- Scope
1.1 these test methods cover the determination of fluoride ion in water. The following two test methods are given :
Sections
Test Method A (Distillation) 7 to 14
Test Method B (Ion Selective Electrode) 15 to 22
1.2 Test Method A covers the accurate measurement of total fluoride in water through isolation of the fluoride by distillation and subsequent measurement in the distillate by use of the ion selective method. The procedure covers the range from 0.1 to 2.6 mg/L of fluoride.
1.3 Test Method B covers the accurate measurement of simple fluoride ion in water by means of an ion selective electrode. With this test method, distillation is eliminated because the electrode is not affected by the interference common to colorimetric procedures. Concentrations of fluoride from 0.1 to 1000 mg/L may be measured.
1.4 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety problems associated with its use. It is the responsibility of the user of the standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For a specific precautionary statement, see 12.1.1; for a specific cautionary statement, see 12.2.2.
1.5 Former Test Method A, SPADNS Photometric Procedure, was discontinued. Refer to Appendix XI for historical information.
- Referenced Documents
2.1 ASTM-Standards:
D1066 Practice for sampling Steam3.
D 1129 Definitions of Terms Relating to
Water3.
D 1192 Specifications for Equipment for
Sampling Water and Steam3.
D 1193 Specifications for Reagent Water3
D 2777 Practice for Determination of Precision and Bias of Applicable Methods of Committee
D-19 on Water3.
D 3370 Practices for Sampling Water3.
- Definitions
3.1 For definitions of terms used in these test method refer to Definitions D 1129.
- Significance and Use
4.1 Simple and complex fluoride ions are found in natural waters. Fluoride forms complexing ions with silicon, aluminum and boron. These complexes may originate from the use of fluoride compounds by industry.
4.2 Fluoridation of drinking water to prevent dental cavities is practiced by a large number of communities in the country. Fluoride is monitored to assure that an optimum treatment level of 1.4 to 2.4mg/L, depending on the corresponding range of ambient temperatures of 32 to 10°C is maintained.
- Purity of Reagents
5.1 Reagent grade chemicals shall be used in all tests. Unless other indicated, it is intended that all reagents shall conform to the specifications of the Committee of Analytical Reagents of the American Chemical Society where such specifications are available4. Other grades may be used, provided it is first ascertained that the reagent is sufficiently high purity to permit its use without lessening the accuracy of the determination.
5.2 Purity of Water - Unless otherwise indicated, references to water shall be understood to mean Type I reagent water conforming to Specification D 1193.
- Sampling
6.1 Collect the sample in accordance with Practice D 1066, Specification D 1192. or Practices D 3370, as applicable.
Test Method A – Distillation
- Scope
Note: Description of Test Method A will not be discussed further. Using the lonScan F pertains to the Test Method B. Kindly refer to the ASTM for more information on Test Method A.
Test Method B - Ion Selective Electrode
- Scope
15.1 This test method is applicable to the measurement of fluoride ion in finished waters, natural waters and most industrial wastewaters. With this test method, distillation is eliminated and concentrations of fluoride from 0.1 to 1000 mg/L may be measured.
15.2 The test method is not applicable to samples containing more than 10,000 mg/L of dissolved solids.
15.3 This test method was tested on reagent water and wastewater. It is the user's responsibility to ensure the validity of this test method for waters of untested matricides.
- Summary of Test Method
16.1 The fluoride is determined potentiometrically using an ion selective fluoride electrode in conjunction with a standard single junction, sleeve-type reference electrode, and a pH meter having an expanded millivolt scale, or a specific ion meter having a direct concentration scale of fluoride.
16.2 The fluoride electrode consists of a lanthanum fluoride crystal across which a potential is developed by fluoride ions.6 .The cell may be represented by Ag/AgCI, Cl(0.3), F (0.001) LaF3/test solution/ reference electrode.
- Interference
17.1 Extremes of pH interfere; sample pH should be between 5 and 9.
17.2 Polyvalent cations of Si+4, Fe+3, and AI+3 interfere by forming complexes with fluoride. The degree of interference depends upon the concentration of fluoride, and the pH of the sample (See Table 3). Although the addition of a pH 5.0 buffer (see 19.1) containing citrate ion preferentially complexes aluminum (the most common interference), silicon and iron, and eliminates the pH problem in the use of either one of the two selective buffers (see 19.2 and 19.3), recommended when aluminum is present, because it is more effective than the citrate buffers over a greater range of aluminum concentrations.
17.3 Interference usually encountered in other test methods, such as sulfate or phosphate. do not affect this test method.
Table 3: Allowable Interference Levels with Selective Ion Electrode and Buffer4
|
Interfering Ion
|
Maximum AllowableConcentration at I.0mgF-/L mg/L
|
|
Al+3
|
0.5
|
|
Si+4
|
50
|
|
Fe+3
|
65
|
- Apparatus
18.1 pH meter with expanded millivolt scale or specific ion meter.
