Cardinal Laboratories, LLC
Standard Operating Procedure
Procedure No.: 4020 Revision Date: 8/2005
Subject: (Method SM-4500-F,C) Fluoride
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Revision Date Approved By: Date
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8/2005
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Fluoride
Method SM 4500-F C
Description
This method is used to determine fluoride in public drinking water and wastewater. Fluoride concentration is determined by adding TISAB-II buffer to the solution, and measured with an ion-selective electrode. Method detection limit 0.10 mg/L.
Sample Preservation
· Samples must be collected in a clean plastic screw top container (250-1000 mL)
· No preservative is necessary
· Samples should be cooled to 4°C after sampling and during storage; holding times on samples is 7 days.
Interferences
Fluoride can form complexes with several polyvalent cations. Aluminum concentrations greater than 3.0 mg/L and Iron concentrations greater than 200 mg/L can cause interference. Also, samples can be distilled if the dissolved solids exceed 10,000 mg/L. For further lists of interferences, consult Table 4500-F:I in Standard Methods, 18th edition.
Apparatus and Materials
· Orion EA940 Expandable Ion Analyzer
· Orion Single Junction Reference Electrode
· Orion Fluoride Electrode
· 250 mL reusable plastic beakers
· Magnetic Stir Plate
· Teflon coated stirring bars
· 2 25 mL graduated cylinders
· Class “A” volumetric pipets and flasks for standard preparation
Reagents and Standards
· Stock fluoride solution (1000 ppm), obtained commercially.
· Fluoride working standard (10 ppm):
Dilute 1 ml stock fluoride solution to 100 mL with deionized water. Prepare daily.
· TISAB II buffer solution, commercially available.
· QC check standard in the range of 0.5 to 1.5 mg/L, commercially available. For example, quality control samples from Environmental Resource Associates.
· Fluoride toothpaste
Procedure:
1. Sample Preparation
1.1. All standards and samples must be buffered with TISAB solution prior to calibration/analysis.
1.2. Allow standards and samples to come to room temperature before analysis.
1.3. Using a graduated cylinder, measure 25 mL of sample/standard and pour into a 250 mL plastic beaker. Rinse the cylinder with deionized water.
1.4. Using the other graduated cylinder, measure 25 mL of TISAB II into the beaker.
2. Instrument Calibration
2.1. Prepare the following calibration standards by pipetting the following volumes of the working standard into 100 mL volumetric flasks:
mL working standard Concentration (mg/L)
5.0 0.50
10.0 1.00
50.0 5.00
Calibration will be performed with the 0.50 and 5.00 mg/L standards. The 1.00 mg/L used as a check after calibration is completed.
2.2. Prepare the QC Check standard according to the Check standard provider’s instructions.
2.3. Prepare standards according to step 1.1-1.4 above.
2.4. Prepare a blank sample using deionized water according to steps 1.1-1.4.
2.5. Connect the electrodes to the Orion ion analyzer. Prep the fluoride electrode by wiping it with a small amount of fluoride toothpaste.
2.6. On the Orion EA940, select the correct electrode and begin calibration.
2.7. When asked if you wish to calibrate by direct measurement, select YES.
2.8. When asked for the number of standards, select 2.
2.9. When asked if you wish to use blank correction, select YES.
2.10. Place a magnetic stir bar in the blank beaker and put it on the magnetic stir plate. Place the electrode in the beaker and begin stirring.
2.11. After a period of time, the blank reading will stabilize. A READY indicator will be displayed.
2.12. Thoroughly rinse the electrodes with deionized water and blot dry between each standard to prevent cross contamination.
2.13. Place the 0.5 mg/L standard on the stir plate and begin stirring. Place the electrodes in the standard.
2.14. Meter reading should stabilize within three minutes. After the standard is finished reading, you will be asked to calibrate the electrode using a standard value. If the value matches the concentration of the standard being measured, select YES. If not, select NO, then enter the value of the standard being measured using the keypad. You should be prompted again to calibrate with the new value. Select YES.
2.15. Repeat steps 2.13-2.14 with the 5.0 mg/L standard.
2.16. After calibrating with the second standard, the electrode will be calibrated and the slope will display. Acceptable slope is –54 to –60 mV/DEC.
- Sample Analysis
3.1. Prepare all samples according to steps 1.1-1.4 above.
3.2. Place a magnetic stir bar in the sample. Place the sample beaker on the magnetic stir plate and begin stirring.
3.3. Place the electrodes in the sample. The meter should stabilize within three minutes. Record concentration in mg/L directly off the screen.
3.4. Thoroughly rinse the electrodes with deionized water and blot dry between each sample to prevent cross-contamination.
3.5. Read the 1.0 mg/L standard using steps 3.1-3.4 above. The reading must be within ± 10% or the meter should be recalibrated.
3.6. Read the QC check standard using steps 3.1-3.4 above. The reading should be within ± 10%. If not, check equipment and standards for error.
3.7. Read each sample according to steps 3.1-3.4 above.
4. Documentation
4.1. Record all calibration information, QC results, and sample results in the Fluoride analytical logbook. The record must be correctly dated and accompanied by the analysts’ initials.
4.2. Document all calibration results and QC checks on Form 4020-A.
4.3. Record standard information: Date Received, Expiration Date, Date Opened, Date Prepared.
4.4. Record any maintenance or replacement of apparatus.
- QA/QC Requirements
5.1. Calibration slope must be within –54 to –60 mv/DEC.
5.2. The 1.0 mg/L standard must read within ± 10% for calibration to be acceptable.
5.3. A QC check standard in a range of 0.5 to 1.5 mg/L is run with each batch. This standard must be within ± 10% of the actual value.
6. Electrode Storage
6.1. Fluoride electrode: rinse electrode and blot dry. Electrode may be stored in air. For long-term storage, place the protective cap on the electrode to prevent damage to the sensing crystal.
6.2. Reference electrode: For short periods (up to 1 week) the electrode may be stored in its filling solution or deionized water. For longer periods, drain the electrode completely, rinse with deionized water and store dry.
References:
Standard Methods for the Examination of Water and Wastewater 18th Edition
Ohio EPA Laboratory Manual for Chemical Analyses of Public Drinking Water, 2000 ed.
Procedure 4020 Procedure 6600
Revised 8/05