The pH Meter

In use, initial calibration of the meter is accomplished by setting the PSW to the calibration point (usually pH 7.0), the switch to cal., and adjusting R for null indication of the meter. The switch is then set to use and pH is determined by adjusting the PSW until the meter nulls. Should the PSW have to be recalibrated (different pH electrodes, etc.), buffered solutions of known pH are used. 

Many degrees of sophistication exist in modern pH meters. Some are direct reading and operate on the basis of a voltage offset against a standard voltage reference in the instrument. 

Electrometric pH Determinations, John Wiley and Sons, New York. 

Bishop, E. and Dhaneshwar, R. G., 1963, Silver- and halide-ion responsive electrodes. Analyst 88 424-445. 

and Janz, G. J., 1961, Reference Electrodes, Academic Press, New York. 

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A pH electrode is normally standardized using two buffers one near a pH of 7 and one that is more acidic or basic depending on the sample s expected pH. The pH electrode is immersed in the first buffer, and the standardize or calibrate control is adjusted until the meter reads the correct pH. The electrode is placed in the second buffer, and the slope or temperature control is adjusted to the-buffer s pH. Some pH meters are equipped with a temperature compensation feature, allowing the pH meter to correct the measured pH for any change in temperature. In this case a thermistor is placed in the sample and connected to the pH meter. The temperature control is set to the solution s temperature, and the pH meter is calibrated using the calibrate and slope controls. If a change in the sample s temperature is indicated by the thermistor, the pH meter adjusts the slope of the calibration based on an assumed Nerstian response of 2.303RT/F. 

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 pH meter usually is coupled to a data recording device and often to a pneumatic or electric controller. The controller governs the addition of reagent so that the pH of the process stream is maintained at the desired level. 

Prepare the solutions and measure the pH at one temperature of the kinetic study. Of course, the pH meter and electrodes must be properly calibrated against standard buffers, all solutions being thermostated at the single temperature of measurement. Carry out the rate constant determinations at three or more tempertures do not measure the pH or change the solution composition at the additional temperatures. Determine from an Arrhenius plot of log against l/T. Then calculate Eqh using Eq. (6-37) or (6-39) and the appropriate values of AH and AH as discussed above. 

Prepare the solutions, thermostat them at the temperatures to be used in the rate study, and then adjust them all to the same pH value by the addition of small volumes of concentrated strong acid or base. The pH meter must be correctly calibrated at each temperature. Now carry out the kinetic study and calculate Eobs. Because this procedure has set d In (H )/d(l/T) = 0 experimentally, use Eq. (6-36) in the form = Eqh +... 

Prepare the solutions, thermostat them at the rate study temperatures, and measure the pH at each temperature, taking the correct precautions concerning calibration of the pH meter. Now convert each pH to (OH ), using (OH ) = A, /(H ), where... 

Similar to the pH meter, gas meters employ specific ion electrodes. The electrodes generate a potential proportional to the activity of a specific ion in solution. The calibration is achieved in standard solution and results read in mV or concentration in mg/L or ppm on the meter. The water can be adapted to monitor the concentration of carbon dioxide, hydrogen sulfide, ammonia, chloride, calcium, potassium and sodium to name a few. 

The pH of a solution can be measured by an instrument called a pH meter. A pH meter translates the H+ ion concentration of a solution into an electrical signal that is converted into either a digital display or a deflection on a meter that reads pH directly (Figure 13.4). Later, in Chapter 18, we will consider the principle on which the pH meter works. 

The general approach illustrated by Example 18.7 is widely used to determine equilibrium constants for solution reactions. The pH meter in particular can be used to determine acid or base equilibrium constants by measuring the pH of solutions containing known concentrations of weak acids or bases. Specific ion electrodes are readily adapted to the determination of solubility product constants. For example, a chloride ion electrode can be used to find [Cl-] in equilibrium with AgCl(s) and a known [Ag+]. From that information, Ksp of AgCl can be calculated. 

The Nernst equation shows that the glass electrode potential for a given pH value will be dependent upon the temperature of the solution. A pH meter, therefore, includes a biasing control so that the scale of the meter can be adjusted to correspond to the temperature of the solution under test. This may take the form of a manual control, calibrated in 0 C, and which is set to the temperature of the solution as determined with an ordinary mercury thermometer. In some instruments, arrangements are made for automatic temperature compensation by inserting a temperature probe (a resistance thermometer) into the solution, and the output from this is fed into the pH meter circuit. 

