Experiment 17 Colorimetric Analysis

This experiment involves determining the amount of a substance in a solution. A calibration curve is constructed plotting the measured absorbance versus the concentration of a colored substance. The concentration of an unknown solution may be determined by reversing this process. (See the Stoichiometry chapter.) 

Using either a pipet or a buret, quantities of standard solutions are measured. (If a buret is used, separate measurements of the initial and final volumes are needed.) Solvent may be added to dilute the samples if needed. These are the known solutions from which a calibration curve will be constructed. 

The absorbance of each solution is determined with a spectrophotometer. 

Concentrations of the known solutions are calculated using the dilution equation. 

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A good example of a bidentate ligand is the 1,10-phenanthroline molecule, which, since it forms a stable complex ion with Fe2+ ions that is deep orange color, is used in the colorimetric analysis of iron(II) ions (see Experiment 19 in Chapter 7). This ligand is shown in Figure 5.17(a). The two bonding sites are... 

Experiment 19 Colorimetric Analysis of Prepared and Real Water Samples for Iron... 

However, near-stoichiometric Fe " ion binding to NifU-1 or NifU was observable only in experiments conducted at 2°C in anaerobic samples that had been pretreated with dithiothreitol to ensure reduction of any intrasubunit or intersubunit disulfides. At room temperature, <10% of the NifU-1 or NifU was in a Fe bound form, and colorimetric analysis indicates that the remainder of the Fe is in solution was in the form of free Fe " ion. Hence this mononuclear Fe -bound species is more likely to be an intermediate in the reduction of Fe ion by NifU or NifU-1 rather than an initial step in cluster assembly on the NifU-1 domain of NifU. In this connection, it is important to note that Fe is rapidly reduced to Fe by cysteine in aqueous solution (Schubert, 1932). The physiological significance (if any) of the apparent ferric reductase activity associated with the NifU-1 domain of NifU remains to be established. 

In Figure 5.1, the fraction of iron whose concentration is being reported is identified as the total dissolved iron concentration. In practice, this fraction is operationally defined by the analytical method used in its measurement. For the data in Figure 5.1, the total dissolved iron concentration was determined by filtration to remove the solid iron, followed by colorimetric analysis to quantify the solutes. Another analytical technique, such as filtration followed by atomic absorption spectrophotometry, might yield a different total dissolved concentration, so it is important to be aware of the analytical methods used. To address this issue, marine chemists engage in intercalibration experiments to assess differences in results from various analytical methods. 

Attempts to Detect Mannose-6-Phosphate in B-Galactosidase and Inhibitor Glycopeptides. Although colorimetric analysis of the inhibitor glycopeptide fraction for phosphate proved negative, the occurrence of trace quantities of mannose phosphate could not be eliminated. Experiments were therefore undertaken to demonstrate the possible presence of small amounts of mannose-6-phosphate in B-galactosidase and in the inhibitor glycopeptide fraction. 

The second push-pull experiment pushed about 610 L from well B-5 at a rate of about 8.3 L/min into well B-3 with the addition of about 55 g of Fe in the form FeCls. After about one hour, well B-3 was pumped until more than twice the injected volume was recovered. The low pH in the water during the beginning of the recovery period was caused by precipitation of iron oxide, as described by the reaction Fe + 2 H O = FeOOH + 3H. Field colorimetric analysis of arsenic indicated that arsenic was removed... 

Experiments were carried out in a 500-mL photochemical reactor with a quartz immersion well. A 10 x 20 cm2 piece of film was wrapped around the immersion well and tied in place with Teflon tape. Then 350 mL of H20 was placed in the reactor and NaOH was added to adjust the pH to 12. The system was purged continuously with air or N2 and irradiated with a 400-W medium-pressure Hg lamp. After a period of time the solution was tested for NH3 by the indophenol method, and for NOJ/NOJ (combined total nitrite and nitrate) by Cd reduction, followed by colorimetric analysis by the azo-dye method [84]. The azo-dye method, sometimes known as the Griess method or the sulfanilic acid method, has been described elsewhere [87]. 

