Colored solutions, determination

Colorimetric Method. A finely powdered sample treated with sulfuric acid, hydrobromic acid [10035-10-6] and bromine [7726-95-6] gives a solution that when adjusted to pH 4 may be treated with dithi one [60-10-6] ia / -hexane [110-54-3] to form mercuric dithi2onate [14783-59-6] (20). The resultant amber-colored solution has a color iatensity that can be compared against that of standard solutions to determine the mercury concentration of the sample. Concentrations below 0.02 ppm have been measured by this method. 

The diastase activity was traditionally determined according to the Schade method in the earlier years (Schade et al., 1958). One unit of diastase activity (or more specifically, a-amylase), DN, is defined as that amoimt of enz)nne that converts 0.01 g of starch to the prescribed endpoint in 1 h at 37 °C under the experimental conditions. In this assay, a standard solution of starch, which reacts with iodine to produce a color solution, is used as a substrate for honey enzymes under the standard conditions (Rendleman, 2003). A recently developed procedure uses an insoluble, dyed starch substrate (Persano Oddo and Pulcini, 1999). As this substrate is hydrolyzed by ot-amylase, soluble dyed starch fragments are released into solution. After reaction termination and insoluble substrate removal by centrifugation, absorbance of the supernatant solution (at 620 nm) is measured. The absorbance is proportional to the diastase activity. This procedure has been widely adopted in the honey industry due to the convenience of a commercially available substrate and the simple assay format. 

Attention is finally focused on the advantages of conductometric titrations, which include (i) colored solutions where no indicator is found to function satisfactorily can be successfully titrated by this method (ii) the method is useful for titrating weak acids against weak bases, which does not produce a sharp change in color with indications in ordinary volumetric analysis and (iii) more accurate results are obtained because of the graphical determination of the end-point. 

Absorption curves of the colored solutions were run on a Beckman spectrophotometer using 1.00-cm. quartz cells. A typical curve, shown in Figure 1, has an absorption maximum at about 515 millimicrons. The Coleman Junior spectrophotometer was used for routine determinations of Compound 118 throughout this study. 

An excellent example of this type of analysis involves the determination of phosphate in soil extracts. Soil is extracted with an appropriate extractant and added to a solution of acid molybdate, with which the phosphate reacts to produce a purple- or blue-colored solution of phosphomolybdate. Standard phosphate solutions are prepared, reacted with acid molybdate, and the intensity of the phosphomolybdate color produced is measured. A standard curve (also called a calibration curve) is prepared (see Section 14.10) from which the intensity of the color is directly related to the concentration of phosphate in the extract. 

Of course, not all dissolved ions produce colored solutions, and therefore not all ions in solution can be quantified by colorimetry. Noncolored solutions can sometimes, however, be converted to colored solutions by introducing chromophore species which complex with (i.e., attach themselves to) the target ion to produce a colored solution, which may then be measured by UV/visible colorimetry. An important archaeological example of this is the determination of phosphorus in solution (which is colorless) by com-plexation with a molybdenum compound, which gives a blue solution (see below). The term colorimetry applies strictly only to analytical techniques which use the visible region of the spectrum, whereas spectrophotometry may be applied over a wider range of the electromagnetic spectrum. 

The value of an equilibrium constant is calculated by measuring (or calculating) the equilibrium concentrations of the reactants and products. A calibration curve is constructed by measuring the absorbance of a colored solution versus its concentration. Known quantities of the reactants are mixed, and the calibration curve is used to determine the concentration of the colored substance in the resultant solution. (See the Equilibrium chapter.)... 

Condensation of isoniazid (isonicotinic acid hydrazide) with any a" -3-ketosteroid in acidic medium affords a yellow color conjugate, allowing the determination of 10-40 pg of the steroid [73]. The reagent is prepared by dissolving 0.8 g of isoniazid in 100 mL of methanol containing 1 mL of concentrated HCl, and then 12.5 mL of this solution is further diluted to 100 mL with methanol. To a 2 mL methanolic sample solution of the 3-ketosteroid is added 2 mL of the reagent. The mixture is allowed to stand at room temperature for an hour, and then the absorbance of the resulting colored solution is measured at 380 nm. In a 1 cm spectrophotometer cell, an absorbance of 0.3 will be produced by 42 pg of cortisone, 34 pg of prednisolone, 29 pg of prednisone, or 32.5 pg of dexamethasone [72]. 

COLORIMETRY. A method of chemical analysis thal deals with the measurement of the light absorption by colored solutions. Since light absorption depends upon the concentraiion of a specific constituent in solution, colorimetry is frequently used by geologists to determine qualitatively the trace quantities of many elements. 