Mode of operation. Before use, it is obviously necessary to become familiar with the instruction manual issued with the pH meter it is proposed to employ, but the general procedure for making a pH measurement is similar for all instruments, and will follow a pattern such as that detailed below. 

Prepare the buffer solutions for calibration of the pH meter if these are not already available the potassium hydrogenphthalate buffer (pH 4), and the sodium tetraborate buffer (pH 9.2) are the most commonly used for calibration purposes. 

Measurement of pH was performed using a Metrohm model 691 pH meter equipped with a Metrohm combined LL micro pH glass electrode calibrated prior to use with pH = 2 and 9 buffers. The checkers found that adjustment to a lower pH led to product with higher amounts of inorganic impurities. The checkers also found that the use of pH paper results in different pH values as compared to the pH meter. 

The MUF resin pH was determined using pH meter model pH 340-A/SET l-MTM. The pH meter was calibrated before it was used to determine the pH of the resin. The viscosity was determined using the Cole-Parmer 98936-15 viscometer (R2 spindle, lOOrpm speed). The storage life was a test of shelf life of the MUF resin under the ambient environment. Resin was first stored at ambient room temperature. Viscosity of the resin was checked for every three to four days. The ratio of water that can be added into resin before it turned turbid or precipitated is called resin solubility. The resin solubility was determined by divide the weight of resin and the weight of water added into resin before it turned turbid or precipitated. The curing period of a resin was defined as the time period for the resin to be hardened after application in a 30°C and 1.0% of NH4CI powder (as hardener). 

A pH meter uses a probe to make electrical measurements between a reference solution containing acid at known concentration and the sample being tested. The pH meter is calibrated by dipping the probe in a solution of known pH and adjusting the meter to read the correct value. Then the probe is dipped into the sample solution, and the pH appears on a digital display. Example illustrates how to convert from pH measurements to ion concentrations. 

In the case of the capillary blood, it is extremely important that the specimen not be allowed to stand for extensive periods of time before centrifugation. If the blood is to be transferred to the pH meter, then the collecting tube is sealed at both ends during transportation. It is then aspirated into the pH instrument as soon as practicable since one needs a smooth even flow in order to aspirate a specimen into the conventional micro pH meter. After the whole blood has been sampled for various purposes, it is important that the remaining blood be centrifuged promptly. If not, it will clot. Subse-quentially, centrifuging with a clot will tend to hemolyze the blood. Erythrocytes will adhere to the wall and as they are pulled down by the clot, they will be ruptured. Those who do not observe these precautions will find that it is rather difficult to obtain unhemolyzed blood. 

As in normal potentiometry one uses and indicator electrode versus a reference electrode, the electrodes should, especially in pH measurements, be those recommended by the supplier of the pH meter in order to obtain a direct reading of the pH value displayed. In redox or other potential measurements any suitable reference electrode of known potential can be applied. However, a reference electrode is only suitable if a junction potential is excluded, e.g., an Ag-AgCl electrode in a solution of fixed Ag+ concentration or a calomel electrode in a saturated KC1 solution as a junction in many instances a direct contact of Cl" with the solution under test (possibly causing precipitation therein) is not allowed, so that an extra or so-called double junction with KN03 solution is required. Sometimes micro-electrodes or other adaptations of the surface are required. 

By calibration on pHb at Tc the pH meter scale expresses the voltage in pH units at that temperature, so difficulties may arise when the test solution is measured at a deviating temperature T. If we assume for the present that the true pH value is not significantly altered by temperature variation (see later) and that the error in the pH read from the scale bears a linear relationship to the relative temperature difference we can correct pH, by... 

Whether the assumption about this linear relationship can be used for the zero shift as such is doubtful the situation becomes more reliable if the internal and external reference electrodes are equal so that E°mer and °uter cancel, hence eqn. 2.95 becomes En = (- 2.3026RT/F) pHinncr. Therefore, the zero shift can be eliminated instrumentally by setting the mechanical zero of the pH meter to pHjnncr (if previously known). With a non-combined glass electrode the external... 

It is clear from the Nemst equation that the temperature of the solution affects the response slope (2.303A7//0 of the calibration curve. The electrode voltage changes linearly in relationship to changes in temperature at a given pH therefore, the pH of any solution is a function of its temperature. For example, the electrode response slope increases from 59.2mV/pH at 25°C to 61.5 mV/pH at a body temperature of 37°C. For modem pH sensing systems, a temperature probe is normally combined with the pH electrode. The pH meter with an automatic temperature compensation (ATC) function automatically corrects the pH value based on the temperature of the solution detected with the temperature probe. 

