USGS Test Method I-2327-85

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Method Name:
Fluoride Ion in Water by Ion-Selective Electrode, Dissolved. Official Name: Fluoride, Electrometric, Ion-Selective Electrode, Automated, Segmented-Flow

Fluoride is determined potentiometrically in a buffered sample with use of an ion-selective (fluoride) electrode in conjunction with a standard calomel reference electrode (SCE) and with a pH meter having an expanded millivolt scale (Frant and Ross, 1968; Harwood, 1969; Bellack, 1958). This electrode deteriorates in time and must be replaced when results become erratic. The method includes a distillation step to decompose organic fluoride compounds and attack minerals such as fluorspar in water-suspended sediment. A fixed volume of sample is added to a sulfuric acid solution having a specific boiling point and is distilled until an identical volume is recovered (Bellack, 1958). The fluoride is distilled as fluosilicic acid.

This method may be used to analyze water and water-suspended sediment containing from 0.1 to 3.0 mg/L of fluoride with conductivities less than 20,000 uS/cm. Samples with higher conductivities need to be distilled. Suspended total fluoride is calculated by subtracting dissolved fluoride from total fluoride.

Methods for the Determination of Inorganic Substances in Water and Fluvial Sediments, Techniques of Water-Resources Investigations of the United States Geological Survey, Book 5, Chapter A1 Edited by Marvin J. Fishman and Linda C. Friedman

The ion-selective electrode measures fluoride ion activity; thus, high concentrations of dissolved solutes (which lower the ion-activity coefficient) cause an error in the determination. Addition of a buffer solution that contains a high concentration of dissolved solutes effectively masks minor variations in the salt content of the samples and, therefore, minimizes this error. The optimum pH for measurement is between 5.0 and 8.5. Below this range, hydrofluoric acid is only slightly dissociated, and above a pH of 8.5, hydroxyl ion interferes. The addition of the buffer solution will adjust the pH of most samples to between 5.0 and 5.5. Several polyvalent cations capable of complexing fluoride ion interfere. These include iron(III), aluminum(III), and silicon dioxide. The extent of their interference is proportional to their concentration, so that dilution of the sample with an equal volume of buffer solution reduces the interference. The (1,2 cyclohexylenedinitrilo) tetra-acetic acid (CDTA) in the buffer solution complexes up to 10,000 ug iron(III), 2,000 ug aluminum(III), and 100 mg silicon dioxide. Orthophosphate-phosphorus concentrations of 25 mg/L and sulfate and chloride concentrations of 3,000 mg/L do not interfere (Harwood, 1969).

QC Requirements:
Quality-control samples area analyzed at a minimum of one in every ten samples. These QC samples include at least one of each of the following: blanks, quality control samples, third party check solutions, replicates, and spikes. Correlation coefficients for calibration curves must be at least 0.99. QC samples must fall within 1.5 standard deviations of the mean value. If all of the data-acceptance criteria in the SOPs are met, then the analytical data are acceptable.

Maximum Holding Time:
180 days from sampling



0.1 - 3.0 (undiluted)

The standard deviation for dissolved fluoride within the range of 0.33 to 1.28 mg/L for nine samples was found to be independent of concentration. The 95-percent confidence interval for the average standard deviation of 0.07 mg/L ranged from 0.05 to 0.09 mg/L. Precision for dissolved fluoride for three of the nine samples as performed by other laboratories is expressed as follows: 3, 8 and 4 laboratories found mean recoveries of 0.33, 1.09 and 1.28 mg/L with 18, 6 and 7 %-RSD, respectively.

Method detection limits (MDLs) were determined for this method using the procedures of the U.S. Environmental Protection Agency (1997).

Revision Number:

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