A mass of 0.200g of a putative catalyst was suspended in 300mL of deionized water in an immersion-well photochemical reactor under a slow stream of N2. The mixture was irradiated by a 400-W medium-pressure Hg lamp which produced more than 5 x 1019 photons per second. Control experiments under argon or without irradiation were performed. At the end of the irradiation, lOmL of 0.10M NaOH was added to the reaction flask. The contents were distilled into a receiving flask which contained 10 mL of HC1 and analyzed for NH3 by the indophenol method. Then 10 mL of the reactor solution was centrifuged and analyzed for N03 by Cd reduction followed by colorimetric analysis by the azo-dye method. 

We have shown a new concept for selective chemical sensing based on composite core/shell polymer/silica colloidal crystal films. The vapor response selectivity is provided via the multivariate spectral analysis of the fundamental diffraction peak from the colloidal crystal film. Of course, as with any other analytical device, care should be taken not to irreversibly poison this sensor. For example, a prolonged exposure to high concentrations of nonpolar vapors will likely to irreversibly destroy the composite colloidal crystal film. Nevertheless, sensor materials based on the colloidal crystal films promise to have an improved long-term stability over the sensor materials based on organic colorimetric reagents incorporated into polymer films due to the elimination of photobleaching effects. In the experiments... 

The carbohydrate being eluted from a GPC column can be detected by a number of physical or chemical means (e.g., variation in refractive index or viscosity and colorimetric or fluorometric spectroscopic analysis). For the purpose of these experiments, the cellulose was tagged with a fluorescent label, dichlorotriazinylaminofluorescein (DTAF), which permits easy detection of very small quantities. The chromatographic system was set up to allow for convenient analysis of cellulose with a maximum resolution of the molecular weight distribution and a minimum of change to the sample. 

The major serum electrolytes—sodium, potassium, calcium, magnesium, chloride, and bicarbonate (CO2)—are fairly easy to determine. The metals are most readily determined by the use of fiame-spectrophotometiic or atomic absorption methods, although colorimetric methods exist for calcium and magnesium. Calcium and, less frequently, magnesium are also titrated with EDTA. Ion-selective electrodes are used for the routine analysis of sodium, potassium, and calcium. Bicarbonate is analyzed also by titration against standard acid (see Experiment 8) in addition to a manometric method. Chloride is widely determined by automatic coulometric titration with electrogenerated silver ion. 

Several flavonoids isolated from tea have been analyzed and their structure determined using NMR. There are several problems with the classical methods of analysis of flavonoids in tea. Due to the presence of complex mixtures of flavonoids in tea, they are often characterized as total polyphenols . The colorimetric method for analysis of total phenols can interfere with other reducing compovmds. LC can well resolve peaks for individual flavonoids however, there are only a few standards available commercially, making the assignment of peaks vmcertain in many cases. Thus, the structure of flavonoids giving rise to peaks in LC is often determined using various ID- and 2D-NMR experiments. 

A further check on the occurrence of systematic errors in a method is to compare the results with those obtained from a different method, if two unrelated methods are used to perform one analysis, and if they consistently yield results showing only random differences, it is a reasonable presumption that no significant systematic errors are present. For this approach to be valid, each step of the two experiments has to be independent. Thus in the case of serum chromium determinations, it would not be sufficient to replace the atomic-absorption spectrometry step by a colorimetric method or by plasma spectrometry. The systematic errors would only be revealed by altering the sampling methods also, e.g. by minimizing or eliminating the use of stainless-steel equipment. A further important point is that comparisons must be made over the whole of the concentration range for which an analytical procedure is... 

The most important degradative method for the determination of urea in the natural water samples is based on its conversion to carbon dioxide and ammonia by hydrolysis obtained with a nickel metalloenzyme (urease). In the manual procedure outlined by McCarthy [89] for the analysis in seawater, the enzymatic hydrolysis of urea was carried out at 50°C for 20 min, in the range of pH from 6.4 to 8.0, using a solution of crude lyophilized jack beam urease. After the samples were cooled at room temperature, NH4 concentration was determined by manual colorimetric method after cooling the samples at room temperature. The ambient concentration of NH4 and the analytical blank (NH4 contained in the reagents and in the urease solution) have to be subtracted for any sample to obtain the concentration of urea. In this reference study, the precision (RSD) was 1% at the concentration of urea equal to 1 pmol N A manual indirect methodology was also described by Katz and Rechnitz [209] and the method was revised in other following studies [9,53,197,198]. It persists with minor modifications in recent works on the field and in culture experiments [71,199-202] and for determination of isotope ratio in urea by elemental... 

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