Spectrophotometric standard additions

A fifth spectrophotometric method for the quantitative determination of the concentration of Pb + in blood uses a multiple-point standard addition based on equation 5.6. The original blood sample has a volume of 1.00 mb, and the standard used for spiking the sample has a concentration of 1560 ppb Pb +. All samples were diluted to 5.00 mb before measuring the signal. A calibration curve of Sjpike versus Vj is described by... 

The standard addition method can take several forms as discussed in Section 8C-3 the single-point method was described in Example 8-8. The multiple additions method is often chosen for photometric or spectrophotometric analyses, and this method will be described here. This technique involves adding several increments of a standard solution to sample aliquots of the same size. Each solution is then diluted to a fixed volume before measuring its absorbance. When the amount of sample is limited, standard additions can be carried out by successive addition of increments of the standard to a single measured aliquot of the unknown. The measurements are made on the original solution and after each addition of standard analyte. This procedure is often more convenient for voltammetry. 

Several experiments have been carried out on samples of natural waters and industrial wastewaters (12 for Cu, 9 for Hg and 14 for Fe), with standard addition for some samples [55]. The analysis of the samples was performed both by a reference method (graphite furnace AAS for copper and iron, cold vapour AAS for Hg), and by the proposed spectrophotometric method. 

Figure 27. Comparison between the UV-visible spectrophotometric method and the corresponding reference methods for (a) copper, (b) mercury and (c) iron determination, with or without standard additions of metals in sample.

M.F. Gine, B.F. Reis, E.A.G. Zagatto, F.J. Krug, A.O. Jacintho, A simple procedure for standard additions in flow injection analysis. Spectrophotometric determination of nitrate in plant extracts, Anal. Chim. Acta 155 (1983) 131. 

Y. Israel, R.M. Barnes, Flow injection sample-to-standard additions method. Spectrophotometric determination of hydrochloric acid and orthophosphate, Analyst 114... 

M.S.P. Silva, J.C. Masini, Exploiting monosegmented flow analysis to perform in-line standard additions using a single stock standard solution in spectrophotometric sequential injection procedures, Anal. Chim. Acta 466 (2002) 345. 

Figure 4.7. (a) Two-line manifold for the spectrophotometric determination of calcium. Color reagent (o-cresolphthaleine complexone) and base are mixed and preheated in the first coil before the sample (30 xL) is injected and allowed to react in the second coil, the system being thermostated at 37°C. (b) Spectrophotometric determination of the total calcium content in serum at a sampling rate of approximately 110 samples/h. From left to right are shown a series of aqueous calcium standards containing (/) 25.0, (e) 20.0, (d) 15.0, (c) 10.0, (b) 5.0, and (a) 1.0 ppm, followed by three human serum samples, a serum pool (DSKK-K77), and the same serum pool sample with standard addition (25 p,L of a 500-ppm Ca standard plus 2.50 mL of serum pool). Each standard or sample was injected in quadruplicate. 

An example of the use of the standard-addition method is the determination of ammonia in aquaria and sea water [19] using an ammonia-gas electrode. To a 100 ml sample is added a sufficient number of NaOH pellets to raise the pH above 11 (to convert all ammonium ion to ammonia). The equilibrium potential is read, then a sufficient volume of standard NH4CI solution to approximately double the concentration is added, and the new equilibrium potential is read. The effective detection limit for this method is approximately 1 ppb NH3 but below about 10 ppb considerable time is required for the electrode to stabilize, making the method somewhat impractical at the 1-10 ppb level. Results compare favorably with the phenol-hypochlorite spectrophotometric method for ammonia. 

Another modification of the catalytic kinetic spectrophotometric method has been established for the determination of iodine using the principle that potassium periodate oxidize rhodamine B (RhB) to discolor and 1 has a catalytic effect on the reaction. The absorbance difference (AA) is linearly related with the concentration of iodine in the range of 0 - 2.6 pg/mL and fits the equation AA = 0.1578 C(C pg/mL) + 0.0052, with a regression coefficient of 0.9965. The detection limit of the method is 7.10 ng/mL. The method was used to determine iodine in kelp, potato, tap water, and rain water samples. The relative standard deviation of 13 replicate determinations was 1.81-2.10%. The recovery of the standard addition of the method was 96.2-99.2% (Zhaiet al., 2010). 

