Junction sample-electrode

Figure 18.3—Principle of ISE measurement of fluoride ions in solution using a double junction reference electrode. The reference electrode is inserted into a separate chamber that contains the auxiliary electrolyte in order to avoid osmosis of KC1 into the sample solution. Also, 1 M KN03 can be used for F , Cl, CN or Ag+ determination. The measurement involves the use of a high impedance millivoltmeter (pH meter type). A version of an all-solid fluoride electrode is shown on the right.

A measurement method based on an ion-selective electrode determination of chloride is suggested. This method is reasonably simple, rugged, and can be run for long times without attention. Several variations of this technique are considered. One variation is a conventional electrode measurement except that the flowing-junction reference electrode can be replaced by a fluoride ion selective electrode as a nonflowing reference. Another method was developed and evaluated for the measurement of very small aerosol samples the measurement of chloride ion concentration is made on 50-/>tL samples. When the major-constituent composition of sea salt is known, the total aerosol weight can be calculated. The small volume allows measurable concentrations to be attained with short sample collection times. This technique is described in detail later. 

Tests for S include the antimony test, silver-silver sulfide electrode test and the lead acetate paper and silver foil tests. In the antimony test, the color produced by treating a 200 ml test sample with 0.5 ml saturated potassium antimony tartrate and 0.5 ml 6 N HCl is compared with colors produced when solutions containing known amounts of S are treated in the same way. In the silver-silver sulfide electrode test, the test sample is diluted 1 1 with an alkaline solution of an oxidizing agent and the electrode potential relative to a double-junction reference electrode is measured. 

The salt analysis consisted of soaking the reference sample in deionised water for 8h, to redissolve all the salt, then determining the salt concentration in the deionised water by titration with silver nitrate (0-0.5 molar) using an Orion EA940 expandable ion analyser link to a single junction reference electrode. Reproducibility of the salt measurement using this equipment was 0.001 mg/cm2. 

The electrode that houses the ion-selective membrane, the inner electrolyte, and the irmer reference electrode is called the indicator electrode or working electrode. If the external reference electrode contains an interchangeable bridge electrolyte, it is often termed a double junction reference electrode to indicate that two separate hq-uid junction potentials are present (one between the reference electrolyte and the bridge electrolyte, and the second between the bridge electrolyte and the sample solution). The use of combination pH glass... 

When thinking about the accuracy and precision of direct potentiometry, it must be kept in mind that 0.5 mV difference in cell voltage translates to almost 2% difference in sample concentration for univalent ions. For divalent ions, it is close to 4%, while for a trivalent ion, in which case S 20mV/decade, it is close to 6%. Therefore, for high accuracy, very well controlled measuring parameters such as cell temperature, junction potential, electrode selectivity, and so on, are needed. 

The limited number of well functioning, classical or spectrophotometric methods is available for measuring fluoride ion concentration in different samples. Therefore, after the invention of lanthanum fluoride crystal-based ISE [15], its use as a detector in standardized methods becomes almost general. For example, the Environmental Protection Agency (ERA) METHOD 9214 [44] is for measuring the concentration of fluoride ions in water samples as well as in soil extracts. It is a direct potentiometric method using the ion-selective fluoride electrode and the conventional or double junction reference electrode. The lower limit of detection is 0.025 mg dm. Fluoride concentrations from 0.025 to 500 mg dm can be measured. 

EPA METHOD 9212 [52] for measuring the chloride ion concentration in water samples as well as in soil extracts. In this direct potentiometric method, the ion-selective chloride electrode and double junction reference electrodes are used. The dynamic range of the analysis is between 2 and 1000 mg/dm . ... 

The methods of reducing the liquid junction potential have already been discussed in Chapter 3. Basically, they include proper selection of the junction or altering the filling solution. Most often, a sleeve junction reference electrode will afford the greatest stability and the least junction potential in this type of sample. It is also very easy to clean, which becomes important with a sticky-type sample that is apt to clog the normal fiber-type junction. 

If the sample solution might react with chloride ions, for example silver or lead salts, then a double junction reference electrode may be used, with an additional liquid junction of KNO3. 

Other problems occur in the measurement of pH in unbuffered, low ionic strength media such as wet deposition (acid rain) and natural freshwaters (see Airpollution Groundwatermonitoring) (13). In these cases, studies have demonstrated that the principal sources of the measurement errors are associated with the performance of the reference electrode Hquid junction, changes in the sample pH during storage, and the nature of the standards used in caHbration. Considerable care must be exercised in all aspects of the measurement process to assure the quaHty of the pH values on these types of samples. 

Samples that contain suspended matter are among the most difficult types from which to obtain accurate pH readings because of the so-called suspension effect, ie, the suspended particles produce abnormal Hquid-junction potentials at the reference electrode (16). This effect is especially noticeable with soil slurries, pastes, and other types of colloidal suspensions. In the case of a slurry that separates into two layers, pH differences of several units may result, depending on the placement of the electrodes in the layers. Internal consistency is achieved by pH measurement using carefully prescribed measurement protocols, as has been used in the determination of soil pH (17). 

Changes in the reference electrode junction potential result from differences in the composition of die sample and standard solutions (e.g., upon switching from whole blood samples to aqueous calibrants). One approach to alleviate this problem is to use an intermediate salt bridge, with a solution (in the bridge) of ions of nearly equal mobility (e.g., concentrated KC1). Standard solutions with an electrolyte composition similar to that of the sample are also desirable. These precautions, however, will not eliminate the problem completely. Other approaches to address this and other changes in the cell constant have been reviewed (13). 

A representative ISE is shown schematically in Fig. 1. The electrode consists of a membrane, an internal reference electrolyte of fixed activity, (ai)i , ai and an internal reference electrode. The ISE is immersed in sample solution that contains analyte of some activity, (ajXampie and into which an external reference electrode is also immersed. The potential measured by the pH/mV meter (Eoe,) is equal to the difference in potential between the internal (Eraf.int) and external (Eref.ext) reference electrodes, plus the membrane potential (E emb), plus the liquid junction potential... 

E,j) that exists at the junction between the external reference electrode and the sample solution. 

The commercial SCE depicted in Fig. 18a.4 is generally an H-cell. One arm contains mercury covered by a layer of mercury(II) chloride (calomel). This is in contact with a saturated solution of potassium chloride a porous frit is used for the junction between the reference electrode solution and the sample solution at the end of the other arm. Similar to the silver/silver chloride reference system, a calomel electrode also warrants precautionary measures to maintain the chloride concentration in the reference electrode. 

The EMF across the entire potentiometric cell, shown in Fig. 18a.l, is the sum of the individual potentials that include the reference electrode potential and other sample-independent potentials (Econst), the liquid junction potential (Ej) and the membrane potential (EM) ... 

A second approach is the standard addition method, which is commonly employed when the sample is unknown. The potential of the electrode is measured before and after addition of a small volume of a standard to the known volume of the sample. The small volume is used to minimize the dilution effect. The change in the response is related only to the change in the activity of the primary ion. This method is based on the assumptions that the addition does not alter the ionic strength and the activity coefficient of the analyte. It also assumes that the added standard does not significantly change the junction potential. 

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