Color of solution—Determine the absorbance of the solution prepared for the Completeness of solution test at 440 nm, in 1-cm cells, using methylene chloride as the blank the absorbance is not greater than 0.1. 

To standardize the solution, perform the following in triplicate Pipet 25.0 mL of 0.1000 N Potassium Iodate Solution into a wide-mouthed Erlenmeyer flask. Add 2.00 0.01 g of potassium iodide, and shake the flask to dissolve the potassium iodide crystals. Pipet 5.0 mL of concentrated hydrochloric acid into the flask, and titrate the free iodine with Sodium Thiosulfate Solution to a light yellow color. Add a few drops of Starch Solution, and continue the titration until 1 drop produces a colorless solution. Determine the Sodium Thiosulfate Solution normality using the following formula ... 

Insoluble Substances and Organic Matter A 1 20 aqueous solution is complete, clear, and colorless to slightly colored. Lead Determine as directed under Lead Limit Test, Appendix TTTB, using 2 pg of lead (Pb) ion in the control and the following as the Sample Solution Dissolve 1 g of sample in a mixture of 5 mL of water and 11 mL of 2.7 N hydrochloric acid, and cool. 

Organometallic compounds containing gallium rings were first reported in 1995 sodium metal reduction of (2,6-Mes2C6H3)GaCl2 resulted in a deeply red colored solution from which a red (almost black) crystalline compound was isolated (equation 10). These crystals were determined to be Na2 [(2,6-Mes2C6H3)Ga]3... 

The rate at which this colored solution enters the flask is determined by the size of the funnel stem, not how fast the solution is poured,... 

Hydroxylamine hydrochloride was used as an indicator for the determination of Co(acac)2. The absorbance of the resulting colored solution was measured at 590 nm . Iridium -diketonates were fluorinated oxidatively with BrFs in Freon 113, followed by decomposition in 6M HCl to IrCle and spectrophotometric determination at 488 nm. ... 

Figuie 19. Absolute configurations and signs of rotation (tetrahydrofuran) for resolved K-region derivatives of polycyclic aromatic hydrocarbons. References are superscripted. Rotation of the free cis-4,5-dihydrodiol of benzo[a]pyrenc was not determined because of facile autoxidation which resulted in colored solutions. With liver microsomal enzymes from 3-methylcholanthrene-treated rats, the frar7s-(R,R)-dihydrodiol greatly predominates (96%) from benzo[a]pyrene, predominates (68%) from benz[a]anthracene, appears to be the minor enantiomer from 12-methylbenz[a]anthracene, and is the minor enantiomer (42%) from phenanthrene. ... 

In 1950 Seligman and his co-workers (S13) suggested the use of sodium j8-naphthyl phosphate as a substrate for the determination of acid or alkaline phosphatase activity. For the former, 1 ml of 1 20 diluted serum was added to 5 ml of 0.4 mM sodium 8-naphthyl phosphate in 0.1 M acetate buffer of pH 4.8, and the reaction was allowed to proceed for 2 hours at 37.5°C. The addition of 4 drops of IM sodium carbonate solution served to retard the reaction as well as to raise the pH to the optimal level for coupling with 1 ml of a solution of tetrazotized ortho-dianisidine. After 3 minutes, the protein was precipitated with trichloroacetic acid, the dye extracted with ethyl acetate, and the color density determined in the region of 540 nm. The unit of phosphatase activity was defined as that amount of enzyme which liberates the color equivalent of 10 ml of j8-naphthol per hour at 37.5° in 1 hour. The serum acid phosphatase in a group of normal adults ranged from 0.7 to 1.6 units and averaged 1.0 unit per 100 ml of serum. 

In this collaborative study fourteen laboratories analyzed six different commodities containing aflatoxin B, at two concentration levels as blind duplicates. Two standards (15 and 50 ng B,/g) were provided, and collaborators were asked to report their results as <15 or >15 ppb. Laboratories with microtiter well readers were asked to determine aflatoxin concentrations both visually and spectrophotometrically. In this kit aflatoxin B,-antibodies are coated onto plastic microtiter wells. The aflatoxin-containing sample is extracted with Me0H-H,0 (55+45). The extract is defatted with hexane, and the MeOH extract mixed with the aflatoxin-enzyme conjugate and added to the well of the antibody-coated microtiter plate. The aflatoxin in the extract and the aflatoxin-enzyme complex compete for the antibody binding sites. The enzyme substrate(ABTS) and HjOj solution are then added, the reaction leading to a colored product in the presence of enzyme. The intensity of color is determined visually or spectrophotometrically at 580nm. 

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