there is a significiant difference in the fluodide concentration in the seawater sample without...

7/27/2019 There is a Significiant Difference in the Fluodide Concentration in the Seawater Sample Without TISAB Added and

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1. There is a significiant difference in the fluodide concentration in the seawater

sample without TISAB added and that containing TISAB. Based on the results

tabulated in Table 3, the difference between the concentration of both these

samples are about 1.42 M(9.64 8.22 M). Sample without

TISAB added has higher concentration value. However, the high values does

not indicate that the seawater sample contains high concentration of fluoride.

It might probably due to the presence if interfering ions in the sample solution

tested, for instance hydroxide ion. According to Skoog, Holler and Crounch

(2007), ion-selective electrodes (ISEs) are not ion-specific. All are sensitive to

other ions to some extent. Hence, TISAB is applied in the analysis of fluoride

in water to eliminate the interference and thereby give reliable results.

2. The difference between the concentration values of the result ( 8.75 M)

and the accepted value for the Quality Control ( MF) is about

1.25 M. By looking the figure, the experimental result which is the

measured concentration seems quite close to the true value. However,

when the accuracy is quantified by calculating the percent error, using the

formula:

% error = 100,

It shows that the experimental values differ from the accepted values by 12.5%.

The error calculation involves using the absolute difference of the experimental

and theoretical values. If the percent error is rewritten by removing the absolute

function, a negative percent error will be obtained as the experimental values is

less than accepted values ( Miller & Miller, 2005 ; Uus & Heckard, 2006 ). Hence,the % error shows the experimental result is moderately accurate.

3. The results obtained in this practical considered precise. The precision of the

results is determined by the replication of the measurement and is reflected


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through the % relative standard deviation ( Housecroft & Constable 2006 ).

The standard deviation calculates shows that the triplet readings taken during

the measurement are closely to each other. Low &RSD ( refer Table 3)

indicates the precision of the results.

1. TISAB is known as total ionic strength adjustment buffer. Occasionally, it

contains other components which actively suppress interfering ions and

control the pH value. In the analysis of fluoride in water, it is added to buffer

the sample solution to acidic condition, about pH5.0 to 5.5, and hence no

hydroxide ion will interfere with the fluoride electrode.

Meanwhile, it helps to avoid the formation of complexes that are meant to

be measured. Complexation would lower the activity and therefore the electrode

response. TISAB is able to overcome this problem by complexing with aluminium

(III) or iron (III), thus releasing bound fluoride that is able to be sensed by fluoride

electrode.

In addition to that, by adding sufficient of TISAB, it prevents a fluctuation in

the activity coefficient og the ion being measured and gives a uniform ionic

strength and standards. When the ionic strength kept constant, the activity will

proportional to the concentration (Skoog, Holler and Crounch 2007). The straight

line calibration curve can then be constructed using concentration uniots and the

unknown concentrations can be read directly from the calibration graph.

2. The electrode behaves in a Nerbstian fashion as plot of measured potential

against log [ F] shoes linear response ( refer Figure 1).

The potential measured by fluoride ISE is given by a form of the Nernst

equation:

E= constant + 59.2 ( pF ).


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This equation shows when one changes the concentration of the fluoride

sample by a factor of ten, the potential changes by 59.2mV ( where the

values if 2.303RT/F is 59.2mV at 25 C ).

The slopes if the calibration graph plotted above is -57.01 1.03mV, which

is between -58.04 and -55.98mV. The range of the possible values is quite

closely in the theoretical values -59.2mV. hence, this indicates that the

electrode is functioning properly and the electrode is said ti be behaving in a

Nernst fashion.

3. The useful concentration range if the fluoride electrode us 0.02 to 2000 mg/L,

or equivalent to 1.05 M. This is proven when the standard solution 5 with

the fluoride concentration of the M appears as an outlier in the plot of measure potential versus log [F]. Fluoride ion -selective electrode can only

measure the concentration maximum down to 1.05 M. Standard solution

5 is located outside if the concentration range.

4. The fluoride concentration found for the tap water sample is consistent with

that reported by Melbourne Water. The acceptable fluoride concentration

range for Melbourne is between 0.7 and 1.2 mg/L. The concentration of

fluoride detected in tap water sample in the experiment is around

0,4503mg/L, which is much lesser than range referred to.

Conclusion

Fluoride ion-selective electrode , which is a type of potentiometric

measurements, provides a highly selective method for the determination if

fluoride species in solution. Total ionic strength adjustment buffer, TISSAB is

used to maintain a high and constant ionic strength which allows

concentration to be used instead of activity, as well as to overcome

complexation and maintain the pH of solution. The activity of fluoride ion is

proportional to concentration only applied in solution with certain


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