Solutions, Colorimetric

Such expressions as 1 10 or 10% mean that 1 part by volume of a liquid or 1 part by weight of a solid is to be dissolved in a volume of the diluent or solvent sufficient to make the finished solution 10 parts by volume. Directions for the preparation of colorimetric solutions (CS), test solutions (TS), and volumetric solutions (VS), are provided in the section on Solutions and Indicators under General Tests and Assays, following Appendix X. 

Colorimetric solutions are used in the preparation of colorimetric standards for certain chemicals and for the carbonization tests with sulfuric acid that are specified in several monographs. Directions for the preparation of the primary colorimetric solutions and Matching Fluids are given under the test for Readily Carbonizable Substances, Appendix IIB. Store the solutions in suitably resistant, tight containers. 

In the test method (ASTM D-565), the mineral oil is treated with concentrated sulfuric acid under prescribed conditions and the resulting color is compared with a reference standard to determine whether it passes or fails the test. When the oil layer shows no change in color and when the acid layer is not darker than the reference standard colorimetric solution, the oil is reported as passing the test. A bluish haze or a slight pink or yellow color in the oil layer should not be interpreted as a change in color. This is a pass-fail, and the more fully refined the oil, the lighter the color of the acid layer. 

Bismuth standard solution (quantitative color test for Bi) dissolve 1 g of bismuth in a mixture of 3 mL of concentrated HNO3 and 2.8 mL of H2O and make up to 100 mL with glycerol. Also dissolve 5 g of KI in 5 mL of water and make up to 100 mL with glycerol. The two solutions are used together in the colorimetric estimation of Bi. 

Reagents, Indicators, and Solutions. This section includes the specifications and testing methods for reagents to be used in the tests specified in the USP-NF, and directions for making the various indicator, buffer, colorimetric, test, and volumetric solutions used in the testing. Reagents for which ACS specifications exist are referenced to the ACS book (7). 

In view of the chromophoric character of the elemental iodine itself, many colorimetric methods have been proposed for the deterrnination of inorganic iodine (88—92). These methods use the visible portion of the spectmm in reading iodine concentrations. In the visible range the extinction coefficient for iodine is not high enough to be used for minute quantities of iodine in water and other solvents (93). Higher sensitivities have been reported for elemental iodine in potassium iodide solutions in the ultraviolet (93,94). 

Titrations with dibutylamine [111-92-2] can also be used to determine the NCO content of isocyanates and prepolymers. Generally, an excess of amine in a suitable solvent such as chlorobenzene [108-90-7] is added to the sample. The resulting solution is allowed to react and the unreacted amine is back- titrated with dilute hydrochloric acid. For low NCO content levels, a colorimetric method is often used. The isocyanate-containing species is titrated with amine and the unreacted amine is deterrnined using malachite green [569-64-2]. 

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. 

Analytical and Test Methods. o-Nitrotoluene can be analyzed for purity and isomer content by infrared spectroscopy with an accuracy of about 1%. -Nitrotoluene content can be estimated by the decomposition of the isomeric toluene diazonium chlorides because the ortho and meta isomers decompose more readily than the para isomer. A colorimetric method for determining the content of the various isomers is based on the color which forms when the mononitrotoluenes are dissolved in sulfuric acid (45). From the absorption of the sulfuric acid solution at 436 and 305 nm, the ortho and para isomer content can be deterrnined, and the meta isomer can be obtained by difference. However, this and other colorimetric methods are subject to possible interferences from other aromatic nitro compounds. A titrimetric method, based on the reduction of the nitro group with titanium(III) sulfate or chloride, can be used to determine mononitrotoluenes (32). Chromatographic methods, eg, gas chromatography or high pressure Hquid chromatography, are well suited for the deterrnination of mononitrotoluenes as well as its individual isomers. Freezing points are used commonly as indicators of purity of the various isomers. 

Ozone can be analyzed by titrimetry, direct and colorimetric spectrometry, amperometry, oxidation—reduction potential (ORP), chemiluminescence, calorimetry, thermal conductivity, and isothermal pressure change on decomposition. The last three methods ate not frequently employed. Proper measurement of ozone in water requites an awareness of its reactivity, instabiUty, volatility, and the potential effect of interfering substances. To eliminate interferences, ozone sometimes is sparged out of solution by using an inert gas for analysis in the gas phase or on reabsorption in a clean solution. Historically, the most common analytical procedure has been the iodometric method in which gaseous ozone is absorbed by aqueous KI. 

