Liquid membrane selective electrodes Part 2

Liquid membrane selective electrodes Part 2

 Liquid membrane selective electrodes

The second part

Dr. Tahseen Ali Zaidan              Dr. Ismail Khalil Al-Hiti

    Liquid membrane selective electrodes based on ion-bearing neutral ligand complexes.

Liquid Membrane Electrodes Based on Neutral Carriers

 The carrier is defined as a compound that has the ability to form a selective complex of ions such as M+zm with the interfering ion N+zn. Scientists have assumed that the process of complex formation takes place in three successive steps:

(1) Transfer of free carriers from the membrane to the boundary layers. (2) formation of MSn+2m complexes and (3) migration of complexes to the membrane phase.

As with liquid films with charged ligands, the solvents used with neutral carriers must be of low vapor pressure, sufficiently high viscosity, and low solubility in water to prevent rapid loss of the active substance from the film. The selectivity coefficient is independent of the type of solvent when the intervening ion and the primary or main ion are complexes with neutral carriers of similar conjugate ratios.

In any case, when the valences are different, the selectivity depends seriously on the polarity of the solvent as reflected by the values ??of the dielectric constant and on the carrier concentration. Therefore, the selectivity is preferable to single positive ions in solvents of lower polarity, but the increase in polarity makes the selectivity preferable to binary cations.

 

 

 

The use of neutralizers began with the efforts of Stefanac and Simon in 1967, who were the first to realize the importance of macromolecular cyclic antibiotics, which showed high potassium ion selectivity, which facilitated the preparation of a potassium ion selective electrode.

 

 

 

This antibiotic is used to form a stable complex with potassium ion and has been invested in making a highly selective potassium ion liquid electrode and has been invested in the estimation of potassium in blood and serum.

 

 

Figure 3: Structure of a polycyclic dihexylcycloether-18-kr6

(Cyclopolyetherdicyclohexyl-18-Crown 6)

 

Several classes of compounds can be distinguished, including (Macrotetrolides) which are tetralactone for each of nonacitinic acid such as nonactin, monactin, dinactin, trinactin, which possess eight oxygen atoms in a flexible ring (Fig. 6-2) and these compounds are complexes with K+ being more It is stable from its complexes with Na+. It is a necessary prerequisite for obtaining selective carriers towards alkali metals and alkaline earth metals including ammonium ions.

1- The carrier molecule must have both polar and non-polar sites or groups.

2- It is preferable that the carrier contains 5-8 coordination sites with metal ions, but not more than 12 sites such as oxygen atoms.

3- The holder must take a stable shape so that the gap (cavity) is inward with the polar aggregates and the polar aggregates form a hydrophobic sheath around the symmetry sphere. The ion should barely fit or fit into the gap shown by the holder.

4- High selectivity can be obtained in the presence of bridge structures or hydrogen bonds, making the resulting organization or arrangement more stable and more stable. On the other hand, the ligands must be flexible and elastic enough to allow rapid ion exchange.

Potassium Ion Selective Electrode

 

     It can be obtained commercially and is based in its manufacture on the antibiotic valinomaycine which is dissolved in diphenyl ether. The linear electrode response is in the range 10-1-10-5 molar with a slope of 58.5-59.0 mV/decade and the minimum detection limits are 10-10 6 molary. If vigorous shaking of the potassium-containing solution is used, the electrode response time takes a few seconds, and if natural shaking is used, the response time takes a few minutes.

It was found that the potassium electrode prepared with valonomycin is more selective than the glass potassium electrode due to the interference of sodium ions and that this high selectivity is desirable when estimating serum potassium. Also, the electrode selectivity for alkaline earth metal ions is acceptable and reliable. The electrode response is affected by I-, OH-, C2O42-, CrO42- ions. It is strongly affected by the presence of tetraphenylborate.

Ammonium Ion Selective electrode

The manufacture of these electrodes is all based on macro tetrolides, which are nonactin and monactin compounds. It was found that the order of selectivity for this ion versus the other ions was as follows:

 

 NH4+ > K+ > Rb+ > H+ > Cs+ > Li+ > Na+ > Ca2+

A Nernist response was observed in the 10-1-10-5 molar range. The limited success of the applications of this electrode appears by comparing it with two commercially obtainable ammonium ion selective electrodes, which are the ammonium ion selective glass ammonium electrode and to the highly ammonia selective electrode which is very sensitive to ammonia gas.

Electrodes of liquid ion exchangers embedded in an inert fabric:

They are semi-solid ion exchangers or liquid ion exchangers that may be prepared by mixing or incorporating them with an inert carrier tissue. This design offers the benefit of avoiding the problems associated with the presence of a liquid or liquid ion exchanger. PVC films consist of the ion exchanger as a sensitizer with the plasticisers or it is also called the mediator and an appropriate amount of PVC all completely dissolved in a suitable solvent, the ion exchanger is mixed separately with the plasticizer while the PVC is dissolved in a volatile solvent such as tetrahydrofuran ( THF) The two solutions are then mixed together with continuous shaking until the mixture becomes homogeneous. Pour the homogeneous mixture into a clean, dry glass mold set on a tin plate

 

Prepare a polyethylene tube of suitable diameter and length, dip its end in THF solvent, hold vertically and press on a clean, dry glass plate with rotation until a smooth, homogeneous end is obtained. Cut a film of suitable diameter from the main film proportional to the diameter of the polyethylene tube. The film cut from the main film is affixed to the polished or cleaned flat end of a polyethylene tube and the film is attached to the tube with a mixture of PVC with THF solvent. The tube with the membrane is left to dry and then tied to a glass tube proportional to the diameter of the polyethylene tube as shown in Figure (4). Fill the glass tube to two thirds with the solution of the internal source containing KCl and the salt of the cation or anion of the sensor provided that the solution is saturated with silver chloride. Electrode preparation is completed by inserting the cone of the AgCl/Ag electrode into its designated place as shown in Figure (5).

 

 

The PVC fabric-based ion exchanger electrode exhibits the same characteristics as the liquid ion exchanger electrode in terms of stability, measurement conformity, linear response range, response time and minimum detection limits. However, it is distinguished over the electrodes of ion exchangers by the following characteristics:

It is characterized by its long life due to the holding of the effective sensor within the membrane.

2- Easy to manufacture and handle.

The membrane can be replaced with a new one when its response becomes inaccurate and impractical.

The chance of membrane contamination is less, so the electrode works in suspended and colloidal solutions.

Small amounts of ion exchanger are used compared to the large amount of liquid used in the liquid ion exchanger electrode. Small amounts of solvent and plasticizer are also used.

 

 

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