Fluoride spectrophotometric methods

The potentiometric method also surpasses the others for speed, simplicity, precision and accuracy as indicated in Table 12.1. Furthermore, it is particularly suited to the continuous monitoring of fluoride levels in drinking water. The spectrophotometric methods are lengthy because of the time required to develop a stable colour (up to 1 hour), the alizarin red-S complex being especially poor in this respect. It was noted, however, that for the three bleaching methods (1-3) the rate of change of absorbance by the blank closely followed that of solutions containing fluoride, i.e. the difference between the blank and a sample absorbance is nearly constant. 

A further spectrophotometric method [3, 4] for water soluble boron in soil, boron is extracted from soil with boiling water. Borate in the extract is converted to fluoroborate by the action of orthophosphoric acid and sodium fluoride. The concentration of fluoroborate is measured spectrophotometrically as the blue complex formed with methylene blue and which is extracted into 1, 2-dichloroethane. Nitrates and nitrites interfere they are removed by reduction with zinc powder and orthophosphoric acid. 

Table 1. The effect of 1 mM NaF + 20 pM A1C13 on the acetylcholinesterase activity (AChE) in freshly prepared intact RBC and in hemolysate of patients with AD (mean age 72.5 5.1 years), age-matched healthy controls (AM-HS) (72.1 1.6 years), and the group of young healthy subjects (YS) (35.9 8.5 years). Whole venous blood samples were drawn from each subject after overnight fasting., always at 07 30 AM. Red blood cells (RBC) were isolated from the blood of patients with AD, AM-HS, and YS by centrifugation [68], RBC AChE activity was evaluated in intact freshly prepared RBC or hemolyzate following the spectrophotometric method [45] with modifications. Buffer was Tris-HCl, pH 7.5 in the solution of 154 mmol L 1 NaCl, acetylthiocholine iodide was a substrate. Measurement of enzymatic activity was performed in fluorimeter polystyrene cuvettes for 3 min (UV/VIS spectrophotometer Shimadzu, Japan). The effects of 1 mmol L-1 NaF in the presence of 20 pmol L 1 A1C13 were measured. Data are expressed in percentage of the AChE activity in the absence of aluminum and fluoride ions. No differences between the AChE activity were found between the investigated groups...

Fluoride Determine as directed in Method III under Fluoride Limit Test, Appendix IIIB, except in the Procedure, use 10 mL of 1 N hydrochloric acid to dissolve the sample. Lead Determine as directed in the Flame Atomic Absorption Spectrophotometric Method under Lead Limit Test, Appendix IIIB, using a 10-g sample. 

There are also indirect spectrophotometric methods, in which the element determined provokes a change in colour. This group comprises most of the methods for the determination of fluoride. Being capable of forming stable complexes with some metals, fluoride anions can decompose colour complexes of those metals. Thus, in the method involving a sulphosalicylate complex of Fe(lII) the solution is discoloured by F and a change of colour is observed in the method based on the use of the Zr complex with ECR. Still another example is the determination of phosphate with the use of lanthanum chloranilate. Phosphate anions react to form the less soluble LaP04 and release coloured chloranilate ions. 

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. 

Direct spectrophotometric methods proposed for fluoride (similar to the Alizarin Complexone method) involve use of Arsenazo lE-Zr reagent [62,63], Sulphochlorophenol S-Zr reagent (e = 3.0-10 ) [64], Xylenol Orange-Al (extraction of ternary fluoride-containing complex into CHCI3 in the presence of TOA, e = 4.MO" ) [65], and Xylenol Orange-Zr reagent (e = 3.4-10 ) [66]. 

