Interference fluoride

The chief interference is from Fe(lll), which forms a green complex with chromotropic acid. Before the determination of Ti, larger quantities of iron should be separated or smaller ones reduced with ascorbic acid or sulphite. Vanadium in quantities not exceeding those of titanium has no appreciable effect on the determination of Ti. Molybdenum at concentrations below 50 pg/ml does not interfere. Fluoride interferes by masking titanium, but can be removed by fuming with H2SO4. Oxidants (e.g. HNO3) must be absent because chromotropic acid is fairly easily oxidized. 

Colored salts like copper, chromium, cobalt, and nickel will reduce the sensitivity of the test, and all heavy metals are expected to interfere. No elements are stated to give a false positive reaction, but a number of ions can interfere. Reductants, like tin(II), can reduce Fe to Fe and will (ultimately) give a false negative result. Ions capable of forming strong complexes with the ions of the test are another cause of interference. Fluoride, acetate, oxalate, and tartrate are examples. 

Small quantities of Iron may be oomplexed with acetic, laotlo, or malonlc acid. Low yields may result from the use of malonlo acid. Ammonium, i>otassl im, and alkaline earths retard the rate of precipitation. Complexlng Ions such as oxalate, tartrate, sulfate, and fluoride dn large quantities, also Interfere. Fluoride Ion may be conplexed with aluml-... 

Many samples containing silicon also contain aluminum and iron. After dehydration, these metals are present as AI2O3 and Fe203. These oxides are potential interferents since they also are capable of forming volatile fluorides. 

Fluoride. A fluoride concentration of ca 1 mg/L is helpful in preventing dental caries. Eluoride is deterrnined potentiometrically with an ion-selective electrode. A buffer solution of high total ionic strength is added to the solution to eliminate variations in sample ionic strength and to maintain the sample at pH 5—8, the optimum range for measurement. (Cyclohexylenedinitrilo)tetraacetic acid (CDTA) is usually added to the buffer solution to complex aluminum and thereby prevent its interference. If fluoroborate ion is present, the sample should be distilled from a concentrated sulfuric acid solution to hydrolyze the fluoroborate to free fluoride prior to the electrode measurement (26,27). 

Several colorimetric procedures for fluoride are available, but it is usually desirable to distill the sample from concentrated sulfuric acid prior to analysis to eliminate interferences. One method is based upon bleaching a dye formed by the reaction of zirconium and sodium 2-(p-sulfophenylazo)-l,8-dihydroxy-3,6-naphthalenedisulfonate (SPADNS reagent) (28). 

Fluorides. Most woddwide reductions in dental decay can be ascribed to fluoride incorporation into drinking water, dentifrices, and mouth rinses. Numerous mechanisms have been described by which fluoride exerts a beneficial effect. Fluoride either reacts with tooth enamel to reduce its susceptibihty to dissolution in bacterial acids or interferes with the production of acid by bacterial within dental plaque. The multiple modes of action with fluoride may account for its remarkable effectiveness at concentrations far below those necessary with most therapeutic materials. Fluoride release from restorative dental materials foUow the same basic pattern. Fluoride is released in an initial short burst after placement of the material, and decreases rapidly to a low level of constant release. The constant low level release has been postulated to provide tooth protection by incorporation into tooth mineral. 

Fluoride analysis requires a previous separation process step, which is known as Willard-Winter distillation using a mixture of HClOyH PO, to isolate fluoride from interferences like Al, Cl, etc. 

Ion-selective electrodes are a relatively cheap approach to analysis of many ions in solution. The emf of the selective electrode is measured relative to a reference electrode. The electrode potential varies with the logarithm of the activity of the ion. The electrodes are calibrated using standards of the ion under investigation. Application is limited to those ions not subject to the same interference as ion chromatography (the preferred technique), e.g. fluoride, hydrogen chloride (see Table 10.3). 

Note. Under the above conditions of determination the following elements interfere in the amount specified when the amount of Mo is 10 fig (error greater than 3 per cent) V, 0.4 mg, yellow colour [interference prevented by washing extract with tin(II) chloride solution] Cr(VI), 2 mg, purple colour W( VI), 0.15 mg, yellow colour Co, 12 mg, slight green colour Cu, 5 mg Pb, 10 mg Ti(III), 30 mg (in presence of sodium fluoride). 

Take 5mL of a solution containing about 15 mg of aluminium and adjust the pH to between 4 and 6. Equilibrate the solution for 10 minutes with two successive 5 mL portions of a solution made up of equal volumes of acetylacetone (pure, redistilled) and chloroform. Combine the organic extracts. Fluoride ion causes serious interference to the extraction and must be previously removed. 

Discussion. This method is based upon the precipitation of lead chlorofluoride, in which the chlorine is determined by Volhard s method, and from this result the fluorine content can be calculated. The advantages of the method are, the precipitate is granular, settles readily, and is easily filtered the factor for conversion to fluorine is low the procedure is carried out at pH 3.6-5.6, so that substances which might be co-predpitated, such as phosphates, sulphates, chromates, and carbonates, do not interfere. Aluminium must be entirely absent, since even very small quantities cause low results a similar effect is produced by boron ( >0.05 g), ammonium (>0.5 g), and sodium or potassium ( > 10g) in the presence of about 0.1 g of fluoride. Iron must be removed, but zinc is without effect. Silica does not vitiate the method, but causes difficulties in filtration. 

