Back extraction

When extracting adds and bases from an aqueous phase, the pH is adjusted such that these compounds are uncharged. During extraction, these compounds, together 

If the organic solvent used in the extraction process is unsuitable for spraying into the flame, the analyte may be extracted back into the aqueous phase from the organic phase. In back extraction the organic extract is shaken with a weakly acidified aqueous solution. In this procedure it is normally possible to obtain complete separation of the analyte from the concomitants so that an almost pure aqueous solution of the analyte is obtained. 

In plasma atomic emission the number of organic solvents which can be sprayed directly into the atomizer is bigger than in FAAS (Table 44). 

3 Standards and Blank Samples. When extraction systems are used, it must be taken into account that the analyte is not always transferred completely into the organic phase. The extraction efficiency depends on the conditions such as pH of the aqueous phase and metal-ligand ratio. In addition to the organic solvent used, the ligand may also have an influence on the sensitivity of the analyte in FAAS. Thus, the sensitivity of the analyte may differ in the same solvent when different ligands are used, since the metal—ligand bonds in metal complexes formed possess different thermal properties. For example, the Mn sensitivity in MIBK is better with the cupferron complex than with the diethyldithiocarbamato or 8-hydroxyquinolato complexes. 

For the reasons above, samples, standards, and blanks must be extracted in exactly the same way. In addition, standards and blanks must be pretreated, as much as possible, in the same way as the samples in order to obtain matching matrices. 

If the extraction is not quantitative, several successive extractions are needed and the organic extracts are then combined. The disadvantage of this procedure is the diminished analyte concentration in the increased solution volume. However, organic phases may be easily concentrated by evaporation. 

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If you netralize the formic acid mix with 25% NaOH the layers separate out nicely. It takes 75 / of 25% NaOH to neutralize the soln for 150grm 88% formic, so you ll need a big sepatory funnel. After you hit ph 4.5 add it rery carefully cause it ll run away to 9+ real quick. You can then back extract the water with DCM, or I guess preferably ether. If you use too much DCM when extracting it sinks to the bottom and some product floats on the top, so you end up with three layers... But then my lab tech SUXSI (not that I d partake in iilegal activities. p"... 

The component C in the separated extract from the stage contact shown in Eigure 1 may be separated from the solvent B by distillation (qv), evaporation (qv), or other means, allowing solvent B to be reused for further extraction. Alternatively, the extract can be subjected to back-extraction (stripping) with solvent A under different conditions, eg, a different temperature again, the stripped solvent B can be reused for further extraction. Solvent recovery (qv) is an important factor in the economics of industrial extraction processes. 

The extent of extraction or back-extraction is therefore governed by the pH of the aqueous phase. 

Therefore the extent of extraction or back-extraction is governed by the concentration of X ia the aqueous phase, the distribution coefficients, and selectivities depending on the anion. In nitrate solutions, the distribution coefficient decreases as the atomic number of the REE increases, whereas ia thiocyanate solutions, the distribution coefficient roughly increases as the atomic number of the REE increases. The position of yttrium in the lanthanide series is not the same in nitrate and thiocyanate solutions, and this phenomenon has been used for high purity yttrium manufacture in the past. A combination of extraction by carboxyUc acids then by ammonium salts is also utilized for production of high purity yttrium. 

In TBP extraction, the yeUowcake is dissolved ia nitric acid and extracted with tributyl phosphate ia a kerosene or hexane diluent. The uranyl ion forms the mixed complex U02(N02)2(TBP)2 which is extracted iato the diluent. The purified uranium is then back-extracted iato nitric acid or water, and concentrated. The uranyl nitrate solution is evaporated to uranyl nitrate hexahydrate [13520-83-7], U02(N02)2 6H20. The uranyl nitrate hexahydrate is dehydrated and denitrated duting a pyrolysis step to form uranium trioxide [1344-58-7], UO, as shown ia equation 10. The pyrolysis is most often carried out ia either a batch reactor (Fig. 2) or a fluidized-bed denitrator (Fig. 3). The UO is reduced with hydrogen to uranium dioxide [1344-57-6], UO2 (eq. 11), and converted to uranium tetrafluoride [10049-14-6], UF, with HF at elevated temperatures (eq. 12). The UF can be either reduced to uranium metal or fluotinated to uranium hexafluoride [7783-81-5], UF, for isotope enrichment. The chemistry and operating conditions of the TBP refining process, and conversion to UO, UO2, and ultimately UF have been discussed ia detail (40). 

The procedure of simultaneous extracting-spectrophotometric determination of nitrophenols in wastewater is proposed on the example of the analysis of mixtures of mono-, di-, and trinitrophenols. The procedure consists of extraction concentrating in an acid medium, and sequential back-extractions under various pH. Such procedures give possibility for isolation o-, m-, p-nitrophenols, a-, P-, y-dinitrophenols and trinitrophenol in separate groups. Simultaneous determination is carried out by summary light-absorption of nitrophenol-ions. The error of determination concentrations on maximum contaminant level in natural waters doesn t exceed 10%. The peculiarities of application of the sequential extractions under fixed pH were studied on the example of mixture of simplest phenols (phenol, o-, m-, />-cresols). The procedure of their determination is based on the extraction to carbon tetrachloride, subsequent back-extraction and spectrophotometric measurement of interaction products with diazo-p-nitroaniline. 

A total volume of 2 L of hexane washes results, accompanied by the gradual precipitation of a yellow solid from the hexane washes. The acid-wash procedure frequently leads to emulsions and gunny yellow solid in both phases back-extraction of the "aqueous" layer with hexane may be necessary. 

