Precipitation titrations photometric

The photometric end point has also been adapted to precipitation titrations. The suspended solid product causes a decrease in the radiant power of the light source by scattering from the particles of the precipitate. The equivalence point occurs when the precipitate stops forming, and the amount of light reaching the detector becomes constant. This type of end point detection is called turbidimetry because the amount of light reaching the detector is a measure of the turbidity of the solution. 

Photometric titrations have also been adapted for determining the end-point of precipitation titrations. The suspended solid produced by the titration reaction has the effect of reducing the energy of the radiation due to scattering, and so titrations can be carried out till a state of constant turbidity has been obtained. 

Matthews and Riley [99] preconcentrated iodide by co-precipitation with chloride ions. This is achieved by adding 0.23 g silver nitrate per 500 ml of seawater sample. Treatment of the precipitate with aqueous bromine and ultrasonic agitation promote recovery of iodide as iodate which is caused to react with excess iodide under acid conditions, yielding I3. This is determined either spectrophotometrically or by photometric titration with sodium thiosulfate. Photometric titration gave a recovery of 99.0 0.4% and a coefficient of variation of 0.4% compared with 98.5 0.6% and 0.8%, respectively, for the spectrophotometric procedure. 

Macro quantities of selenium can be determined gravimetrically after reduction to the elemental form by various reagents such as tin (II) chloride, potassium iodide, or ascorbic acid (I). Ooba described a technique whereby the element is precipitated from perchloric acid solution with hydrazine (2). Selenium may be titrated with standard solutions of sodium thiosulfate, iodide, and ferrous, chromous, or trivalent titanium salts after oxidation to Se(VI) (I). Photometric and fluorometric methods based on formation of the piaselenol with diaminobenzidine or 2,3-diaminonaphthalene has been used for the determination of selenium (I, 3,4,5). Interfering elements such as As, Co, Cr, Cu, Fe, Hg, and Ni, are masked with EDTA or other chelating agents. 

The use of amperometry as an endpoint detection method has been widely adopted for the titration of substances at the millimole per liter level, with excellent analytical precision ( 1% RSD at 0.01 mmol 1 level). The majority of these titrations involve the formation of precipitates, while others involve complexometric and redox titrations. At the millimole per liter level, the accuracy of this method is better than that achievable with other electroan-alytical methods and as good as those of spectro-photometric titration. 

Other example is the determination of fuel parameters including the diene value and mercaptan sulfur. First, the robot weighs the sample, refluxes it in the presence of maleic anhydride, and extracts the analytes into an aqueous phase, which is poured into the titration vessel, where the robot inserts the photometric or potentiometric probe. For the determination of mercaptan sulfur, the robot also weighs the sample and removes sulfide by precipitation with a CdS04 solution and liquid-liquid extraction. Once the fuel is sulfide free, which is checked by using a photometric probe, it is poured into the titration vessel by the robot arm, which also plunges an Ag electrode prior to addition of the titrant (an AgNOs solution). In both cases, the automated titrator acts as a module of the robotic station and is operated by the robotic arm. 

Procedures especially suited to silicate rocks, minerals, and refractory silicates and aluminosilicates have been described by Bennett and Reed (290). A history of analytical methods was published by Andersson (291), and three new spectro-photometric procedures were developed. Available chemical methods listed by Meites (292), in addition to the conventional gravimetric and colorimetric methods, also include precipitation of the silicate ion as the cobalt salt, which is then determined by chelometric titration, and as a nitrogen base salt which is titrated with perchloric acid. 

Assay of Ribonuclease. Several kinds of assay have been found useful for studying ribonuclease (RNAase). A spectrophotometric assay is based on the empirical, unexplained observation that there is a decrease in optical density at 300 m/ during hydrolysis of RNA. RNA can be precipitated from solutions of its degradation products by uranyl acetate in trichloroacetic acid the soluble nucleotides can be measured spectro-photometrically or chemically. A manometric assay determines the formation of secondary acid groups through release of CO2 from a bicarbonate buffer acid groups have also been determined by titration. Recently cyclic nucleotides have been used as substrates in place of nucleic acid. ... 

Bismuth(III) compounds in solution can be titrated with EDTA solution or precipitated as orthophosphate or they can be electrolytically determined as Bi. A suitable spectro-photometric method is based on complex formation with iodide. 

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