Filling solutions

Fig. 7. Schematic diagram of digoxin antibody sensing electrode (a) PVC membrane containing digoxin-carrier conjugate (b) inner filling solution, 0.01 M KCI (c) plasticizer, dibutyl sebacate (d) digoxin antibodies. (From 152, with permission)...

Another parameter essential for quantitative applications of micropipettes is the internal ohmic resistance, R. It is largely determined by the solution resistance inside the narrow shaft of the pipette, and can be minimized by producing short (patch-type) pipettes. The micropipette resistance has been evaluated from AC impedance measurements. Beattie et al. measured the resistance of micropipettes filled with aqueous KCl solutions (0.01, 0.1, and 1 M) [18b]. The value obtained for a 3.5/am-radius pipette was within the range from 10 to 10 As expected, the tip resistance was inversely proportional to the concentration of KCl in the filling solution. In ref. 18b, the effect of pipette radius on the tip resistance was evaluated using a constant concentration of KCl. The pipette resistance varied inversely with the tip radius. The iR drop was found to be 4.5-8 mV for the pipette radii of 0.6 to 19/rm when 10 mM KCl was used. 

In experiments with dual-pipette electrodes, the purpose of silanizing the outer glass wall is to prevent the formation of an aqueous film between the two pipettes and mixing of the filling solutions. The procedure is similar to the above except that argon should be passed through both barrels of the pipette. 

Another way to detect the short-circuit is by sweeping the potential of one of the pipettes while the second pipette is disconnected from the potentiostat. When the surface film is present, the voltammograms obtained in this way are practically identical for both pipettes. This can be expected because both curves are produced by IT across the same interface (i.e., the interface between the aqueous film and DCE) and the filling solutions in... 

Unlike the assisted IT processes, no mediator species are involved in a simple IT reaction, e.g., transfer of a cation (M ) from organic phase into the aqueous filling solution inside the pipette ... 

Accordingly, the standard potential of the ISE depends on the specific choice of the internal reference electrode (e.g. Ag-AgCl) and also that of the fixed A" concentration of the filling solution, so that simply... 

Ag AgCl KClsat 3M KC11 sample solution membrane inner filling solution AgCl Ag... 

Sample Polymeric membrane Inner filling solution... 

FIGURE 4.6 Schematic view of the equilibria between sample, ion-selective membrane, and inner filling solution for three important classes of solvent polymeric ion-selective membranes. Top electrically neutral carrier (L) and lipophilic cation exchanger (R ) center charged carrier (L-) and anion exchanger (R+) and bottom cation exchanger (R-). 

FIGURE 4.7 Calibration curves for Ca2+-selective membrane electrodes made from a monolith of low porosity (i.d., 200 pm, no PVC) with inner filling solution of 0.1 M CaCl2 (a) and time response of the monolithic Ca2+-ISE (b). (Figures adapted from [32].)... 

An inner filling solution and internal reference electrode are used in macro ISEs due to a very good stability of the potential at the inner membrane-solution interface in such a setup (see Fig. 4.4). However, the presence of a solution inside a sensor could be a serious limitation for development of microelectrodes and may be undesired for a variety of other reasons, including ionic fluxes in the membrane and limited temperature range of sensor operation. There are several requirements for such an inner contact. First of all, a reversible change of electricity carriers ions-electrons must take place at the membrane-substrate interface. The potential of the electrochemical reaction, ensuring this transfer, has to be constant, stable, and must not depend on the sample composition. At last, the substrate must not influence the membrane analytical performance. 

The capsule fill solution may be injected directly. The amount of calcitriol in the capsule is determined by comparison to a calcitriol reference standard, prepared in a medium similar to the capsule fill (3). 

McCarron We clamped the ATP via the patch pipette filling solution. 

Fig. 18a.l. Schematic diagram of a potentiometric cell with an ion-selective electrode (ISE) as the indicator electrode. EM is the electrical potential of the sensing membrane and IFS the internal filling solution. 

The vent hole may also be used prior to the experiment to refill the outer tube with more fill solution (often supplied by the manufacturer), as this solution is lost with time or is sometimes drained out for long-term storage or cleaning. In addition, if the undissolved KC1 in the SCE disappears, more solid KC1 can be added through the vent hole. 

