Potassium Ions interference

Possible applications of enzyme electrodes include the determination of urea in blood (Equation (20)) and for which the optimum pH is about 7 at which 99.7 per cent of the ammonia product exists as NH4 cation, the only form among the species generated that elicits a potential response from the Beckman cation-selective glass electrode. Any sodium or potassium ion interference introduced with the enzyme (or substrate) can easily be detected because of the large positive potential reading that it causes. In such instances Dowex 50 cation exchanger may be used to remove the interferences beforehand [372]. Average differences between the urea values in blood and urine as determined by the urea-urease electrode and a spectrophotometric technique were 2.8 and 2.3 per cent m/m, respectively [373]. 

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

D interferes with the inward displacement of potassium ions through active cell membranes... 

Potassium also can be measured by ICP/AES. The wavelengths at which it can be analyzed without interference from other metals are 766.49 and 769.90 nm. Other wavelengths may be used. Potassium ion in aqueous solution can be identified quantitatively by using a potassium ion-selective electrode attached to a pH meter having an expanded millivolt scale or to a specific ion meter having a direct readout concentration scale for potassium. 

Elemental composition K 28.22%, Cl 25.59%, and 0 46.19%. An aqueous solution is analyzed for potassium by AA, ICP, and other methods (see Potassium). Perchlorate ion may be analyzed by ion chromatography or a liquid-membrane electrode. Iodide, bromide, chlorate, and cyanide ions interfere in the electrode measurement. Alternatively, perchlorate ion may be measured by redox titration. Its solution in 0.5M H2SO4 is treated with a measured excess standard ferrous ammonium sulfate. The excess iron(II) solution is immediately titrated with a standard solution of potassium dichromate. Diphenylamine sulfuric acid may be used as an indicator to detect the end point ... 

Figure 45.3 shows the response surface obtained for ammonium ISE in presence of potassium ion as interference. This surface has been generated from the experimental data of the seven previous calibration sequences, fitting them to the Nicolsky-Eisenman equation with SigmaPlot 2000 software and plotted with 3D visualization options. The experimental data also appear in the figure as black symbols. In the same way, the fitted response surfaces for potassium and generic ISEs can be easily generated. 

Funazo et al. [812] have described a method for the determination of cyanide in water in which the cyanide ion is converted into benzonitrile by reaction with aniline, sodium nitrite and cupric sulphate. The benzonitrile is extracted into chloroform and determined by gas chromatography with a flame ionisation detector. The detection limit for potassium cyanide is 3 mg L 1. Lead, zinc and sulphide ion interfere at lOOmg L 1 but not at lOmgL-1. 

The precipitate is almost insoluble in water (0-053 g -1, Ks = 2-25 x 10 8) potassium is precipitated quantitatively if a small excess of the reagent is applied (01-0-2 per cent). The precipitate is soluble in strong acids and alkalis, and also in acetone. Rubidium, caesium, thallium(I), and ammonium ions interfere. 

The bicyclic heteroaromatic bases 191 possess arrays of complementary donor-donor-acceptor and acceptor-acceptor-donor hydrogen bond sites and an additional benzocrown ether moiety. Structure and dimension of the nanotubes were determined by NMR, dynamic light scattering, small-angle X-ray scattering, and transmission electron microscopy. Addition of sodium or potassium ions did not interfere with the stability of the multichannel... 

An example is provided by a comparison of the diffraction patterns of the isostructural chlorides of sodium and potassium (see Figure 6.20). It is noted that alternate rows of diffraction spots are very faint in the potassium chloride diffraction pattern, unlike the situation for sodium chloride. This alternating pattern of intensity is due to the fact that potassium and chloride ions are isoelectronic (with 18 electrons), and therefore have approximately identical powers to scatter X rays. On the other hand, the difference in scattering power between a sodium ion (10 electrons) and a chloride ion (18 electrons) is appreciable. Therefore those diffraction spots in which scattering from the metal ion interferes with scattering from the chloride ion will have a measurable intensity for diffraction by crystals of sodium chloride but almost no intensity for diffraction by crystals of potassium chloride. 

Thallium s mechanism of toxicity is related to its ability to interfere with potassium ion functions. Thallium interferes with energy production at essential steps in glycolysis, the Kreb s cycle, and oxidative phosphorylation. Other effects include inhibition of sodium-potassium-adenosine triphosphatase and binding to sulfhydryl groups. 

A valinomycin-based potassium ion-selective electrode is evaluated for sodium interference using the fixed interference method. A potassium calibration curve is prepared in the presence of 140 mM sodium. The straight line obtained from extrapolation of the linear portion deviates from the experimental curve by 17.4 mV at a potassium concentration corresponding to 1.5 X 10 M. If the linear slope is 57.8 mV per decade, what is ATnok for the electrode ... 

A potassium opto-sensor was recently described [75] for the continuous determination of electrolytes. Certain fluorescent dyes respond to an electric potential at the interface between the aqueous and lipid phases. This potential is created by the neutral ion carrier. The lipid layer is formed on a glass support by the Langmuir-Blodgett thin-fllm technique. This layer incorporates Rhodamine B as a dye and valinomycin as the carrier. The lipid membrane is made of arachidic acid. The fluorescence intensity decreases when this layer is exposed to potassium ions (linearity between 0.01 and 10 mM). This optode is also sensitive to sodium ions [76]. The selectivity factor of potassium in comparison with sodium ions varies from 10 - to 10 , and in relation to ammonium ions by 10. Interferences can be compensated for by a reference optode. However, better selectivity is obtained with new lipid membrane compositions (octadecan-l-ol-valinomycin) [77]. Tetralayers (Figure 17-9) give a maximum response for K". The K /Na selectivity is about 10 in a wide range (0.01-100 mM). 

A related problem is ionization interference. If the analyte atoms are ionized in the flame, they cannot emit atomic emission wavelengths, and a reduction in atomic emission intensity will occur. For example, if potassium is ionized in the flame, it cannot emit at its atomic emission line at 766.5 nm and the sensitivity of the analysis will decrease. If a large amount of a more easily ionized element, such as cesium, is added to the solution, the cesium will ionize preferentially and suppress the ionization of potassium. The potassium ions will capture the electrons released by the cesium, reverting to neutral potassium atoms. The intensity of emission at 766.5 nm will increase for a given amount of potassium in the presence of an excess of cesium. The added cesium is called an ionization suppressant. Ionization interference is a problem with the easily ionized elements of groups 1 and 2. The use of ionization suppressants is recommended for the best sensitivity and accuracy when determining these elements. Of course, as ionization increases, ion emission line intensity increases. It may be possible to use an ionic emission line instead of an atomic emission line for measurements. 

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