Sodium chloride liquid junction

The liquid junction is open to transfer of all ions. We have indicated that charge will be carried both by chloride anions from the right to the left and sodium cations from left to right. The potential of the junction remains constant. 

For most potentiometric measurements, either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few mV. The silver/silver chloride electrode also finds application in non-aqueous solutions, although some solvents cause the silver chloride film to become soluble. Some experiments have utilised reference electrodes in non-aqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for non-aqueous systems as are any of the prototypes that have been developed to date. When there is a need to exclude water rigorously, double-salt bridges (aqueous/non-aqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the non-aqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence, the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause dramatic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

The reference electrode (system) and its stability in clinical analyzers is a crucial problem because all typical ISSs (and GSSs) use this electrode. Currently silver chloride electrodes are used. They work in two systems an open liquid junction or a constraint liquid junction with concentrated KC1 (>2M) or sodium formate (4M) as the equitransferent hypertonic electrolyte bridge. The latter better serves whole blood measurements. 

For the above reasons, the IFCC recommendations on activity coefficients [19] and the measurement of and conventions for reporting sodium and potassium [21] and chlorides [25] by ISEs were developed. At the core of these recommendations is the concept of the adjusted active substance concentration (mmol/L), as well as a traceable way to remove the discrepancy between direct and indirect determinations of these electrolytes in normal sera. Extensive studies of sodium and potassium binding to inorganic ligands and proteins, water binding to proteins, liquid-junction effects and the influence of ionic strength have demonstrated that the bias between sodium and potassium reports obtained from an average ISE-based commercial... 

Much of the uncertainty concerning activity coefficients and liquid junctions can be minimised by calibrating the ion-selective electrode with calcium chloride standards (approx. 0.5—0.2 mmol dm ) in 0.150 mol dm sodium chloride solutions. This corresponds to an emf span of just approx. 18 mV, thus emphasising the need for using precise emf-measuring instruments and attention to electrode care and solution details. 

In polarographic practice the most important reference electrodes are separated calomel electrodes, a mercurous sulphate electrode, or, especially for small volumes, a silver chloride electrode immersed into an electrolysed solution containing OT M chlorides. This electrode proved satisfactory over the pH-range 1-13 when sodium or potassium chloride was added to the buffer solutions. Measurements in solutions forming complexes with silver e.g. glycine, veronal or ammonia buffers are precluded. The use of this electrode eliminates the uncertainty concerning the liquid junction potential. 

An inner solution with a constant activity of the respective measured ion, sodium or potassium, is contained in the tip of the electrode. To deliver the potential further, a reference element, usually Ag/AgCl, dips into the inner solution forming a reference cell without liquid junction. The inner solution must contain a constant chloride ion activity so that the equilibrium Galvani potential of this reference electrode remains constant. There are also now micro-flow-cells made with sodium-selective glass (Fig. 

Place a small crystal of sodium sulflte in a solution of 0.3-0.4 gram of stannous chlorid in pure HCl, sp. gr. 1.13. Float the liquid to be tested on the surface of this mixture. If As be present a yellow band is formed at the junction of the two liquids, and gradually increases upward. 

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