Potentiometric recorder

Recorder potentiometric strip chart, 0-1 millivolt, 1 second F.S. response, with integrator... 

Potential measurements were made on cells A, B, C, and D with a Model E436 Metrohm Potentiograph recording potentiometric titrator. The sensitivity was set to 50-mV full scale so that potentials were readable to 0.1 mV. The glass... 

In Fe(II)-dichromate titrations, Winter and Moyer observed a time dependence of the potential after the end point. When potential readings were taken soon after each addition, an asymmetrical titration curve was observed, but when a time interval of 10 to 15 min was allowed after each addition, the curve approached the theoretical shape. We have noted that automatically recorded titration curves for the Fe(II)-dichromate titration show a considerably smaller potential jump than manually observed curves, the difference being due to lower potentials after the end point. But curves plotted with 15 s of waiting for each point differed only slightly from curves plotted with 150 s of waiting. Ross and Shain also studied the drift in potential of platinum electrodes with time and noted hysteresis effects in recorded potentiometric titration curves. These effects, due to oxidation and reduction of the platinum surface, are discussed below. 

CHO-CHOCOO" + COj CO2 + H2O - HCO3" + H. Graph (a) shows experiments in which proton uptake is followed spectrophotometrically in the presence of cresol red and graph (i>) shows proton uptake recorded potentiometrically. Both sets of experiments show that free CO2 is liberated by carboxyligase. In the absence of carbonic anhydrase the second reaction is slow. 

Proceed with the automatic titration and record potentiometric curves or derivative curves as the case may be. 

Piezoelectric biosensors exploit variations in the resonance frequency of a quartz crystal. In order to vibrate the crystal, an oscillator is connected to electrodes placed on both sides of the crystaL The vibrational frequency of the ciystal is monitored using a frequency meter, which has a resolution of about 0.1 Hz and displays the results directly. The data can also be recorded potentiometrically using a digital to analog converter. 

The significance of amine value varies with the product. The procedure below, based on simple titration to a visual end point, is useful for quality control of a well-characterized material. For quaternaries, a high amine value may indicate that the compound is not completely neutralized, or it may indicate that primary, secondary, or tertiary amine is present. For new products, it is preferable to conduct the analysis with a recording potentiometric titrator. In the case of a quaternary amine hydroxide, two inflections may be observed. The first is due to the strongly basic quaternary, the second to more weakly basic amines. 

The significance of amine value varies with the product. The procedure given in Chapter 3 for analysis of cationic surfactants is based on simple titration to a visual end point and is useful for quality control of a well-characterized material. For new products, it is preferable to conduct the analysis with a recording potentiometric titrator. 

Experimental arrangement for recording a potentiometric redox titration curve. 

The potentiometric titration curve shown here was recorded on a 0.4300-g sample of a purified amino acid that was dissolved in 50.00 ml of water and titrated with 0.1036 M NaOH. Identify the amino acid from the possibilities listed in the following table. 

Ey and E2 are the indicator electrodes. These may consist of a tungsten pair for a biamperometric end point for an amperometric end point they may both be of platinum foil or one can be platinum and the other a saturated calomel reference electrode. The voltage impressed upon the indicator electrodes is supplied by battery B (ca 1.5 volts) via a variable resistance Rs N records the indicator current. For a potentiometric end point Ey and E2 may consist of either platinum-tungsten bimetallic electrodes, or Ey may be an S.C.E. and E2... 

To measure the e.m.f. the electrode system must be connected to a potentiometer or to an electronic voltmeter if the indicator electrode is a membrane electrode (e.g. a glass electrode), then a simple potentiometer is unsuitable and either a pH meter or a selective-ion meter must be employed the meter readings may give directly the varying pH (or pM) values as titration proceeds, or the meter may be used in the millivoltmeter mode, so that e.m.f. values are recorded. Used as a millivoltmeter, such meters can be used with almost any electrode assembly to record the results of many different types of potentiometric titrations, and in many cases the instruments have provision for connection to a recorder so that a continuous record of the titration results can be obtained, i.e. a titration curve is produced. 

In such reactions, even though the indicator electrode functions reversibly, the maximum value of AE/AV will not occur exactly at the stoichiometric equivalence point. The resulting titration error (difference between end point and equivalence point) can be calculated or can be determined by experiment and a correction applied. The titration error is small when the potential change at the equivalence point is large. With most of the reactions used in potentiometric analysis, the titration error is usually small enough to be neglected. It is assumed that sufficient time is allowed for the electrodes to reach equilibrium before a reading is recorded. 

Titrations can be carried out in cases in which the solubility relations are such that potentiometric or visual indicator methods are unsatisfactory for example, when the reaction product is markedly soluble (precipitation titration) or appreciably hydrolysed (acid-base titration). This is because the readings near the equivalence point have no special significance in amperometric titrations. Readings are recorded in regions where there is excess of titrant, or of reagent, at which points the solubility or hydrolysis is suppressed by the Mass Action effect the point of intersection of these lines gives the equivalence point. 

