Weak-acid anion exchangers

Weakly acidic anion exchangers were used in the separation of metals in chloride [164] and thiocyanate [165] media. 

Weak Base. Weak base anion-exchange resins may have primary, secondary, or tertiary amines as the functional group. The tertiary amine -N(CH2)2 is most common. Weak base resins are frequentiy preferred over strong base resins for removal of strong acids in order to take advantage of the greater ease in regeneration. 

Most weak base anion exchangers adsorbweak organic acids such as formic acid [64-18-6] and acetic acid [64-19-7] but do not remove weak organic acids such as carbonic acid [463-79-6] or siHcic acid [7669-41 ]. 

Fig. 1. Demineializei systems consist of various unit processes arranged to meet the system needs. I Lstrong acid cation exchanger I I Strong ha anion exchanger 0 Degasifier I Mixed bed I Weak acid cation exchanger 1 1 Weak base anion exchanger and I IConnterflow cation.

Strong-acid cation exchanger weak base anion exchangers... 

Weakly acidic cation exchangers (e.g. polymethylacrylic acid resins). These resins (Zerolit 226, Amberlite 50, etc.) are usually supplied in the hydrogen form. They are readily changed into the sodium form by treatment with 1M sodium hydroxide an increase in volume of 80-100 per cent may be expected. The swelling is reversible and does not appear to cause any damage to the bead structure. Below a pH of about 3.5, the hydrogen form exists almost entirely in the little ionised carboxylic acid form. Exchange with metal ions will occur in solution only when these are associated in solution with anions of weak acids, i.e. pH values above about 4. 

Simultaneous determination of both cations and anions in acid rain has been achieved using a portable conductimetric ion-exclusion cation-exchange chromatographic analyzer.14 This system utilized the poly(meth-ylmethacrylate)-based weak acid cation exchange resin TSK-Gel OA-PAK-A, (Tosoh , Tokyo, Japan) with an eluent of tartaric acid-methanol-water. All of the desired species, 3 anions and 5 cations, were separated in less than 30 minutes detection limits were on the order of 10 ppb. Simultaneous determination of nitrate, phosphate, and ammonium ions in wastewater has been reported utilizing isocratic IEC followed by sequential flow injection analysis.9 The ammonium cations were detected by colorimetry, while the anions were measured by conductivity. These determinations could be done with a single injection and the run time was under 9 minutes. 

As mentioned in the historical synopsis (Section 5.1), Levine121 perfected the compendial partition column procedure in which aspirin in chloroform is first trapped in an immobile phase of sodium bicarbonate on a column of siliceous earth (celite) then eluted with a solution of acetic acid in chloroform and measured spectrophotometrically. This has been also used for separation in combination products.80 For the determination of salicylic acid in presence of aspirin by this method, see Section 5.61. Ion exchange columns filled with strongly or weakly basic anion exchange resin in the acetate or chloride cycle have also been used for separation of aspirin in combination products. 122 123/l2lf This has also been adapted for a student experiment.125 A Sepha-dex-G25 column has been used for the separation of aspirin from salicylic acid.126... 

When passage of a sugar solution at 90°C through a weak-base anion exchange resin was interrupted, an explosion occurred. This was attributed to an exothermic Maillard reaction (interaction of an amino acid with a glycosidic OH group) under the poor heat transfer conditions in a particulate bed without fluid flow. 

Adsorption chromatography is an efficient way to isolate organic acids from large volumes of water. The nonionic, macroporous, Amberlite XAD-8 and the weak-base anion-exchange resin Duolite A-7 are two resins well suited for this purpose. These resins have been successfully used to extract organic acids from natural waters at sites where it was necessary to process thousands of gallons of sample. 

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