Fluoride calibration curves

Plot the observed e.m.f. values against the concentrations of the standard solutions, using a semi-log graph paper which covers four cycles (i.e. spans four decades on the log scale) use the log axis for the concentrations, which should be in terms of fluoride ion concentration. A straight line plot (calibration curve) will be obtained. With increasing dilution of the solutions there tends to be a departure from the straight line with the electrode combination and measuring system referred to above, this becomes apparent when the fluoride ion concentration is reduced to ca 0.2 mg L-1. 

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. 

A calibration curve for the range 0.2-10 mg fluoride ion per 100 mL is constructed as follows. Add the appropriate amount of standard sodium fluoride solution, 25 mL of 2-methoxyethanol, and 10 mg of a buffer [0.1 Af in both sodium acetate and acetic (ethanoic) acid] to a 100 mL graduated flask. Dilute to volume with distilled water and add about 0.05 g of thorium chloranilate. Shake the flask intermittently for 30 minutes (the reaction in the presence of 2-methoxyethanol is about 90 per cent complete after 30 minutes and almost complete after 1 hour) and filter about 10 mL of the solution through a dry Whatman No. 42 filter paper. Measure the absorbance of the filtrate in a 1 cm cell at 540 nm (yellow-green filter) against a blank, prepared in the same manner, using a suitable spectrophotometer. Prepare a calibration curve for the concentration range 0.0-0.2 mg fluoride ion per 100 mL in the same way, but add only 10.0 mL of 2-methoxyethanol measure the absorbance of the filtrate in a 1 cm silica cell at 330 nm. 

Treat the fluoride sample solution in the same manner as described for the calibration curve after removing interfering ions and adjusting the pH to about 5 with dilute nitric acid or sodium hydroxide solution. Read off the fluoride concentration from the calibration curve and the observed value of the absorbance. 

Figure 15-18 Calibration curve for fluoride ion-selective electrode. [Data from M. S. Front and J. W. Ross, Jr., Electrode tor Sensing Fluoride Ion Activity in Solution," Science 1966, 154, 1553.]...

Prepare a standard calibration curve using fluoride standards from 0 to 1.5 mg F /L. A 50-mL volume of standard solutions is treated with 10 mL of zirconyl-... 

Fluoride Determine as directed in Method III under Fluoride Limit Test, Appendix IIIB, using a 1-g sample, accurately weighed. Use 1.0, 5.0, and 10.0 mL of the Sodium Fluoride Solution (equivalent to 5.0, 25.0, and 50.0 mg/kg of fluoride, respectively) to prepare the Calibration Curve, and use 10 mL of water and only 10 mL of 1 N hydrochloric acid to dissolve the sample as directed under Procedure. 

Procedure Pipet a 20-mL aliquot of Sample Solution into a 100-mL plastic beaker, add 10 mL of 0.2 N EDTA/0.2 N TRIS Solution, and measure the solution potential as described under Calibration Curve (above). From the measured potential of the Sample Solution, calculate the concentration, in milligrams per kilogram, of fluoride ion from the Calibration Curve. 

Procedure Transfer 1.00 g of sample into a 150-mL glass beaker, add 10 mL of water, and, while stirring continuously, slowly add 20 mL of 1 A hydrochloric acid to dissolve the sample. Boil rapidly for 1 min, then transfer into a 250-mL plastic beaker, and cool rapidly in ice water. Add 15 mL of 1 M sodium citrate and 10 mL of 0.2 M disodium EDTA, and mix. Adjust the pH to 5.5 0.1 with 1 A hydrochloric acid or 1 A sodium hydroxide, if necessary transfer into a 100-mL volumetric flask dilute to volume with water and mix. Transfer a 50-mL portion of this solution into a 125-mL plastic beaker, and measure the potential of the solution with the apparatus described under Calibration Curve. Determine the fluoride content, in micrograms, of the sample from the Calibration Curve. 

Figure 19.4 Typical phantom calibration curve measurement, (a) 191 MR spectrum of compound and reference standard (left) tecastemizole (right) potassium fluoride, (b) Calibration curve of tecastemizole concentration measured analytically by LC-MS versus that measured using l9F MR.

When electrolyte concentrations are not too great, it is often useful to swamp both samples and standards with a measured excess of an inert electrolyte. The added effect of the electrolyte from the sample matrix becomes negligible under these circumstances, and the empirical calibration curve yields results in terms of concentration. This approach has been used, for example, in the potentiometric determination of fluoride ion in drinking water. Both samples and standards are diluted with a solution that contains sodium chloride, an acetate buffer, and a citrate buffer the diluent is sufficiently concentrated so that the samples and standaids have essentially identical ionic strengths. This method provides a rapid means of measuring fluoride concentrations in the part-per-million range with an accuracy of about 5% relative. 

Notes. 1. This method of measuring the absorbance gives a calibration curve as for direct spectrophotometric methods, i.e., zero absorbance corresponds to absence of fluoride. 

In the preparation of the calibration curve, the standard solution of fluoride is added to the Zr-ECR reagent solution in a 25-ml volumetric flask, the solution is diluted to the mark with water, and the absorbance is measured as stated above. 

Fluoride in a water sample is determined by measurement with a fluoride ion-selective combination electrode (contains reference electrode built in). First, you will determine whether the electrode response is Nemstian over a wide range of concentrations. Then, you will determine fluoride in the uiikiiovra by comparing potential measurements with standards over a narrower range, bracketing the unknown a calibration curve will be prepared. 