Unfortunately, neither the computer nor the potentiometric recorder measures the primary variable, volume of mobile phase, but does measure the secondary variable, time. This places stringent demands on the LC pump as the necessary accurate and proportional relationship between time and volume flow depends on a constant flow rate. Thus, peak area measurements should never be made unless a good quality pump is used to control the mobile phase flow rate. Furthermore, the pump must be a constant flow pump and not a constant pressure pump. 

When measuring a signal, one records the magnitude of the output or the response of a measurement device as a function of an independent variable. For instance, in chromatography the signal of a Flame Ionization Detector (FID) is measured as a function of time. In spectrometry the signal of a photomultiplier or diode array is measured as a function of the wavelength. In a potentiometric titration the current of an electrode is measured as a function of the added volume of titrant. 

In routine analysis, often a one-dimensional so-called end-point titration can be automatically carried out up to a pre-set pH or potential value and with a previously chosen overall titration velocity in order to avoid overshoot, the inflection point should be sufficiently sharp and the titrant delivery must automatically diminish on the approach to that point in order to maintain equilibrium, and stop in time at the pre-set value. For instance, the Metrohm 526 end-point titrator changes both the dosing pulse length and its velocity by means of a pulse regulator in accordance with the course of the titration curve in fact, the instrument follows the titration two-dimensionally, but finally reports only a one-dimensional result. The Radiometer ETS 822 end-point titration system offers similar possibilities. However, automated titrations mostly represent examples of a two-dimensional so-called eqilibrium titration, where the titration velocity is inversely proportional to the steepness of the potentiometric titration curve hence the first derivative of the curve can usually also be recorded as a more accurate means of determining the inflection... 

Considering the related Mettler DL 40 and DL 40 RC MemoTitrators, the DL 40 can be used for ten different volumetric and potentiometric methods titration to a pre-selected absolute (EPA) or relative (EPR) end-point equilibrium titration (EQU)-recording titration (REC)-incremental titration (INC) Karl Fischer water determination (KF)-controlled dispensing (DOSE) pH and pX measurements (pX/E) multi-level titrations and back-titrations with automatic calculation (CALC) and manual titration (MAN)-automatic calibration of electrodes (CAL). 

Differential ionic chromatography. In a potentiometric method for recording ion-exchange elution curves, a dual-channel membrane cell is used as a differential detector186 for following the eluate composition in comparison with the eluent. In the chromatography of alkali metal ions over a... 

After passing through the column, the separated solutes are sensed by an in-line detector. The output of the detector is an electrical signal, the variation of which is displayed on a potentiometric recorder, a computing integrator or a vdu screen. Most of the popular detectors in hplc are selective devices, which means that they may not respond to all of the solutes that are present in a mixture. At present there is no universal detector for hplc that can compare with the sensitivity and performance of the flame ionisation detector used in gas chromatography. Some solutes are not easy to detect in hplc, and have to be converted into a detectable form after they emerge from the column. This approach is called post-column derivatisation. 

Chloride can also be estimated by potentiometric titration using standard silver nitrate [27]. The results are recorded directly and evaluated by means of a computer program based on the Gran extrapolation method. The determinations have a precision of 0.02% and since many samples can be titrated simultaneously, the time for a single determination can be reduced to less than 5 min. 

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. 

Potentiometric titrations are readily automated by using a motor-driven syringe or an automatic burette coupled to a chart recorder or digital printout system. This is described in more detail in Chapter 12. A micro-processor-controlled titrator is discussed in Chapter 13. 

Figure 16.2 illustrates a typical assembly for carrying out a potentiometric titration. Broadly speaking, the titration essentially comprises of measuring and subsequently recording a cell potential in terms of either mV or pH, after each sequentially known addition of reagents. 

In general, the signal from a gas chromatograph is recorded continuously as a function of time by means of a potentiometric device. Most frequently, a recorder of 1-10 mV full-scale deflection ( 10 inches) and having a response time 1 second or less is quite adequate. 

The signal emerging from the detector of a HPLC is recorded continuously as function of time most commonly with the help of a potentiometric recorder. Invariably, a recorder of 1 to 10 mV full-scale deflec-... 

TTie pH signal is fed via a pH meter to a potentiometric recorder and to a neutrahzation controller, which compares the electrode e.m.f. with pre-selected values and opens or closes gas-control valves accordingly. Depending on the pH, it is possible to obtain a flow of air, ammonia or a mixture of both gases. By careful adjustment of the potentiometers and the flow-rates of ammonia and air, it is possible to control the final pH and also to keep the digest within a fairly closely defined pH range during the neutrahzation process. 

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