TISAB

Another approach to matrix matching, which does not rely on knowing the exact composition of the sample s matrix, is to add a high concentration of inert electrolyte to all samples and standards. If the concentration of added electrolyte is sufficient, any difference between the sample s matrix and that of the standards becomes trivial, and the activity coefficient remains essentially constant. The solution of inert electrolyte added to the sample and standards is called a total ionic strength adjustment buffer (TISAB). 

The following data were collected for the analysis of fluoride in tap water and in toothpaste, (a) For the analysis of tap water, three 25.0-mL samples were each mixed with 25.0 mL of TISAB, and the potential was measured with an F ISE relative to a saturated calomel electrode. Five 1.00-mL additions of a standard solution of 100.0-ppm F were added to each, measuring the potential following each addition. 

Direct analysis with the fluoride lon-selective electrode requires addition of total ionic strength adjustor buffer solution (TISAB) to the standard and to unknown samples Some advantages of this addition are that it provides a constant background ion strength, ties up interfenng cations such as aluminum or iron, which form a complex with fluoride ions, and maintains the pH between 5 0 and 5 5 According to the manufacturer s claim, reproducibility of direct electrode measurement IS 2 0%, and the accuracy for fluonde ion measurement is 0 2% [27]... 

Total Ionic Strength Adjustment Buffer (TISAB). Dissolve 57 mL acetic acid, 58 g sodium chloride and 4g cyclohexane diaminotetra-acetic acid (CDTA) in 500 mL of de-ionised water contained in a large beaker. Stand the beaker inside a water bath fitted with a constant-level device, and place a rubber tube connected to the cold water tap inside the bath. Allow water to flow slowly into the bath and discharge through the constant level this will ensure that in the... 

Pipette 25 mL of solution B into a 100 mL beaker mounted on a magnetic stirrer and add an equal volume of TISAB from a pipette. Stir the solution to ensure thorough mixing, stop the stirrer, insert the fluoride ion-calomel electrode system and measure the e.m.f. The electrode rapidly comes to equilibrium, and a stable e.m.f. reading is obtained immediately. Wash down the electrodes and then insert into a second beaker containing a solution prepared from 25 mL each of standard solution C and TISAB read the e.m.f. Carry out further determinations using the standards D and E. 

Now take 25 mL of the test solution, add 25 mL TISAB and proceed to measure the e.m.f. as above. Using the calibration curve, the fluoride ion concentration of the test solution may be deduced. The procedure described is suitable for measuring the fluoride ion concentration of tap water in areas where fluoridation of the supply is undertaken. 

Complexation of fluoride by metal ions in seawater has previously been overcome by the addition of TISAB solution. The reagent is presumed to release the bound fluoride by preferential complexation of the metal ions with EDTA type ligands present in the TISAB. Examination of the metal ions present in seawater [66,67] suggests that magnesium is the major species forming fluoride complexes. Theoretical calculations demonstrate that even this species is unlikely to interfere. 

Calculated via addition of TISAB and calibration versus synthetic seawater. 

Total ionic strength adjustment buffers TISABs) are used to equalize Ionic activity n different solutions. 

It is often more convenient to relate the potentiometer reading directly to concentration by adjusting the ionic strength and hence the activity of both the standards and samples to the same value with a large excess of an electrolyte solution which is inert as far as the electrode in use is concerned. Under these conditions the electrode potential is proportional to the concentration of the test ions. The use of such solutions, which are known as TISABs (total ionic strength adjustment buffers), also allows the control of pH and their composition has to be designed for each particular assay and the proportion of buffer to sample must be constant. 

To leam that the change of y with ionic strength is a major cause of error in electroanalytical measurements, and so it is advisable to buffer the ionic strength (preferably at a high value), e.g. with a total ionic strength adjustment buffer (TISAB). 

Often, the potentiometric determination of concentration requires a preferred pH range. If pH is also important, then the ionic strength adjuster can conveniently function additionally as a pH buffer. Such tablets are called total ionic strength adjustment buffers (or TISABs). ... 

Incidentally, ionic strength adjusters and TISABs also decrease all junction potentials (see Section 3.6.5). 

The TISAB also slops complexation between fluoride and other ions in solution, particularly Al . Fe and silicate species. 

The fluoride content of a sample of toothpaste is unknown. Accordingly, a sample of the toothpaste was digested in acid solution, filtered to remove the white gritlike solid and then buffered with a total ionic strength adjustment buffer (TISAB) to pH 6. A fluoride electrode is immersed in the clear solution and the emf recorded when the reading was steady. 

