. 

The potentiostatic technique discussed here involves the polarisation of a metal electrode at a series of predetermined constant potentials. Potentio-stats have been used in analytical chemistry for some time Hickling was the first to describe a mechanically controlled instrument and Roberts was the first to describe an electronically controlled instrument. Greene has discussed manual instruments and basic instrument requirements. 

The necessary equipment for a pH stat is easily available, but to obtain highly reliable measurmients with a minimum of fluctuations, the greatest care should be used to select instruments of especially high reliability and precision. In all work quoted here the following instruments from Radiometer International A/S were used ... 

In most voltammetry, the power source is a polarograph or potentiostat. The name potentiostat comes from the roots poten- (voltage) and stat- , implying holds steady . We will see later why such a steady potential is essential. A polarograph is an instrument (hence -graph , in this context) that polarizes. 

In addition to automated analysers for general use, sophisticated single purpose instruments have been developed and marketed, chiefly for clinical analyses (for example Astra 4 and Astra 8 from Beckman for the determination of sodium and potassium in blood or Orion Space-Stat SS-20 or SS-30 for the determination of calcium in blood). 

Donker H, Smit W, Blasse G (1988) Phys Stat Sol (b) 148 413-419 Donker H, Smit W, Blasse G (1989) J Electrochem Soc 136 3130-3135 Dorenbos P (2003) J Physics Condensed Matter 15 2645-2665 Dubessy J, Orlov R, McMiUan P (1994) In Marfunin A (ed) Advanced Mineralogy 2, Methods and Instrumentation. Springer, Berlin Heidelberg New York, pp 138-146... 

A pH-stat or automatic titration instrument greatly simplifies titrimetric analyses in that no buffer is required and the time course of tritrant addition may be available as a continuous recording (facilitating estimates of initial rates see Critical Parameters). However, one limitation of the automatic titrator compared to the... 

A related form of an automatic potentiometric titrator is instrumentation that permits the maintenance of the acidity or basicity of a solution over a period of time. Such devices are known as pH-stats, and find application in kinetic studies of hydrolysis reactions. The general approach is (by either manual or automatic means) to add either acid or base such that the pH in the solution is maintained constant over a period of time. Normally the amount of acid or base added as a function of time is sought in order that kinetic measurements may be made for the system. In its simplest form the acidity of the solution is monitored with a pH meter and controlled at a preselected value by the addition of acid or base from a burette the quantity delivered as a function of time is recorded in a notebook. Obviously for the fast reactions this becomes difficult and dependent on the dexterity of the individual. 

This demonstrative TAT data, along with a multitude of other process management information, is readily available from the TPID database. This information allows the user to monitor work load and productivity trending that can be used to adjust staffing and instrument usage to meet the demands of the day s work. When instruments are interfaced with the TPID software, locating a particular case of interest in any step of the histological process is made easier. This flexibility allows you to minimize the time required to locate and move to the front of the line a STAT case, whenever it is determined to be so. 

Quantifying Reactions 7.64 and 7.65 requires fixing the pH and partial pressure of 02 (p02) at some predetermined value and providing OH" upon demand. This is accomplished with a pH-stat technique. The technique utilizes a pH electrode as a sensor so that as OH" is consumed, (during Mn2+ oxidation), the instrument measures the rate of OH consumption and activates the autoburete to replace the consumed OH . It is assumed that for each OH" consumed, an equivalent amount of Mn2+ is oxidized. 

Automatic Recording Titrimeter Use an instrument operating in the pH stat mode and equipped with a jacketed titration cell (Radiometer Titralab, or equivalent). 

All government publications, including the Medicines Act and its subordinate legislation, are obtainable from Stationery Office bookshops. UK statutory instruments may also be found on the HMSO website at http //hmso.gov.uk/stat.htm... 

Potentiometry. Potentiometric methods rely on the logarithmic relationship between measured potential and analyte concentration. The most common involves an instrument called a pH-Stat , in which a glass (pH) electrode follows reactions that either consume or produce protons. Since pH changes cause changes in enzyme activity, the pH is maintained at a constant value by the addition of acid or base. The rate of titrant addition is then proportional to the rate of the enzymatic reaction. Precise measurements using the pH-Stat require low buffer concentrations in the enzymatic assay mixture. 