Once in solution, the preferred method for measurement of boron is inductively coupled plasma atomic emission spectroscopy (ICP-AES) or inductively coupled plasma mass spectrometry (ICP-MS). The most widely used nonspectrophotometric method for analysis of boron is probably ICP-MS because it uses a small volume of sample, is fast, and can detect boron concentrations down to 0.15 pgL . When expensive ICP equipment is not available, colorimetric or spectrophotometric methods can be used. However, these methods are often subject to interference (e.g., nitrate, chloride, fluoride), and thus must be used with caution. Azomethine-H has been used to determine boron in environmental samples (Lopez et al. 1993), especially water samples. Another simple, sensitive spectrophotometric method uses Alizarin Red S (Garcia-Campana et al. 1992). 

The advantage of discrete analyzers is that sample crossover in the system itself is the lowest possible. Volumes of 75 pi of reagent and sample volumes as large as 100 pi are sufficient. In an automated system with a throughput of 200 determinations per hour in the same sample 6 to 10 components (such as ammonium, alkalinity, aluminum, boron, bromide, calcium, chloride, chromium(VI), cyanide, fluoride, iron, magnesium, nitrate, nitrite, phosphate, etc.) can be determined. In discrete analyzers normally conventional spectrophotometric methods are used. These methods are prone to interference of the matrix of the sample. As a good concept for interference studies still is not available, interferences are as yet not sufficiently studied systematically even for routine analyses. 

A spectrophotometric method was worked out by Wang and coworkers [53] for the determination of trace level fluoride concentration of water samples. In this procedure, the samples are mixed with a reagent mixture (alizarin-3-methylimino-N,N-diacetic acid/ sodium acetate/12.5% acetic acid buffer of pH 4.1/1 mM lanthanum nitrate (1 1 1 )). After a reaction time the colored complex is extracted with 5% N,N-dimethylaniline solution in 3-methylbutan-l-ol and the absorbance is detected in a special, long capillary at 580 nm. 

A Spectrophotometric Determination with Microdistillation Combined FIA Method La(III)-alizarine complexone (La-ALC) fluoride ternary complex formations based spectrophotometric fluoride determination method was reported by Shimaada et al. [54]. The spectroscopic detection does not have the selectivity needed in the low concentration ranges therefore, a separation step was inserted into the analytical procedme. 

The limited number of well functioning, classical or spectrophotometric methods is available for measuring fluoride ion concentration in different samples. Therefore, after the invention of lanthanum fluoride crystal-based ISE [15], its use as a detector in standardized methods becomes almost general. For example, the Environmental Protection Agency (ERA) METHOD 9214 [44] is for measuring the concentration of fluoride ions in water samples as well as in soil extracts. It is a direct potentiometric method using the ion-selective fluoride electrode and the conventional or double junction reference electrode. The lower limit of detection is 0.025 mg dm. Fluoride concentrations from 0.025 to 500 mg dm can be measured. 

Many of its hydroxy derivatives form strong complexes with metal ions. For example. Alizarine (1,2-Dihydroxyanthraquinone) forms coloured suspensions or solutions by complexing with one of a wide range of metal ions, including Zr, Th, Ga, In, Ti and U. The red-violet Zr IV) chelate is decolourised by F , by preferential complexation of F with Zr(/V), thus providing the basis of a popular spectrophotometric method for the determination of fluoride. 

A method for the determination of fluorine in fluorinated polymers such as polytetrafluoroethylene (PTFE) is based on decomposition of the sample by oxygen flask combustion followed by spectrophotometric determination of the fluoride produced by a procedure involving the reaction of the cerium(III) complex of alizarin complexan (1,2-dihydroxy-anthraquinone-3-ylmethylamine N,N-diacetic acid). The blue colour of the fluoride-containing complex (maximum absorption, 565 nm) is completely distinguishable from either the yellow of the free dye (maximum absorption, 423 nm) or the red of its cerium(III) chelate (maximum absorption, 495 nm). 

The spectrophotometric procedure with alizarin complexone is the most widely used method for the determination of fluoride in seawater. Small modifications have been made to the method first proposed by Greenhalgh and Riley (1961). 

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