18.2 Fluoride Ion Selective Electrode.7
18.3 Reference Electrode, a single-junction sleeve-type.
18.4 Mixer,magnetic,with a TFE-Fluorocarbon coated stirring bar.
- Reagents
19.1 Buffer Solution (pH from 5.0 to 5.5) - To approximately 500 mL of water, add 57 mL of glacial acetic acid (sp. gr. 1.06), 58g of sodium chloride (NaCl). and 0.30g of sodium citrate dihydrate in a 1000-mL beaker. Stir the solution to dissolve and cool to room temperature. Adjust the pH of the solution to between 5.0 and 5.5 with 5N sodium hydroxide (NaOH) (about 150 mL will be required). Transfer the solution to a 1000 mL volumetric flask and dilute to the mark with water.
19.2 Buffer A for Aluminum - To approximately 500 mL of water, add 84 mL of reagent grade hydrochloric acid (sp. gr. 1.19), 242g of tris-(hydroxymethyl)-aminomethane (THAM) (also known as 2-amino-2-(hydroxymethyl)-1,3-propanediol) and 230g of sodium tartrate (Na2C4H4O62H20). Stir to dissolve, and cool to room temperature. Transfer the solution to a 1000 mL volumetric flask and dilute to the mark with water.
19.3 Buffer B for Aluminum8 - Dissolve 60 g of citric acid monohydrate, 210g of sodium citrate dihydrate. and 53.5g of ammonium chloride in 500 mL of water. Add 67 mL of ammonium hydroxide (sp.gr. 0.90). Transfer the solution to a 1000-mL volumetric flask and dilute to the mark with water.
19.4 Sodium Fluoride Solution, Standard (1.0 ml = 0.01 mg of F) - Dissolve 0.221 Og of NaF in water, dilute to 1.0L. Dilute 100 mL of this solution to 1.0 L of water, store ir Borosilicate glass or Polyethylene.
- Calibration
20.1 Prepare a series of three standards. 0.5, 1.0 and 2.0 mL using the fluoride standard solution (see 19.4). Dilute the following volumes to 100 mL:
|
Fluoride Solution, Standard. mL(1.0mL=0.01 mg F)
|
Concentration mg F/L
|
|
5.00
|
0.50
|
|
10.00
|
1.00
|
|
20.00
|
2.00
|
20.1.1 For unusual waters containing high concentrations of fluorides, the range standards may be expanded up to 1000 mg/L, if necessary.
20 2 Pipet 50 mL of each standard into a 150mL beaker, using a pipet, add 50 mL of buffer. Mix each solution well using a magnetic. stirrer.
20.3 Calibration of pH meter - Immerse the fluoride electrode in each stand solution, starting with the lowest concentration, and measure the developed potential while mixing. The electrodes must remain in the solution for at least 3 min., or until the reading has stabilized. Using a semi-logarithmic graph paper, plot the concentration of fluoride in milligrams per liter on the logarithmic axis versus the electrode potential developed in the standard on the linear axis, starting with the lowest concentration at the bottom of the scale.
20.4 Calibration of Specific Ion Meter: Follow the directions of the manufacturer for the operation of the instrument, See 20.1 and 20.1.1 for selection of standards.
- Procedure
21.1 Place 50.0 mL of the sample and 50.0 mL of buffer into a 150 mL beaker. Place the solution on a magnetic stirrer and mix at medium speed. Immerse the electrodes in the solution and observe the meter reading while mixing The electrodes must remain in the solution for at least 3 min., until the reading has stabilized. At concentrations less than 0.5 mg/L of fluoride, it may take as long as 5 min. to reach a stable meter reading; higher concentrations stabilize more quickly. If a pH meter is used, record the potential measurement for each unknown sample and convert the potential reading to the fluoride ion concentration of the unknown using the standard curve. If a specific ion meter is used. read the fluoride level in the unknown sample directly in milligrams per litre on the fluoride scale.
- Precision and Bias9
22.1 The precision of this test method, using the buffer solution described in 19.1. was tested by 111 laboratories and found to be 0.030 at a concentration of 0.85 mg/L of fluoride.8
22.2 The precision of the test method using the buffer solution described in 19.2 was tested by six operators in four laboratories in the presence of 20 mg/L of Al obtained from 247 mg/L of aluminum sulfate [Al2(SO4)318H2O]. The overall precision of the test method within its designated range varies with the concentration being tested; refer to Figs.2 and 3 and Table 4 which apply to the low and high-concentration ranges, respectively. The determination of bias is shown in Table 5.
22.3 The precision of the test method using the buffer solution described in 19.3 was tested by seven operators in five laboratories using the same quantity of aluminum as described in 22.2 The overall precision of the method within its designated range varies with the concentration being tested; refer to Figs 4 and 5 and Table 4, which apply to the low- and the high-concentration ranges, respectively. The determination of bias is shown in Table 6.
22.4 The precision of the test method was determined in reagent water and wastewater matrices. The data given may not apply to other types of matrices.
22.5 This precision and bias statement confirms to Practice D 2777-77, which was in place at the time of round robin testing It does not meet the requirements of Practices D 2777-86.