Accuracy is a measure of how close the result is to the true value while reproducibility or precision is a measure of how close a series of measurements on the same sample are to each other. The accuracy and reproducibility of pH measurements can be highly variable and are dependent on several factors electrode stability (drift and hysteresis), response slope/calibration curve, and accuracy of the pH meters. While some of these factors are determined by the properties of electrodes, some measures can be taken to improve measurement accuracy and reproducibility. 

Methods in which the cell potential for the sample solution is compared with that for one or more standards are rapid, simple and readily automated. The measurement of pH is the most common application of this type, one or more buffer solutions serving to calibrate the pH-meter (potentiometer). In all such measurements, calibration involves the evaluation of the constant in the equation... 

Figure 1.3. The pH scale showing the region unavailable to chemists before the development of the pH scale and the pH meter.

It would not be until the development of the pH meter and pH electrode that soil scientists had a good way to measure soil pH. In 1934, A. O. Beckman introduced the first pH meter and started a company to build and sell the meter. This sparked an intense study of soil pH and its relationship to plant nutrient availability [14],... 

The pH meter can be analog, digital, or, as mentioned earlier, attached to a computer. As shown in Figure 9.6, each electrode has a different connector illustrating the need to match electrode connector with meter. Regardless of the make, pH meters are usually robust and should need little maintenance. 

Measurements made using pH meters and electrodes are temperature-sensitive that is, the pH reading obtained depends on the temperature of the solution. Some pH meters have a temperature probe (see Figure 9.6 [A]), such that a temperature correction is automatically made during the measurement. However, if this is not the case, the pH meter must be set to the proper temperature if accurate measurements are to be obtained. 

This experiment is an acid-base titration similar to those performed in Chapters 4 and 5. It is the titration of 0.10 M HC1 with 0.10 M NaOH, as in Experiment 8, but a combination pH probe will be used to monitor the pH during the titration, as in Experiment 10. The pH meter to be used has an RS232 output to interface with a microcomputer. We will use this special feature of the pH meter to feed the pH data directly to a microcomputer in an example of data acquisition by computer, and observe the titration curve traced on the screen in real time. 

Prepare the pH meter and a combination pH electrode. Place the electrode in the beaker such that the tip of the electrode is immersed, but suspended with a clamp or pointed to the side of the beaker to avoid contact with the stirring bar. You may have to add some distilled water in order for the tip (including the contact to the reference electrode) to be completely immersed. 

The pH meter is standardized (calibrated) with the use of buffer solutions. Usually, two buffer solutions are used for maximum accuracy. The pH values for these solutions should bracket the pH value expected for the sample. For example, if the pH of a sample to be measured is expected to be 9.0, buffers of pH = 7.0 and pH = 10.0 should be used. Buffers with pH values of 4.0,7.0, and 10.0 are available commercially specifically for pH meter standardization. Alternatively, of course, homemade buffer solutions (see Chapter 5) may be used. In either case, when the pH electrode and reference electrode are immersed in the buffer solution being measured and the electrode leads are connected to the pH meter, the meter reading is electronically adjusted (refer to manufacturer s literature for specifics) to read the pH of this soluiton. The electrodes can then be immersed into the solution being tested and the pH directly determined. 

In order to use the pH electrode described above, two half-cells (probes) are needed—the pH electrode itself and a reference electrode, either the SCE or the silver-silver chloride electrode—and two connections are made to the pH meter. An alternative is combination pH electrode. This electrode incorporates both the reference probe and pH probe into a single probe and is usually made of epoxy plastic. It is by far the most popular electrode today for measuring pH. The reference portion is a silver-silver chloride reference. A drawing and a photograph of the combination pH electrode is given in Figure 14.7. 

FIGURE 14.9 Photographs of the electrical connections to the combination pH electrode. Left, the bnc type of connection. Right, two separate connections to be made to the pH meter. 

Obtain a pH meter and combination pH electrode. Standardize the pH meter. 

Once the electrodes have been prepared for a given aqueous-organic solvent, the pan determinations can be made at each temperature, either graphically or by direct reading on commercial pH meters calibrated for poH measurements. In this procedure the pH meter is used as a milli-voltmeter. A solution A (10 M HCl in the aqueous-organic solvent considered) is selected as the standard reference solution, its pan being calculated for any temperature according to the Debye-Hilckel formula. After the electrodes have been immersed in this solution, the... 

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