SO Electrode. A gas-sensing SO2 electrode marketed by Ionics, Inc. was used to provide additional VLE data at 25°C as a function of composition. Aqueous SO2 equilibrates across a polymeric membrane with a filling solution containing about 0.1 M NaHSO-j. Ionic species do not diffuse across the membrane. A small combination glass electrode measures the pH of the filling solution. The SO2 activity (Pso ) is proportional to the activity of H+ (10"PH), because the bisulfite activity is constant ... 

Water also diffuses across the polymer membrane to a limited extent. Therefore the electrode response is unstable and unreliable if there is a significant difference between the osmotic pressure of the filling solution and the unknown solution. To partially alleviate this problem,data were taken with filling solutions containing 0, 1.0, and 2.0 M additional KC1. 

Internal filling solution (typically NaF of known concentration)... 

The liquid-membrane electrode is another important type of ion-selective electrode. The internal filling solution contains a source of the ion under investigation, i.e. one for which the ion exchanger is specific, while also containing a halide ion to allow the reference electrode to function. The physico-chemical behaviour of the ISE is very similar to that of the fluoride electrode, except that ise and the selectivity are dictated by the porosity of a membrane rather than by movement through a solid-state crystal. 

Potentiometric detection is based on selective transfer of an ion in the solution into a lipophilic membrane phase, which generates a potential difference between the internal filling solution of the sensor and the sample solution. The electrode is generally placed at the outlet of the detector, and the membrane does present ion-selective characteristics. ... 

Printing inks are highly filled solutions of resins. The classic printing inks were drying oil-based systems but the trend in this almost billion-dollar business is towards solvent-free inks. 

Thus, the system comprising membrane, solution 2 of constant composition (internal filling solution), and electrode 2 (internal reference electrode) constitutes an ion selective electrode. The electrically neutral carrier antibiotics of the valinomycin group and related lipid-soluble compounds can serve as the active components of highly selective liquid... 

Another method used for nitrate determination on dried and milled herbage employs the nitrate selective electrode. One of the first published methods was that of Paul and Carlson (1968). Other anions, especially chloride, can interfere. These authors removed chloride with silver resin, but Barker ef al. (1971) omitted the resin because it tended to foul the electrode and cause excessive drift. Normally the Cl N03 ratio is so low as not to interfere, but saline precipitation from coastal plots could affect this. The method was further modified to allow storage of extracts for up to 64 h by adding a preservative of phenyl-mercuric acetate and dioxane, both very toxic (Baker and Smith, 1969). This paper mentions the need to change the electrode s membrane, filling solution and liquid ion exchanger every 2 months to minimize chloride interference. It is easy to overlook electrode maintenance between batches of nitrate analyses, and this can lead to errors and sluggish performance. 

Internal filling solution phosphate buffer pH 7.0 containing KCI and saturated with AgCI... 

When the temperature of the testing solution is different from that of the electrode, the temperature of the electrode will drift until they are equal. The speed of this change depends on the physical design of the electrode and the heat transferring ability of the fill solution inside the electrode. As indicated in equation (15.4), the slope of the electrode is temperature dependent. A change in temperature from 50°C to 100°C will result in a slope change of about 10 mV/pH unit (Figure 15.3). 

Step 1 Remove the electrode from its storage solution and rinse with purified water. Dab the electrode with tissue paper to remove excess water that could dilute the solution to be tested. Do not wipe the glass bulb. Place the electrode (and the temperature probe if applicable) into the first buffer to be tested. Wait for a stable response from the instrument. (Note If using a refillable electrode, open the fill-hole cover during calibration or any measurement to allow a uniform flow of electrode filling solution. Close the fill hole when the electrode is not in use.)... 

Step 2 Calibrate the meter to read the temperature-corrected value of the first buffer. (Note The filling solution level must be higher than the sample level to maintain the flow of filling solution. At least 1 inch above sample height is recommended.)... 

A regular maintenance schedule and proper storage of the pH electrode ensures proper performance, helps extend the life of the electrode, and avoids the cost of replacements. On a weekly basis the electrodes should be inspected for scratches, cracks, deposits, and membrane-junction deposits. The reference chamber of refillable electrodes should be drained, flushed with fresh filling solution, and refilled. (Note The chamber should only be filled to about 75 to 80% of capacity, to allow expansion during temperature change.) This maintenance procedure will keep the electrode ready to use and improve the lifetime of the electrode. A standard glass electrode in normal use can last for up to two years. 

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