Constant interferent concentration method

Method 1 is the constant interferent concentration method (mixed solution technique) [123], The EMF of a cell consisting of an ISE and a ref erence electrode is measured at constant interferent activity [Pg.83]

The latter method should only be used when the constant interferent concentration method is unsuitable or impracticable. 

The carbohydrates are a problematical group of compounds from the point of view of detection, having neither useable spectrophotometric or fluorescent properties. The method of choice for detection of carbohydrates in pure solution has been refractometry but interference from other compounds in body fluids and its relative insensitivity has limited its application in clinical chemistry. The development of pulsed amperometric detection at noble metal electrodes and improvements in anion-exchange columns has now made possible the separation and quantitation of most carbohydrates of interest in urine samples. As ion-exchange columns require a constant ionic concentration to achieve consistent separations, urines are first desalted with... 

Walters [24] examined the effect of chloride on the use of bromide and iodide solid state membrane electrodes, and he calculated selectivity constants. Multiple linear regression analysis was used to determine the concentrations of bromide, fluorine, and iodide in geothermal brines, and indicated high interferences at high salt concentrations. The standard curve method was preferred to the multiple standard addition method because of ... 

Wong [10,11] has studied this in further detail and found that carrying out the titration at pH 2 yields a true concentration of total residual chlorine after correction for naturally occurring iodate. The effectiveness of sulfamic acid in this method for removal of the nitrite interference is shown in Fig. 4.1. In this experiment, all the solutions contained 30 pmol/1 nitrite, and about 0.5 pmol/1 of iodate. The absorbance of the solution at 353 nm decreased with increasing amounts of added sulphamic acid. A constant absorbance was recorded when 3 ml or more of 1% (w/v) sulphamic acid was added to the solution, and this absorbance was identical with that in a sample containing the same amount of iodate and no nitrite. A concentration of nitrite of 30 pmol/l is unlikely to occur in estuarine water and seawater ... 

Kuroda and Tarui [498] developed a spectrophotometric method for molybdenum based on the fact that MoVI catalyses the reduction of ferric iron by divalent tin ions. The plot of initial reaction rate constant versus molybdenum concentration is rectilinear in the range 0.01-0.3 mg/1 molybdenum. Several elements interfere, namely, titanium, rhenium, palladium, platinum, gold, arsenic, selenium, and tellurium. 

For APHA Methods 2350-C and 4500-CL02-E, amperometric analyzers are used to measure chlorine dioxide in water. Amperometric analyzers measure the current that is necessary to maintain a constant concentration of titrant as chlorine dioxide reduces the titrant (e.g., phenylarsine oxide). This method is limited by interference from compounds that might react with the titrant (e.g., chlorine and chloroamine) (APHA 1998). 

Because of the small concentration of the 2 1 complex the last term can be ignored. From the extreme rate values in the absence of zinc and with an excess of zinc, 2i and 22 are determined as 2.4 X 104 min.-1 and 1.57 min.-1 respectively. These values can be combined with the trend in the rate constants to give the stability constant of the reactive complex, presumably Zn(OR)(OAc), as 3 X 107. For the simple zinc complex in water the literature values of the stability constant for the 1 1 complex vary from 2.5 X 108 to 6.3 X 108. The diazo coupling reaction of the complex indicates the smaller effect of coordination vis a vis protonation since this reaction is very sensitive to such effects and does not proceed with phenols. Unfortunately the choice of cations for such a reaction is restricted since the cation should not interfere with the analytical methods used to obtain the kinetic data nor should it introduce additional reactions such as occur with transition metal cations which can catalyze the decomposition of the diazonium salt via a redox process. 

In the MIP literature the most widespread method for establishing the selectivity of an MIP for its template against an interferent has been to determine by HPLC the corresponding a value. In the section on MIP HPLC we have shown, however, that the k values determined in MIP HPLC depend on the injected sample concentration. Therefore they do not provide a distribution ratio. Indeed, the distribution ratio is obviously concentration dependent when the isotherm is not linear. It is not self-evident but it has been shown [35] that the ratio of the apparent k values (as determined from the peak maximum positions) for two compounds with nonlinear isotherms is not a constant and such an a value is not suitable to quantitate the separation selectivity. 

The iodide reaction was first studied by incremental addition of thiosulphate ( The method of constant rates ) . This use of the method was attacked by Bell . Bray s work established the two rate terms shown in Table 28. The reaction has also been studied in the presence of arsenite , which does not interfere as thiosulphate does, and kj =3.3x10 l .mole .sec at 25 °C. Catalysis of the reaction has been demonstrated in presence of vanadium(IV) , ruthenium(III) , rhenium(II) osmium tetroxide " and iron(III) , together with retardation by Mn(II) - Ni(II) and chloride . These measurements were made at moderate acidities and appear to involve quite different dependence of rate on acid concentration from the reaction in absence of catalyst (half-order for Ru(OH), first-order for Re ). The mechanisms may also be quite different. 

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