Total ionic strength adjustment buffer 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). 

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... 

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

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). 

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. 

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). 

A total ionic strength adjustment buffer (TISAB) is used to control the ionic strength and the pH of samples and standards in ion-selective electrode measurements. 

Problems occurring in chemical analyses using ISEs are mainly caused by the chemical environment of the analyte. The ionic strength of the solution must be kept constant by the use of a total ionic strength adjustment buffer (TISAB). This TISAB should contain additives to adjust the pH and compounds masking possible interferents [40, 41]. For more detailed information on the function of potentio-metric sensors, see Chapter 1.1 (Sect. 1.3) in this volume. 

ISEs are calibrated in a manner similar to pH electrodes. Standard solutions of known concentrations of the ion to be measured are used and a plot of the cell emf versus concentration is made. It is important to keep the ionic strength and other matrix components of the samples and standards the same. This is often done by adding the same amount of a high-ionic-strength buffer to all samples and calibration standards. In some cases, the buffer may also adjust the pH if the form of the ion to be measured is pH dependent. The determination of fluoride ion is an example of this approach. All samples and standards have the same amount of a commercial buffer called total ionic strength adjusting buffer (TISAB) that also controls pH to insure that fluoride is present as F , not HF, because the fluoride electrode does not respond to HF. 

Figure 4 shows the cahbration plot for a copper ion selective electrode, where a total ionic strength adjustment buffer (TISAB), such as IM NaN03, has been added to each solution so that the response is effectively at constant ionic strength and constant activity coefficient. 

---