Thiothienoyltrifluoroacetone [4552-64-1] M 228.2, m 61-62". Easily oxidised and has to be purified before use. This may be by recrystd from benzene or by dissolution in pet ether, extraction into IM NaOH soln, acidification of the aqueous phase with 1-6M HCl soln, back extraction into pet ether and final evapn of the solvent. The purity can be checked by TLC. It was stored in ampoules under nitrogen at 0" in the dark. [Muller and Rother Anal Chim Acta 66 49 1973.]... 

To a suspension of 25.0 g of 11/3,17a,21-trihydroxy-6,16a-dimethyl-4,6-pregnadiene-3,20-dione in 1.5 liters of alcohol-free chloroform cooled to about 5°C in an ice bath is added with constant stirring 750 ml of cold, concentrated hydrochloric acid and then 750 ml of formalin (low in methanol). The mixture is removed from the ice bath and stirred at room temperature for 7 hours. The layers are separated and the aqueous phase is back-extracted twice with chloroform. The combined organic layers are washed twice with a 5% solution of sodium bicarbonate, and twice with a saturated salt solution. The solution is dried over magnesium sulfate and evaporated to dryness under reduced pressure. 

Another disadvantage in the use of liquid ion exchangers is that it is frequently necessary to back-extract the required species from the organic phase into an aqueous phase prior to completing the determination. The organic phase may, however, sometimes be used directly for determination of the extracted species,... 

A flame-dried flask under argon containing a 0.08 M THF solution of 317 mg (1.0 mmol) of the 2-(l- or 2-naphthyl)-substituted 4,5-dihydrooxazole is cooled to a temperature of between — 80 and 0 CC (see ref 7) and is treated with 1.5-2.0 equiv of the alkyllithium. The solution becomes deep red over 2-4 h and is quenched by the dropwise addition of 1.5 equiv of the electrophile (either neat or as a THF solution). The temperature is maintained for 1 h and then the solution is warmed gradually to 0 JC. The solution is diluted with 100 mL of diethyl ether and washed with 5 mL of sat. NH4C1, followed by 3 mL of sat. aq NaCI. The combined aqueous layers are back-extracted with 10 mL of CII2C12, and the combined extracts are dried over Na2S04. Concentration of the filtrate in vacuo provides a yellow oil, which is flash chromatographed over silica gel (1 -10% ethyl acetate in hexane) to yield the desired adducts. The diastereomeric ratios are determined by HPLC (Zorbax Sil column, Du Pont). 

Although the D s for U(VI) and tetravalent actinides are very high, the data in Table VII show that formic acid (HC00H) will readily back-extract these elements as well as Am(III) from all the extractants except in the case of U(VI) with 0 D[IB]CMP0. 

To provide a more generalized picture for achieving separations by solvent extraction one can consider a number of possibilities, according to direction of transfer. Such possibilities are (i) pre-extraction (aqueous — solvent) (ii) extraction (aqueous — solvent), scrubbing (solvent —> aqueous) (iii) stripping/back extraction (solvent — aqueous) and (iv) solvent clean up (solvent —> aqueous — solvent). The direction of transfer has been shown in the parentheses of the four possibilities that have been listed. A reference to Figure 5.14 is relevant in this premise. 

The apparent rate constant kapp depends on the concentration of hydroxide ion as is shown in Fig. 1. The absorption maxima of TcCl2(acac) 2 in chloroform appear at 281,314(sh), 340(sh), 382 and 420 nm. On the other hand, the spectrum of the aqueous phase exhibits absorption maxima at 292,350 and 540 nm. The absorbances at 350 and 540 nm increase with time, but decrease after reaching maxima. This suggests that the chemical species which is formed by the back-extraction of TcCl2(acac)2 decomposes with time. In order to clarify the behavior of chloride ion liberated from the complex, an electrochemical method was introduced for the homogeneous system. In acetonitrile, no detectable change in the spectrum of TcCl2(acac)2 was observed. On the addition of an aqueous solution of hydroxide, however, the brown solution immediately turned red-violet, and exhibited absorption maxima at 292,350 and 540 nm. The red-violet... 

Szathmary and Luhmann [50] described a sensitive and automated gas chromatographic method for the determination of miconazole in plasma samples. Plasma was mixed with internal standard l-[2,4-dichloro-2-(2,3,4-trichlorobenzyloxy) phenethyl]imidazole and 0.1 M sodium hydroxide and extracted with heptane-isoamyl alcohol (197 3) and the drug was back-extracted with 0.05 M sulfuric acid. The aqueous phase was adjusted to pH 10 and extracted with an identical organic phase, which was evaporated to dryness. The residue was dissolved in isopropanol and subjected to gas chromatography on a column (12 m x 0.2 mm) of OV-1 (0.1 pm) at 265 °C, with nitrogen phosphorous detection. Recovery of miconazole was 85% and the calibration graph was rectilinear for 0.25 250 ng/mL. 

The most famous fungal metabolites are, of course, the penicillins and cephalosporins. The association of sulfur and penicillin has a curious history. Penicillin was investigated chemically in 1932 by Harold Raistrick and his colleagues.14 The antibacterial activity could be extracted into ether from acid solution but on solvent evaporation the residue was without antibacterial activity. Clearly, penicillin was not a well-behaved natural product If only Raistrick had carried out a back-extraction from ether into dilute alkali, penicillin might have become available in the 1930s (and Raistrick would have become a Nobel Laureate). 

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