Only slightly less accurate ( 0.3—0.5%) and more versatile in scale are other titration techniques. Plutonium maybe oxidized in aqueous solution to PuO " 2 using AgO, and then reduced to Pu" " by a known excess of Fe", which is back-titrated with Ce" ". Pu" " may be titrated complexometricaHy with EDTA and a colorimetric indicator such as Arsenazo(I), even in the presence of a large excess of UO " 2- Solution spectrophotometry (Figs. 4 and 5) can be utilized if the plutonium oxidation state is known or controlled. The spectrophotometric method is very sensitive if a colored complex such as Arsenazo(III) is used. Analytically usehil absorption maxima and molar absorption coefficients ( s) are given in Table 10. Laser photoacoustic spectroscopy has been developed for both elemental analysis and speciation (oxidation state) at concentrations of lO " — 10 M (118). Chemical extraction can also be used to enhance this technique. 

More specifically, 2-aminophenol can be detected in solution using an iron(II) sulfate—hydrogen peroxide reagent (94). 3- Aminopheno1 has been analyzed colorimetrically by oxidation in base and subsequent extraction of a violet quinoneimide dye (95). A colorimetric method using... 

Nitrogen oxide sampling is simpler. This gas is drawn into an evacuated sample flask containing dilute sulfuric acid and hydrogen peroxide. The flask is shaken and allowed to stand for 16 h before the flask pressure is measured. Then the solution is made alkaline, and the nitrogen oxides are deterrnined by the phenoldisulfonic colorimetric test. 

The sodium hydroxide is titrated with HCl. In a thermometric titration (92), the sibcate solution is treated first with hydrochloric acid to measure Na20 and then with hydrofluoric acid to determine precipitated Si02. Lower sibca concentrations are measured with the sibcomolybdate colorimetric method or instmmental techniques. X-ray fluorescence, atomic absorption and plasma emission spectroscopies, ion-selective electrodes, and ion chromatography are utilized to detect principal components as weU as trace cationic and anionic impurities. Eourier transform infrared, ft-nmr, laser Raman, and x-ray... 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

Analysis of zinc solutions at the purification stage before electrolysis is critical and several metals present in low concentrations are monitored carefully. Methods vary from plant to plant but are highly specific and usually capable of detecting 0.1 ppm or less. Colorimetric process-control methods are used for cobalt, antimony, and germanium, turbidimetric methods for cadmium and copper. Alternatively, cadmium, cobalt, and copper are determined polarographicaHy, arsenic and antimony by a modified Gutzeit test, and nickel with a dimethylglyoxime spot test. 

Replenishment should be done with caie. Massive additions can cause decomposition. Maximum stability of electroless baths is obtained when continuous replenishment is practiced. Colorimetric analy2ers are commonly used to control the addition of replenisher solutions in a set ratio based on the nickel or copper content of the bath. A number of machines are available that continuously analy2e plating baths and make additions based on each separately analy2ed component. 

A samphng probe is placed at any location in the stack, and a grab sample is collected in an evacuated flask. This flask contains a solution of siilfiiric acid and hydrogen peroxide, which reacts with the NO. The volume and moisture content of the exhaust-gas stream must be determined for calculation of the total mass-emission rate. The sample is sent to a laboratoiy, where the concentration of nitrogen oxides, except nitrons oxide, is determined colorimetrically. 

The West-Gaeke manual method is the basis for the U.S. Environmental Protection Agency reference method for measurement of SO2 (6). The method uses the colorimetric principle i.e., the amount of SO2 collected is proportional to the amount of light absorbed by a solution. The collection medium is an aqueous solution of sodium or potassium tetrachloromercu-... 

Pretreatment of the collected particulate matter may be required for chemical analysis. Pretreatment generally involves extraction of the particulate matter into a liquid. The solution may be further treated to transform the material into a form suitable for analysis. Trace metals may be determined by atomic absorption spectroscopy (AA), emission spectroscopy, polarogra-phy, and anodic stripping voltammetry. Analysis of anions is possible by colorimetric techniques and ion chromatography. Sulfate (S04 ), sulfite (SO-, ), nitrate (NO3 ), chloride Cl ), and fluoride (F ) may be determined by ion chromatography (15). 

Quinoxaline-2,3-dithione (2,3-dimercaptoquinoxaline) (90) forms chelates with several transition elements and is used as a colorimetric agent for the detection and quantitative estimation of nickeT and also for the quantitative estimation of palladium. " Nickel gives a pink coloration with (90) in ammoniacal solution, and palladium an orange-red color in AA-dimethylformamide solution containing a little hydrochloric acid. Spectrophotometric studies of the chelate compounds of (90) with Ni(II), Co(II), and Pd(II) in alkaline solu-... 

As an example, take the molecule aminoazobenzene, one of the solutes listed in Table 39. When colorimetric measurements were made at room temperature on very dilute aqueous solutions of HC1, containing a trace of this substance, it was found that neutral molecules and (BH)+ ions were present in equal numbers when the concentration of the HCl was 0.0016 molal.1 At this low concentration the activity coefficient of the HCl is very near unity, and we may use (216) to find how far the vacant proton level provided by the aminoazobenzene molecule in aque-... 

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