Fluoride ion, and weak acids and bases do not interfere, but nitrate, nitrite, perchlorate, thiocyanate, chromate, chlorate, iodide, and bromide do. Since analysis of almost all boron-containing compounds requires a preliminary treatment which ultimately results in an aqueous boric acid sample, this procedure may be regarded as a gravimetric determination of boron. 

Phosphate, arsenate, and vanadate interfere. Borate, fluoride, and large amounts of aluminium, calcium, magnesium, and the alkali metals have no effect in the determination, but large amounts of iron (> 5 per cent) appear to produce slightly low results. 

Interferences in the distillation method are fluoride and large amounts of gelatinous silica. Fluoride interference may be overcome by the addition of calcium chloride. Strong oxidising agents, such as chromate and nitrate, interfere,... 

The cobalt complex is usually formed in a hot acetate-acetic acid medium. After the formation of the cobalt colour, hydrochloric acid or nitric acid is added to decompose the complexes of most of the other heavy metals present. Iron, copper, cerium(IV), chromium(III and VI), nickel, vanadyl vanadium, and copper interfere when present in appreciable quantities. Excess of the reagent minimises the interference of iron(II) iron(III) can be removed by diethyl ether extraction from a hydrochloric acid solution. Most of the interferences can be eliminated by treatment with potassium bromate, followed by the addition of an alkali fluoride. Cobalt may also be isolated by dithizone extraction from a basic medium after copper has been removed (if necessary) from acidic solution. An alumina column may also be used to adsorb the cobalt nitroso-R-chelate anion in the presence of perchloric acid, the other elements are eluted with warm 1M nitric acid, and finally the cobalt complex with 1M sulphuric acid, and the absorbance measured at 500 nm. 

Sulphuric acid is not recommended, because sulphate ions have a certain tendency to form complexes with iron(III) ions. Silver, copper, nickel, cobalt, titanium, uranium, molybdenum, mercury (>lgL-1), zinc, cadmium, and bismuth interfere. Mercury(I) and tin(II) salts, if present, should be converted into the mercury(II) and tin(IV) salts, otherwise the colour is destroyed. Phosphates, arsenates, fluorides, oxalates, and tartrates interfere, since they form fairly stable complexes with iron(III) ions the influence of phosphates and arsenates is reduced by the presence of a comparatively high concentration of acid. 

The colour is unaffected by the presence of phosphate or fluoride. Titanium and molybdenum) VI) (which give colours with hydrogen peroxide) and tungsten interfere. Titanium may be removed by adding fluoride or hydrofluoric acid, which simultaneously remove the yellow colour due to iron(III). If titanium is absent, phosphate may be used to decolorise any iron(III) salt present. Oxalic acid eliminates the interference due to tungsten. In the presence of elements... 

The amount of reddish-purple acid-chloranilate ion liberated is proportional to the chloride ion concentration. Methyl cellosolve (2-methoxyethanol) is added to lower the solubility of mercury(II) chloranilate and to suppress the dissociation of the mercury(II) chloride nitric acid is added (concentration 0.05M) to give the maximum absorption. Measurements are made at 530nm in the visible or 305 nm in the ultraviolet region. Bromide, iodide, iodate, thiocyanate, fluoride, and phosphate interfere, but sulphate, acetate, oxalate, and citrate have little effect at the 25 mg L 1 level. The limit of detection is 0.2 mg L 1 of chloride ion the upper limit is about 120 mg L . Most cations, but not ammonium ion, interfere and must be removed. 

The effect of different ions upon the titration is similar to that given under iron(III) (Section 17.57). Iron(III) interferes (small amounts may be precipitated with sodium fluoride solution) tin(IV) should be masked with 20 per cent aqueous tartaric acid solution. The procedure may be employed for the determination of copper in brass, bronze, and bell metal without any previous separations except the removal of insoluble lead sulphate when present. 

The procedure utilises eriochrome blue black RC (also called pontachrome blue black R Colour Index No. 15705) at a pH of 4,8 in a buffer solution. Beryllium gives no fluorescence and does not interfere iron, chromium, copper, nickel, and cobalt mask the fluorescence fluoride must be removed if present. The method may be adapted for the determination of aluminium in steel. 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

It has been well established that the fluoride electrode is highly selective in its response to the activity of fluoride, with the only common interferant being hydroxide [65]. Nevertheless, although seawater has a uniform pH of 8, possible hydroxide interference was shown not to be a problem. 

Complexation of fluoride by metal ions in seawater has previously been overcome by the addition of TISAB solution. The reagent is presumed to release the bound fluoride by preferential complexation of the metal ions with EDTA type ligands present in the TISAB. Examination of the metal ions present in seawater [66,67] suggests that magnesium is the major species forming fluoride complexes. Theoretical calculations demonstrate that even this species is unlikely to interfere. 

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