Precipitation tests, anion

Perhaps the most striking and widespread of these earlier reports is the frequent absence of characteristic anion precipitation tests found for complexes containing anions. These are especially striking where the... 

Oxidation of the aldehyde group of an aldose to form a carboxyUc acid or carboxyUc acid anion is often used analytically to determine the amount of reducing sugar. The Benedict and Fehling methods measure the amount of reducing sugar present in a fluid. In these reactions, the oxidant, Cu ", is reduced to Cu". Cu" precipitates as CU2O, which can be measured in a variety of ways. In the ToUens test, Ag" is reduced to Ag. ... 

In the incremental or decremental technique, another designation for the standard addition (or subtraction) technique, one adds increments of standard solution to the sample, or vice versa. (In the decremental technique the standard precipitates or complexes the ion under test.) When the sample itself is incrementally added to the standard, the latter may have received a previous addition of ISA and/or pH adjuster, but in the reverse method this addition may be made to the sample. However, for the specific example of a univalent anion we shall show how the normal incremental method works38 and that in fact the addition of ISA is not necessary. 

Phosphorus is a common component of additives and appears most commonly as a zinc dialkyl dithiophosphate or triaryl phosphate ester, but other forms also occur. Two wet chemical methods are available, one of which (ASTM D1091) describes an oxidation procedure that converts phosphorus to aqueous orthophosphate anion. This is then determined by mass as magnesium pyrophosphate or photochemically as molybdivanadophosphoric acid. In an alternative test (ASTM D4047), samples are oxidized to phosphate with zinc oxide, dissolved in acid, precipitated as quinoline phosphomolybdate, treated with excess standard alkali, and back-titrated with standard acid. Both of these methods are used primarily for referee samples. Phosphorus is most commonly determined using x-ray fluorescence (ASTM D4927) or ICP (ASTM D4951). 

Elemental composition P 38.73%, H 1.26%, O 60.01%. The compound may be identified by physical properties alone. It may be distinguished from ortho and pyrophosphates by its reaction with a neutral silver nitrate solution. Metaphosphate forms a white crystalline precipitate with AgNOs, while P04 produces a yellow precipitate and P20 yields a white gelatinous precipitate. Alternatively, metaphosphate solution acidified with acetic acid forms a white precipitate when treated with a solution of albumen. The other two phosphate ions do not respond to this test. A cold dilute aqueous solution may be analyzed for HPO3 by ion chromatography using a styrene divinylbenzene-based low-capacity anion-exchange resin. 

Weak-base resins have been tested from time to time but have not found wide acceptance in the uranium industry, The main reason for this is that the major advantage of weak- over strong-base resins, viz. elution by neutralization, cannot be utilized in uranium processing since it is not possible for the weak-base resin to be converted to the free-base form without diuranate precipitating within the pores of the resin (unless a complexing agent such as carbonate is added to the eluate). In the presence of carbonate, uranium remains in solution as the uranyl carbonate anion, even in very alkaline solution, so is readily eluted from a weak-base resin in the free-base form. This eluate would then be treated as depicted in equations (105) and (106) for the recovery of uranium. Alternatively, weak-base resins can be eluted by ion-exchange mass action. 

More recently, such processes were tested to reduce the acidity of clarified passion fruit (Passiflora edulis v. flavicarpa) juices from pH 2.9 to 4.0 in comparison with other conventional processes, such as calcium citrate precipitation as resulting from CaC03 or Ca(OH)2 addition, or removal via weakly basic IER (Calle et al., 2002 Vera et al., 2003). Whatever the process tested, the physicochemical and sensory properties of the de-acidified juices were quite similar. In spite of the fact that their sodium concentration was higher when using any of the above-mentioned ED processes, the two-compartment stack using bipolar and anionic membranes (Figure 8C) was regarded as optimal, since no chemical consumption was needed and a valuable solution rich in citric acid (89% purity) was recovered (Vera et al., 2003). 

Using the following procedure, you will use precipitation, flame tests, and pH levels to analyze eight unknown solutions to detect the presence of cations or anions in the solutions. Each solution will contain a cation or an anion to be detected. Solutions 1, 2, and 3 contain cations, and solutions 4, 5, 6, 7, and 8 contain anions to be detected. The possible cations are Ag+, Ca +, and Cu +. The possible anions are OH , Cl, S04 , I , and P04. (Each unknown should be one of the solutions listed under materials above.)... 

Cupric ion is said to give highly insoluble precipitates and this has been suggested as a test for the —CON(OH)— bond (146). In the case of iron it is apparent that the number of hydroxamate anions coordinated to the ferric ion is mainly a function of pH, and this in turn has a profound effect on the wavelength maximum and the intensity of absorption. In general, at about pH 1 to 2 the 1 1 complex is favored, the color is purple (maximum 5100 A) and the amM is about 1.0 (Table 2). (35, 116, 142). 

