Automatic standardization, with meter

A pH meter is standardized with buffer solutions of known pH before a measurement of an unknown solution is taken. It should be noted from Equation 2.2 that the voltage depends on temperature. Hence, pH meters must have some means for temperature correction. Older instruments usually have a knob labeled temperature control, which is adjusted by the user to the temperature of the measured solution. Newer pH meters automatically display a temperature-corrected pH value. 

On some manufactured pH meters the standardization adjustment can be done automatically. This is a convenience feature when frequent standardizations are done to a single buffer point. This is accomplished by matching the electrode output to a buffer value which is set to zero millivolts with the zero control. For example, if frequent standardizations with pH 7.41 buffer solution are being performed, the zero control is adjusted until the display indicates 7.41 when in stand-by. When the electrodes are immersed in pH 7.41 buffer solution and the autostandardization mode is initiated, the meter searches for a zero potential by offsetting the... 

Whether manual or automatic standardization is employed, the criteria of this control are resolution, range, and stability. Often a meter which reads to 0.001 pH has two controls, coarse and fine, to provide the needed resolution. The range is typically 100 to 200 mV (>l-3 pH units) to offset aging electrodes with a high asymmetry potential or to standardize an ion-selective electrode to a particular value. The stability is indicated by the amplifier drift specification which is typically less than a millivolt in 12 hours. 

In addition, most devices provide operator control of settings for temperature and/or response slope, isopotential point, zero or standardization, and function (pH, mV, or monovalent—bivalent cation—anion). Microprocessors are incorporated in advanced-design meters to faciHtate caHbration, calculation of measurement parameters, and automatic temperature compensation. Furthermore, pH meters are provided with output connectors for continuous readout via a strip-chart recorder and often with binary-coded decimal output for computer interconnections or connection to a printer. Although the accuracy of the measurement is not increased by the use of a recorder, the readabiHty of the displayed pH (on analogue models) can be expanded, and recording provides a permanent record and also information on response and equiHbrium times during measurement (5). 

Specialized equipment for industrial measurements and automatic control have been developed (18) (see Process control). In general, the pH of an industrial process need not be controlled with great accuracy. Consequendy, frequent standardization of the cell assembly may be uimecessary. On the other hand, the ambient conditions, eg, temperature and humidity, under which the industrial control measurements are made, may be such that the pH meter must be much more robust than those intended for laboratory use. To avoid costiy downtime for repairs, pH instmments may be constmcted of modular units, permitting rapid removal and replacement of a defective subssembly. 

The concrete block walls of the cell housing the generator tube and associated components are 1.7 meters thick. The facility also includes a Kaman Nuclear dual-axis rotator assembly for simultaneous transfer and irradiation of reference and unknown sample, and a dual Na iodide (Nal) scintillation detector system designed for simultaneous counting of activated samples. Automatic transfer of samples between load station to the rotator assembly in front of the target, and back to the count station, is accomplished pneumatically by means of two 1.2cm (i.d.) polyethylene tubes which loop down at both ends of the system and pass underneath the concrete shielding thru a pipe duct. Total one-way traverse distance for the samples is approx 9 meters. In performing quantitative analysis for a particular element by neutron activation, the usual approach is to compare the count rates of an unknown sample with that of a reference standard of known compn irradiated under identical conditions... 

Lebel [224] has described an automated chelometric method for the determination of sulfate in seawater. This method utilises the potentiometric end-point method for back titration of excess barium against EDTA following precipitation of sulfate as barium sulfate. An amalgamated silver electrode was used in conjunction with a calomel reference electrode in an automatic titration assembly consisting of a 2.5 ml autoburette and a pH meter coupled to a recorder. Recovery of added sulfate was between 99 and 101%, and standard deviations of successive analyses were less than 0.5 of the mean. 

There are few models with automatic test capability. Testing is usually limited to hand held devices only 2 meters (7 ft.) from the detector or directly on the lens test unit. It can be ineffective if ice forms on the lens. It is sensitive to modulated emissions from hot black body sources. Most of the detectors have fixed sensitivities. The standard being under five seconds to a petroleum fire of 0.1 square meter (1.08 sq. ft.) located 20 meters (66 ft.) from the device. Response times increase as the distance increases. It cannot be used in locations where the ambient temperatures could reach up to 75 °C (167 °F). It is resistant to contaminants that could affect a UV detector. Its response is dependent on fires possessing a flicker characteristic so that detection of high pressure gas flames may be difficult. 

Let us dwell on Figure 6.4 for a moment. The standards and sample solutions are introduced to the instrument in a variety of ways. In the case of a pH meter and other electroanalytical instruments, the tips of one or two probes are immersed in the solution. In the case of an automatic digital Abbe refractometer (Chapter 15), a small quantity of the solution is placed on a prism at the bottom of a sample well inside the instrument. In an ordinary spectrophotometer (Chapters 7 and 8), the solution is held in a round (like a test tube) or square container called a cuvette, which fits in a holder inside the instrument. In an atomic absorption spectrophotometer (Chapter 9), or in instruments utilizing an autosampler, the solution is sucked or aspirated into the instrument from an external container. In a chromatograph (Chapters 12 and 13), the solution is injected into the instrument with the use of a small-volume syringe. Once inside, or otherwise in contact with the instrument, the instrument is designed to act on the solution. We now address the processes that occur inside the instrument in order to produce the electrical signal that is seen at the readout. 

Pipet 25.00 mL of the degassed soda pop into a 250-mL Erlenmeyer flask Set up the titration experiment with pH probe and pH meter on an automatic stirrer. The standardized 0.10 N NaOH is the titrant. 

The Corning Ion-meter 135 and the Orion 811 are representative examples of pH-meters with and without a built-in microprocessor, respectively. The latter shows the cell voltage at equilibrium. Its temperature sensor automatically corrects the error due to the difference in temperature between the standards and the sample. When the potential difference between two standards varies significantly from the theoretical expectation, the Instrument shows the deviation. The microprocessor of the Ion-meter 135 allows the automatic use of different measurement programs (e.g. standard additions, sample addition, pH determination). Data can be stored In the memory for subsequent calculations. The microprocessor can also be used Independently. 

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. 

The OSL-density meter (OSDM) measures the density of solutions to estimate accurately the mass of a solution contained in a sampled vessel/container. OSDM measures the frequency of an oscillating tube filled with the solution sample. The instrument is calibrated by measuring the frequency of oscillation using air, water, or liquid density standards. A sample solution is injected, or drawn, into the temperature-stabilized tube and a built-in computer automatically converts the frequency measurement into a density reading. 

Usually, production quantities are made up automatically. The standard automatic make-up plant will consist of a mixing vessel, into which water is admitted at a constant rate. The solid polymer is metered out from a hygroscopically secure hopper, using a screw feeder, into the incoming stream of make-up water. Some automatic systems use an air blower to convey the polymer, entering the air stream via a venturi, to a mixer, where the water enters with a cyclone action to keep the powder away from the mixer walls. From the mixer the product falls into a stirred ageing vessel. 

Cool the solution and add 100 mL of acetone. Titrate the solution potentiometrically with standard 0.01 M silver nitrate, using glass versus silver-silver chloride electrodes. If an automatic titrator, such as a Metrohm, is available, use the semi-micro 5-mL piston buret. If the titration is carried out with a manually-operated pH meter, use a 5-mL semi-micro buret which can be estimated to three decimal places in millilitres. 

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