Afkhami A, Zarei AR (2004) Simultaneous spectrophotometric determination of hydrazine and phenylhydrazine based on their condensation reactions with different aromatic aldehydes in micellar media using H-point standard addition method. Talanta 62 559-565... 

Simultaneous determination of Co(II) and Cr(III) is described in J.Chem.Educ, 74(1997)848 75(1998)878. The Authors use a spectrophotometric analysis with a standard additions method. 

A. Direct titration. The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to PH = 10 with NH4-aq. NH3) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexing agent, such as tartrate or citrate or triethanolamine. At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This is generally determined by the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods. 

Potentiometric titrations can also be used to determine stability constants for complexes in some cases, which can be useful as a substitute or complementary approach to spectrophotometric titrations (56). By starting at a low pH, where the relative amount of complex formation is low, and then measuring the pH upon addition of standardized base, one can determine the relative amounts of the species in solution or how many equivalents of H+... 

Various chromogenic reagents have been used for the spectrophotometric determination of boron in seawater. These include curcumin [108,109], nile blue [110], and more recently 3,5 di-tert butylcatechol and ethyl violet [111]. Uppstroem [108] added anhydrous acetic acid (1 ml) and propionic anhydride (3 ml) to the aqueous sample (0.5 ml) containing up to 5 mg of boron per litre as H3BO3 in a polyethylene beaker. After mixing and the dropwise addition of oxalyl chloride (0.25 ml) to catalyse the removal of water, the mixture is set aside for 15-30 minutes and cooled to room temperature. Subsequently, concentrated sulfuric-anhydrous acetic acid (1 1) (3 ml) and curcumin reagent (125 mg curcumin in 100 ml anhydrous acetic acid) (3 ml) are added, and the mixed solution is set aside for at least 30 minutes. Finally 20 ml standard buffer solution (90 ml of 96% ethanol, 180 g ammonium acetate - to destroy excess of protonated curcumin - and 135 ml anhydrous acetic acid diluted to 1 litre... 

Detection, identification and quantification of these compounds in aqueous solutions, even in the form of matrix-free standards, present the analyst with considerable challenges. Even today, the standardised analysis of surfactants is not performed by substance-specific methods, but by sum parameter analysis on spectrophotometric and titrimetric bases. These substance-class-specific determination methods are not only very insensitive, but also very unspecific and therefore can be influenced by interference from other compounds of similar structure. Additionally, these determination methods also often fail to provide information regarding primary degradation products, including those with only marginal modifications in the molecule, and strongly modified metabolites. 

Tecator [20] has described a flow injection system for the determination of nitrate and nitrite in 2 mol/1 potassium chloride extracts of soil samples. Nitrate is reduced to nitrite with a copperised cadmium reductor and this nitrite is determined by a standard spectrophotometric procedure in which the soil sample extract containing nitrate is injected into a carrier stream. Upon the addition of acidic sulfanilamide a diazo compound is formed which then reacts with N-(l-naphthyl)ethylcncdiamine dihydrochloride provided from a second merging stream. A purple azo dye is formed, the intensity of which is proportional to the sum of the nitrate and the nitrite concentration. Nitrite in the original sample is determined by direct spectrophotometry of the soil extract without cadmium reduction. 

A standard official method has been published for the determination of cobalt in plant material [8 ]. The samples are digested with 1 4 v/v perchloric acidmitric acid and the residue dissolved in nitric acid. Cobalt is then extracted into chloroform as the diethyldithiocarbonate. The latter complex is decomposed by bromine and cobalt extracted into dilute hydrochloric acid. Following the addition of a borate buffer, cobalt is then extracted as the o-nitrocresol complex [9]. Excess coupling agent is removed by repeated extraction with copper acetate solution and cobalt determined spectrophotometrically at 360 nm. See Sects. 7.34.1, 7.34.3 and 7.34.4. 

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