Coprecipitation precipitation step

Cranston and Murray [35,36] took samples in polyethylene bottles that had been pre-cleaned at 20 °C for four days with 1% distilled hydrochloric acid. Total chromium Cr(VI) + Cr(III) + Crp (Crp particulate chromium) was coprecipitated with iron (II) hydroxide, and reduced chromium Cr(III) + Crp was co-precipitated with iron (III) hydroxide. These co-precipitation steps were completed within minutes of the sample collection to minimise storage problems. The iron hydroxide precipitates were filtered through 0.4 pm Nu-cleopore filters and stored in polyethylene vials for later analysis in the laboratory. Particulate chromium was also obtained by filtering unaltered samples through 0.4 pm filters. In the laboratory the iron hydroxide co-precipitates were dissolved in 6 N distilled hydrochloric acid and analysed by flameless atomic absorption. The limit of detection of this method is about 0.1 to 0.2 nM. Precision is about 5%. 

The uses of constant-current coulometry for the determination of drugs in biological fluids are few, basically due to sensitivity restriction. Monforte and Purdy [46] have reported an assay for two allylic barbituric acid derivatives, sodium seconal and sodium sandoptal, with electrogenerated bromine as the titrant and biamperometry for endpoint detection. Quantitative bromination required an excess of bromine hence back titration with standard arsenite was performed. The assay required the formation of a protein-free filtrate of serum with tungstic acid, extraction into chloroform, and sample cleanup by back extraction, followed by coulometric titration with electrogenerated bromine. The protein precipitation step resulted in losses of compound due to coprecipitation. The recoveries of sodium seconal and sodium sandoptal carried through the serum assay were approximately 81 and 88%, respectively. Samples in the concentration range 7.5-50 pg/mL serum were analyzed by this procedure. 

First, the RNA must be collected from the transcription mixture by precipitation. We use ammonium acetate in the ethanol precipitation step because this is the quickest method. Coprecipitated NH ions, which interfere with further enzymatic reactions, are removed in the next purification step. The RNA pellet is dissolved in an appropriate loading buffer for denaturing PAGE (7 M urea, 50 mM EDTA), incubated for 5 min at 65 °C, and then immediately loaded on the gel. The gel should be preequilibrated and preheated by preelectrophoresis for 15 min. 

The extent of mixed-ci-ystal contamination is governed by the law of mass action and increases as the ratio of contaminant to analyte concentration increases. Mixed-crystal fomiation is a particularly troublesome type of coprecipitation because little can be done about it when certain combinations of ions are present in a sample matrix. This problem is encountered with both colloidal suspensions and crystalline precipitates. When mixed-crystal formation occurs, the interfering ion may have to be separated before the final precipitation step. Alternatively, a different precipitating reagent that does not give mixed crystals with the ions in question may be used. 

In the classical procedure, scheelite was mainly processed by acid digestion while wolframite was dissolved by an alkaline digestion. The important solid, pure intermediate was, in both cases, tungstic acid, derived either directly or after several precipitation steps. Tungstic acid is formed by a precipitation, which is a more or less instantaneous process. Foreign ions present in solution during the precipitation are partly entrapped or coprecipitated and contaminate the tungstic acid. 

Uranium. The uranium product from the PUREX process is in the form of uranyl nitrate which must be converted to some other chemical depending on anticipated use. One route to MO fuel is to mix uranium and plutonium nitrates and perform a coprecipitation step. The precipitate is... 

The presence of the ammonium chloride prevents the coprecipitation of zinc salts when sodium bicarbonate is added to the solution to precipitate the aminoguanidine as the bicarbonate. If the solution is not clear at this step, it should be filtered. 

As the first step in the coprecipitation process, ferric hydroxide precipitates either from the effect of the changing pH on the solubility of ferric iron,... 

Steps are normally taken to prevent the simultaneous precipitation of materials other than the desired analyte species. Incorporation of impurities into the precipitate may however occur by coprecipitation or post-precipitation. The former arises during the formation of the precipitate, and the latter after it has been formed. The various modes of coprecipitation are summarized in Table 5.16. 

Kruissink et al.56 confirmed the earlier observation of Beecroft et al.s5 that the decomposition of the precipitate occurs in two steps. In the first, between 130 and 230 UC, depending on the anion, the molecular water is removed from the interlayer and there is a slight decrease in the interlayer spacing. The brucite layer is destroyed at a higher temperature ( 350°C) with the evolution of water (from the OH-ions) and for example, C02 or N02, depending on the anions in the coprecipitate. 

Step 6a. Filter the supernate through a 47-mm, 0.45- micron, membrane filter in the larger Millipore filtering apparatus to collect the Fe(OH)3 with the coprecipitated plutonium. Slurry the precipitate remaining in the beaker with 10 mL deionized water and pour through the filter with the precipitate. Discard filtrate. 

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