From the discussions above, we will be aware that the concentration ([F ]) determined will in fact be an activity, i.e. a(F ). It will also be apparent that adding an acid to digest the sample of toothpaste will introduce errors into the calculation since two electrolytes are involved, thereby increasing the ionic strength / (see SAQs 3.10 and 3.11). Since the preferred pH range of the fluoride electrode is 5-6, the ionic strength adjuster (TISAB) can also conveniently function as a pH buffer. 

While this relationship is simple, it introduces more errors because the activity coefficient (or more normally, the mean ionic activity coefficient y ) is wholly unknown. While y can sometimes be calculated (e.g. via the Debye-Huckel relationships described in Section 3.4), such calculated values often differ quite significantly from experimental values, particularly when working at higher ionic strengths. In addition, ionic strength adjusters and TISABs are recommended in conjunction with calibration curves. 

The activity a and concentration c are related by a = (c/c ) x y (equation (3.12)), where y is the mean ionic activity coefficient, itself a function of the ionic strength /. Approximate values of y can be calculated for solution-phase analytes by using the Debye-Huckel relationships (equations (3.14) and (3.15)). The change of y with ionic strength can be a major cause of error in electroanalytical measurements, so it is advisable to buffer the ionic strength (preferably at a high value), e.g. with a total ionic strength adjustment buffer (TISAB). 

TISAB total ionic strength adjustment buffer... 

An acceptable method quite frequently used in practice depends on the cell whose EMF is being measured having a liquid junction with a constant potential value. Such a situation is attained in the determination of the activity of fluoride ions, by adding a constant amount of quite concentrated buffer, for example TISAB, to the studied solution this buffer also fulfills other functions in the analysis (see p. 146). Then the liquid junction potential is a function of the composition of the reference electrode electrolyte and of the buffer composition alone, and not of the concentrations of the other components of the studied solution. 

Frant and Ross [108] recommended sample adjustment using TISAB buffer ( Total Ionic Strength Adjustment Buffer ), obtained by dissolving 57ml glacial acetic acid, 58 g NaCl and 4g 1,2-cyclohexanediaminetetraacetic acid (CDTA), adjustment of the solution pH with sodium hydroxide to 5 to 5.5 and dilution to 1 litre, all to maintain a constant ionic strength and pH between 5 and 5.5 and to complex ions such as Al or Fe that interfere in the determination. A detailed... 

Fluoride release is most frequently determined using an ion-selective electrode. Because such electrodes are incapable of detecting complexed fluoride, a decomplexing agent is generally added to the mixture prior to analysis. This frees up fluoride from most complexes as the F ion, and the total quantity of fluoride released can then be determined by the ion-selective electrode. The usual complexing agent is TISAB (total ionic solubility acid buffer) [250]. 

Fluoride is assumed to complex with aluminium in the form of alumino-fluoride ions (AIF2+ and AIF2+), and such complexes are dissociated in TISAB to liberate all of the fluoride as free F ions. This is consistent with the observation that ion release under acidic conditions tends to show proportionately greater aluminium release yet lower amounts of free fluoride [251],... 

This approach may not account for all the fluoride released by glass-ionomers [252], A recent study has used two methods of decomplexation of fluoride, using the same solutions for both methods, by dividing a given storage volume into two and treating each aliquot differently. One aliquot was diluted with an equal volume of TISAB, as is usual in the determination of fluoride by ion-selective electrode. The other solution was treated with a small volume of 4 M hydrochloric acid, allowed to stand for 3 h, then neutralised with an equal amount of 4 M sodium hydroxide. A volume of TISAB equal to the initial volume of the aliquot was added. This technique is known to liberate fluoride from monofluorophosphate as well as from aluminofluoride complexes [253],... 

The proportion of free and complexed fluoride has been determined in one study of ion release from compomers [279], by measuring the amount of fluoride with and without TISAB. Results obtained are shown in Table 8. 

Total fluorine in fluoride supplements and dental products could be determined with minimal samples pre-treatment as for example by direct acid extraction or heating in TISAB buffer solution and subsequent determination of fluoride using fluoride ISE for the reason that entire fluorine, in these products, should be, by definition, available as free inorganic fluoride. 

There are many methods that allow the determination of the concentration q of an ionic species i in a sample. In the presence of an ionic strength adjuster (ISA) or a buffering solution that can fix the pH (TISAB, Total Ionic Strength Adjustment Buffer) all of these methods are based on application of equation (18.3). 

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