Analysis of unknown. After preparing the calibration curve, obtain an unknown fluoride sample. This may be a synthetic solution, in which case obtain the unknown in a 250-mL volumetric flask. Immediately dilute to volume with distilled deionized water and transfer to a polyethylene bottle. Add 10 mL of the unknown with a pipet to a small plastic beaker followed by 10 mL TISAB. Record the mV reading as above. Make at least three separate runs (separate additions and potential readings). Note The unknown... 

From the spreadsheet calibration curve, determine the concentration of fluoride in the unknown solution. Report the results in parts per million fluoride, along with the standard deviation for the three measured samples. 

A calibration curve for the determination of F in municipal waters is illustrated in Figure 2.8. The 40 mV difference in electrode response to two different water supplies, one with and the other without added fluoride, is easily measured. This measurement system can be readily automated for continuous monitoring of fluoride levels. 

Commercial solid-state potential measuring devices based on the type of op-amp described are often called pH or plon meters and are designed to work with glass pH electrodes, ion selective electrodes, and other indicator electrodes described earlier. Research quality plon meters have built-in temperature measurement and compensation, autocalibration routines for a three-point (or more) calibration curve, recognition of electrodes (so you do not try measuring fluoride ion with your pH electrode ), and the ability to download data to computer data collection programs. The relative accuracy of pH measurements with such a meter is about +0.005 pH units. Meters are available as handheld... 

It is absolutely essential that a calibration curve is prepared and a blank test taken into account for each fluoride determination. 

Read off the appropriate fluoride content from the calibration curve on the basis of the measured extinction, take into account the quantity of water used and the blank test, and convert for 1 litre. 

Check the calibration curve daily with fluoride calibration solutions of appropriate concentration. This is important in particular due to the fact that both the origin and the gradient of the calibration curves of the... 

A CE method with indirect UV detection has been validated for eight anions and two electrolyte systems pyromellitic acid - - hexamethonium hydroxide and chromate- -TTAB. The detection limits are between 1 and 3 mg 1 the repeatability and reproducibility of the measurement differ for different compounds and amounts to 5%, except for fluoride and phosphate. Linear calibration curves have been obtained within a concentration range between 1 and lOmgl. ... 

Figure 15-12 Calibration curve for fluoride ion-selective electrode.

Curie point pyrolysis has been used to carry out quantitative analysis of monomer units in polyhexafluorpropylene-vinylidene fluoride [95]. The polymer composition is calculated from the relative amounts of monomer regenerated and the trifluoromethane produced during pyrolysis. A calibration curve is obtained using samples whose compositions are measured by NMR as standards and a least squares fit calculated. The reproducibility... 

To compensate for the salt error, calibrate the method as described in Section 11.2.8.3 using the artificial seawater. To ISmL aliquots add 0.5,1.0,1.5 and 2.0 mL of the fluoride working standard and 8.0 mL of the alizarin reagent. For seawater samples with different chlorinities, prepare individual calibration curves with chlorinities of 0,5,10,15 and 20 %o. 

This method is based on the bleaching action of fluoride ion content of the sample. The color of fhe red Zr-solochrome cyanine R (Aldrich Cat. No. 23,406-0, Sigma Prod. No. E2502) [52] complex fades as ZrOp2 is formed in the medium. As a matter of fact, no simple stoichiometric relationship exists between the fluoride and the zirconium complex with the dye. Therefore, in order to obfain reliable results, the reaction conditions need to be controlled very carefully. The absorbance of fhe reaction media is measured at 540 nm. The fluoride concenfrafion is evaluated using an absorbance-fluoride concentration calibration curve prepared with standard solutions. The method can be used for samples containing 0-2.5 pg fluoride. 

Procedure A 5 mL water sample was taken into a 50 mL centrifuging tube followed by the addition of 1 mL each of 0.01 M citrate buffer (pH 4.0), 1 M sodium nitrate solution and 0.01 M DCTA, and 2 mL of distilled water. Then, 10 mL of a sapphyrin chloroform solution (1 X 10 M) was added to the sample solution in the centrifuging tube. Fluoride ion was extracted into the chloroform by shaking the tube mechanically for at least 1 h. Fluorescence intensity at 684 run was measiued for the determination of fluoride ion imder excitation at 448 nm. Calibration curve could be used for evaluation. 

Fluoride samples of high concentration can be titrated potentiometrically [60] with lanthanum nitrate or thorium nitrate reagent. The potentiometric standard addition technique with NaF standard solution [61] also was found applicable however, direct potentiometry using calibration curves is most often relied on in water analysis. 

Direct potentiometric method The sample and standard solutions are introduced into the potentiometric measuring cell mixed with background electrolyte. The fluoride ion-selective electrode and appropriate reference electrode (saturated calomel electrode or silver/silver chloride electrode with double jimction is recommended) are dipped into the solution and the electromotive force is measured. An EMF versus log (fluoride ion concentration) calibration curve is used for evaluation. 

The calibration curve is prepared by plotting the cell voltage against the negative logarithm of the fluoride concentration of the standards (pF). The plot is a straight line between pF values of 5 and 2. It deviates from linearity between the pF values of 6 and 5. The sample concentration is determined from the calibration curve, taking the dilution factor into consideration. 

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