Figure III.4.3. Flow Injection Hardware Setup for ammonia determination for use in a unidirectional and/or stopped flow procedure

Figure III.4.3. Flow Injection Hardware Setup for ammonia determination for use in a unidirectional and/or stopped flow procedure.

Reproduced from J. Ruzicka, Flow Injection 2nd Edition (CD) FIAlab Instruments, with permission.

There are also condensed forms of phosphate as pyrophosphate and polyphosphates and organic phosphates. The environmental parameter known as “total phosphorous” TP, means the sum of all these forms. Other operational parameters are total reactive phosphorus (TRP), filterable reactive phosphorus (FRP), and total filterable phosphorus (TFP).

The natural phosphorus cycle does not depend on a significant atmospheric component (unlike nitrogen); being the chemical distribution of phosphorus between water and particulate components via adsorption and precipitation processes. Other bulk phosphorous sources are marine sediments, soils and rocks.

Most methods of phosphorus determination are based on the formation of phosphomolybdate heteropolyacid through the reaction of the analyte (phosphate) with molybdate in acidic medium (Figure III.4.5.). In a second step the heteropolyacid is then reduced to an intensely colored blue compound with a maximum absorbance band at 840 nm (McKelvie, Peat, and Worsfold, 1995).

The phosphorus determined is defined as “molybdate reactive” or soluble reactive phosphorus (SRP). When suspected the presence of other phosphorus containing organic compounds and condensed phosphates the process initiates with chemical, photochemical, thermal or microwave digestion prior to the molybdate reaction.

Some spectrophotometric examples on metal determination are presented below.

Cr (VI) reacts with 1,5-diphenylcarbazide (DCP) yielding a colored complex with maximum absorbance at 540 nm. Cr (III) do not interfere with this reaction and its determination is based on the same reaction after being oxidized to Cr(VI) by Ce (IV). A flow assembly provides the simultaneous or sequential determination of both chromium valences. Figure IV.4.7 depicts the flow-assembly for the sequential speciation. A selecting key allows the passage of the reagent or the oxidant. The first step consists in inserting the sample into a sulfuric acid stream and then merging with the reagent for the determination of Cr(VI). The second step is a new sample insertion to be oxidized by Ce(IV) before merging with the reagent and results in a transient signal proportional to the total chromium amount.

Flow–rates (in mL min^-1) were: 0.37, 0.30 and 1.22 for DPC, oxidant and sulfuric acid, respectively. A water-bath for the oxidation is required.

A flow assembly similar to the depicted has also been proposed for the speciation of As(III) and As(V). The oxidant was the potassium iodate and the colorimetric reaction was with molybdenum blue.

F
igure III.4.10.

The method is based on the catalytic effect of the Pb(II) ions on the redox reaction resazurine – sulfide in alkaline media. The catalytic reactions are very sensitive though lack selectivity. Several metals interfere with this redox reaction: namely, Cu(II), Co(II), Ni(II) and Fe(III) even at very low concentrations. The analytical former applications of this reaction implied the use of a certain number of tedious steps including toxic reagents to avoid interferences. To improve the selectivity and bearing in mind lead forms iodo-complexes in an acid medium unlike the other cited ions, the reported method uses a previous sample pre-treatment with potassium iodide and then the formed iodo-complexes are retained in a solid phase-reactor (placed in the external loop of an auxiliary injection valve) filled with the anionic resin exchanger AG1 X8 (Figure III.4.12). The retained lead iodo-complexes were eluted by 90 L of 0.2 mol L^-1 NaOH solution. The method was applied to drinking water samples and results were compared with the obtained with the electrothermal atomization Atomic Absorption Spectrometry.

An automated method for determination of free chlorine in water samples can be performed based on the oxidation of dianisidine as colorimetric reagent to release a colored product that can be spectrophotometrically monitored at 445 nm.

The manifold comprises a set of three solenoid valves acting as an independent switch. See Figure III.4.13. The sample (channel Q₃) merges with a 0.5 mol L^-1 HCl solution (Q₄); the global chlorine is converted into chlorine and transported through the gas diffusion membrane. Valves 2 and 3 control the time of stopped-flow and then the time of pre-concentration (volume of sample and diffusion of chlorine to a basic solution (Q₂) acting as acceptor solution). The resulting basic solution causes the quantitative decomposition of chlorine into hypochlorite, which facilitates the gradient solution and transport process through the membrane. After this pre-concentration step the volume comprised between valves 2 and 3 is forced to the flow system and merges with the solution of dianisidine (Q₁). A previous solenoid valve is used for recycling the non-inserted reagent solution.

Cyanide is highly toxic to mostly living organisms by preventing the normal activity of metal-containing molecules due to its property of strongly complexing metal cations. However, in low concentrations it is biodegradable by some bacteria. It can be found in industrial wastewater effluents, it is used in mining and different industries.

Hydrogen cyanide and cyanide salts are important environmental problems and there are numerous methods for determination of cyanide in air, water, workplace, etc. Hydrogen cyanide in environmental or workplace air is usually collected by flushing the sample (filtered to distinguish from the particulate cyanide) in sodium hydroxide solution, and then measured by the spectrophotometric procedure. It should be pointed out the problems derived from the instability of the samples (hydrogen cyanide is highly volatile). However, the presence of carbon dioxide from the sampling air may lower the pH and facilitate the releasing of hydrogen cyanide gas. Other problems appear from the oxidizing agents in solution, which may transform cyanide during storage and handling. It is recommended for the storage of cyanide samples to collect the samples at pH 12-12.5 in tightly closed dark bottles and store them as soon as possible in a cool, dark place. It is also recommended that the samples be analyzed immediately upon collection.

Particulate cyanides are known to decompose in moist air with the liberation of hydrogen cyanide. Filters are usually used to trap particulate cyanides, which can be quantified separately after acid distillation.

Inorganic cyanides in water sample can be present both as complexed and free cyanide and the determination of cyanide in water is usually classified in free cyanide, cyanide amenable to chlorination, and total cyanide.

Cyanides are usually measured by a sensitive colorimetric/ spectrophotometric procedure that can detect levels down to about 5 parts per billion in water. Since much of the cyanide in an industrial plant effluent is likely to be bound to metal ions, the sample is acidified and irradiated with UV light to convert metallo-cyanides into simple cyanides. The hydrogen cyanide formed is easily separated from the rest of the matrix when the sample flows through the dialyser provided with a porous membrane; the hydrogen cyanide ion is retained by the diluted alkaline flow stream.

The resulting alkaline stream merges with the chloramine T (N-chloro-p-toluene sulfonamide sodium salt) stream at pH less than 8 and the cyanide is converted into cyanogen chloride (CNCl). Finally the resulting cyanogen chloride gives a reddish-violet color compound by reaction with pyridine and barbituric acid.

The Figure III.4.15 depicts the segmented-flow method for the determination of the cyanide. In this methodology the liquid streams are segmented by air bubbles (periodically inserted). The solutions “closed” between two consecutive air bubbles are homogeneous (as contrary in FIA with the gradient concentration) and the transient signal is not a peak; it is theoretically speaking a rectangle, in practice a curved rectangle.

W, waste; m, porous membrane; D, detector; C, computer; P, peristaltic pump; L, UV lamp; and, R, reactors.

Solutions: 1 and 4, air; 2, sample or reference; 3, acidic medium; 5, alkaline medium; 6, chloramines T; 7, pyridine and barbituric acid solution.

Wavelength measurement at 570 nm

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