The widespread occurrence of iron ores, coupled with the relative ease of extraction of the metal, has led to its extensive use as a constructional material with the result that the analysis of steels by both classic wet and instrumental methods has been pursued with vigour over many years.3 Iron complexes are themselves widely used as the basis of convenient analytical methods for the detection and estimation of iron down to parts per million. Familiar tests for iron(III) in aqueous solution include the formation of Prussian blue as a result of reaction with [Fe(CN)6]4, and the formation of the intensely red-coloured [Fe(H20)5SCN]2+ on reaction with thiocyanate ion.4 Iron(II) forms particularly stable red tris chelates with a,a -diimines such as 1,10-phenanthroline or 2,2 -bipyridine that have been used extensively in spectrophotometric determinations of iron and in the estimation of various anions.5 In gravimetric estimations, iron(III) can be precipitated as the insoluble 8-hydroxyquinoline or a-nitroso-jS-naphthol complex which is then ignited to Fe203.6 In many situations the levels of free [Fe(H20)6]3+ may be controlled through complex formation by addition of edta. 

Formation of aniline blue test Upon heating insoluble oxalates with concentrated phosphoric acid and diphenylamine or upon heating together oxalic acid and diphenylamine, the dyestuff aniline blue (or diphenylamine blue) is formed. Formates, acetates, tartrates, citrates, succinates, benzoates, and salts of other organic acids do not react under these experimental conditions. In the presence of other anions which are precipitated by calcium chloride solution, e.g. tartrate, sulphate, sulphite, phosphate, and fluoride, it is best to heat the precipitate formed by calcium chloride with phosphoric acid as detailed below. 

In the presence of anions which are precipitated by calcium chloride solution, proceed as follows. Precipitate the acetic acid test solution with calcium chloride solution, and collect the precipitate on a filter or in a centrifuge tube. Remove the water from the precipitate either by drying or by washing with alcohol and ether. Mix a small amount of the precipitate with diphenylamine in a dry micro test-tube, add a little concentrated phosphoric acid, and heat gently over a free flame. Calcium phosphate and free oxalic acid are formed, and the latter condenses with the diphenylamine to aniline blue and colours the hot phosphoric acid blue. The colour disappears on cooling. Dissolve the melt in alcohol, when a blue colouration appears. Pour the alcoholic solution into water thus precipitating the excess of diphenylamine, which is coloured light blue by the adsorption of the dyestuff. The dye may be extracted from aqueous solution by ether. 

Cyanides, thiocyanates, hexacyanoferrate(II)s, and hexacyanoferrate(III)s also yield ammonia under these experimental conditions. The reaction is somewhat slower for these anions up to 5 minutes may elapse before ammonia can be detected from hexacyanoferrate(II)s and hexacyanoferrate(III)s. If these are present, or are suspected as a result of the preliminary tests, particularly that with concentrated sulphuric acid, they must first be removed as follows. Treat the soda extract with excess of nitrate-free silver sulphate, warm the mixture to about 60°, shake vigorously for 3-4 minutes, and filter from the silver salts of the interfering anions and excess of precipitant. Remove the excess silver ions from the filtrate by adding excess sodium hydroxide solution and filter off the precipitated silver oxide. Evaporate the filtrate to about half bulk and test with zinc, aluminium or Devarda s alloy. If cyanides alone are present, they may be rendered innocuous by the addition of a little mercury(II) chloride solution. 

V.18 PRELIMINARY TESTS FOR AND SEPARATION OF CERTAIN ANIONS After the systematic separation and detection of cations, the search for anions should be started. At this stage already considerable information is available about the presence or absence of certain anions. Not only the preliminary wet and dry tests supplied this information, but during the separation of cations a number of facts became available thus the presence of phosphate, borate, fluoride, citrate, tartrate, and oxalate has been detected, before the precipitation of Group III cations. Similarly, the presence of chromate (or dichromate), permanganate and arsenate has been established at various stages of these separations. When the systematic examination for anions is carried out, such findings should be kept in mind. 

Cyanide. This should have been detected and confirmed in the preliminary test with dilute sulphuric acid (Prussian blue test or as Section IV.8, reaction 1). Sulphite. This anion will have been detected in the preliminary test with dilute sulphuric acid (potassium dichromate paper or fuchsin solution test). Hexacyanoferrate(II) (and Thiocyanate). Acidify 1 ml of the soda extract with dilute hydrochloric acid and add a few drops of iron(III) chloride solution. A deep-blue precipitate indicates hexacyanoferrate(II) present. Now add 0-5-1 ml iron(III) chloride solution, 0-2 g sodium chloride and half a Whatman filtration accelerator, shake the mixture vigorously and filter. A deep-red filtrate indicates thiocyanate present. 

Procedure C (examination ofresidue C) If the precipitate is white, only organic acids may be present and the other anions need not be tested for. Furthermore, the preliminary tests of heating alone and heating with concentrated sulphuric acid will have indicated the presence of organic acids. If organic acids are indicated or suspected, follow the procedure given in Section IV.45, 20. 

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