In titrimetric methods, LPS catalyzes the hydrolysis of fatty acids from an emulsion of olive oil or oleic acid. The fatty acids liberated are titrated with dilute allcali. Kinetic versions use an automated potentiometric titrator (an instrument commonly referred to as a "pH-stat ). The amount of aUcah used is recorded as a function of time and serves as a measure of fatty acid produced during the reaction. This method has been proposed as reference measurement procedure, useful in the evaluation of new LPS methods. ... 

Recent developments in instrumentation have included stat. profile equipment for point-of-care or bedside... 

A pH electrode connected to an autotitration instrument (pH meter, controller, and autoburet) is often used to maintain a constant pH (pH-stat) during the reaction. If it is desired to monitor pH changes during the reaction, then the pH meter can be connected to a recorder. It should be remembered I hat autotitration units have finite response times that are longer than many surface-controlled reactions. Thus, pH changes can occur before the autotitra-1 ion system can fully respond, and there will be some delay between the reac-lion and return to pH-stat conditions. 

Other biosensor-based diagnostic instruments, such as the enzyme-electrode-based analyzers of YSI for glucose and lactate, are utilized routinely in many clinical laboratories. i-Stat recently introduced a portable analyzer for bedside use which utilizes enzyme-electrode-based assays for glucose and urea, as well as chemical sensor tests for nitrogen, sodium, potassium, and chloride. 

Typical representatives of in situ operating Instruments without final transfer are those of the ASTRA (Automated STAT/Routlne Analyzer) series (Models 4 and 8), marketed by Beckman and featuring four or eight measuring cuvettes. They allow programming of a sequence of up to 80 samples with their corresponding Identification number and assay required at a rate of 70-85 samples/h and with a consumption of 8-50 pL of sample for the different assays. They use both photometric and potentiometrlc detectors. Routine analyses... 

The drawbacks of discrete analyzers are their mechanical complexity and high cost of operation. Sample cups, disposable cuvettes, rotors, and prepacked reagents increase the cost of individual assays above the acceptable limit for the strained budgets of most clinical laboratories. In addition, these machines are seldom used outside the clinical laboratory, because they are designed to handle three dozen of the most frequently required clinical tests. The advantages of the discrete approach are the ability of some of these instruments to perform assays via random access—which allows sequential assay of diverse analytes at will—and the capability of stat operation, which yields the analytical readout within 5-10 min after the machine has been switched on and a sample has been inserted by a technician. 

It should be added that if the FIA instrument is to be used only intermittently, yet always be ready for use (as for standby or stat operation), it is not necessary to wash it between each series of assays. The system should, however, be kept wet i.e., completely filled with reagent solutions (or, preferably, water) in all lines. 

The experimental apparatus for a potentiometric titration can be quite simple only a pH or millivolt meter, a beaker and magnetic stirrer, reference and indicator electrodes, and a burette for titrant delivery are really needed for manual titrations and point-by-point plotting. Automatic titrators are available that can deliver the titrant at a constant rate or in small incremental steps and stop delivery at a preset endpoint. The instrument delivers titrant until the potential difierence between the reference and indicator electrodes reaches a value predetermined by the analyst to be at, or very near, the equivalence point of the reaction. Alternatively, titrant can be delivered beyond the endpoint and the entire titration curve traced. Another approach to automatic potentiometric titration is to measure the amount of titrant required to maintain the indicator electrode at a constant potential. The titration curve is then a plot of volume of standard titrant added versus time, and is very useful, for example, for kinetic studies. The most extensive use of this approach has been in the biochemical area with so-called pH-stats—a combination of pH meter, electrodes, and automatic titrating equipment designed to maintain a constant pH. Many enzymes consume or release protons during an enzymatic reaction therefore, a plot of the volume of standard base (or acid) required to maintain a constant pH is a measure of the enzyme activity, the amount of enzyme present. 

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