Weak acid anions

Effect on Oxide—Water Interfaces. The adsorption (qv) of ions at clay mineral and rock surfaces is an important step in natural and industrial processes. SiUcates are adsorbed on oxides to a far greater extent than would be predicted from their concentrations (66). This adsorption maximum at a given pH value is independent of ionic strength, and maximum adsorption occurs at a pH value near the piC of orthosiUcate. The pH values of maximum adsorption of weak acid anions and the piC values of their conjugate acids are correlated. This indicates that the presence of both the acid and its conjugate base is required for adsorption. The adsorption of sihcate species is far greater at lower pH than simple acid—base equihbria would predict. 

Sjogren. A. and Dasgupta, P. K., Two-dimensional conductimetric detection in ion chromatography. Analyte identification, quantitation of very weak acid anions, and universal calibration, Anal. Chem., 67, 2110, 1995. 

Many IC techniques are now available using single column or dual-column systems with various detection modes. Detection methods in IC are subdivided as follows [838] (i) electrochemical (conductometry, amper-ometry or potentiometry) (ii) spectroscopic (tJV/VIS, RI, AAS, AES, ICP) (iii) mass spectrometric and (iv) postcolumn reaction detection (AFS, CL). The mainstay of routine IC is still the nonspecific conductometric detector. A significant disadvantage of suppressed conductivity detection is the fact that weak to very weak acid anions (e.g. silicate, cyanide) yield poor sensitivity. IC combined with potentiometric detection techniques using ISEs allows quantification of selected analytes even in complex matrices. The main drawback... 

Because of its capacity to separate effectively cations and anions ion chromatography has also found application in chromatographic practice. A nonlinear model has been developed for the prediction of the retention of polyvalent weak acid anions in anion chromatography. In the case of strong acid anions containing only one acidic group eluted by a mobile phase with monoanionic additive, the retention can be described by... 

F. Houton, L. Yaoping and X. Minjie, Nonlinear function retention model in weak acid anion chromatography. J. Chromatogr.A 945 (2002) 97-102. 

BrOj-, IO3-, CIO4-, Br04 , IO4-, HSO4-, HSe04 , NO3-, H2PO4-, and H2As04. Almost all other anions are weak-acid anions, and their salts will give basic solutions when they are dissolved. The extent to which weak acid anions produce OH can be determined as illustrated above. 

Kp values may be found in the literature or they may be calculated from AG° values of the weak acid anion and the weak acid. Salts can be formed between cations and the A1(0H)4 anion, these being called tetrahydrox-oaluminates. Those of the alkali metals are soluble, but most others are insoluble. 

Weak acid anions are more strongly affected by... 

The response is greater for ultraviolet detection by a factor of 5-30. A relatively stronger, negative ultraviolet response occurs for weakly acidic anions because there is no dependence on their ionisation. 

The ion chromatographic determination of weak acid anions is complicated by ion exclusion in the suppressor column, resulting in faster elution and sharper peaks, directly proportional to the degree of exhaustion of the suppressor column [7], A lOmg L 1 nitrite standard showed a 37% increase in peak height over an 8h period when monitored with the conductivity detector while on only a minor 2% increase in peak height was observed over the same time period by using the ultraviolet detector after the separator column. 

Kuster, B. F. M. and Temmink, H. M. G., Dehydration Of D-fructose (formation of 5-hydroxymethyl-2-furalde-hyde and levulinic acid). 4. Influence of PH and weak-acid anions on dehydration of D-fructose. Carbohydrate Res 1977, 54(2), 185-191. 

This synthesis nicely demonstrates the loss of F- from a weak acid anion (BFj or PFft) to Ag2+. 

Alkalinity = (HCOs -b CO + OH -b Org -b Other weak acid anions)... 

Strong base cations) — (Strong acid anions) = alkalinity = acid neutralizing capacity = y (Weak acid anions) — (Weak acid cations) (6)... 

Alkaline diuresis will favor the excretion of weak acids (anions) such as salicylate or phenobarbital. Indeed, the more the drug is ionized, the more it is trapped in the tubular lumen and consequently is not reabsorbed, hence eliminated in the urine. This mechanism can play a role in the treatment of severe intoxications. The reverse being true for weak bases (cations) such as methadone. Acidification of the urine facilitates the reahsorption of weak acids and will re-... 

In Section 4.4 we saw that most soilwaters that feed rivers and groundwater have near-neutral pH, with I ICO, as the major anion. This results from the dissolution of C02 in water (see eqn. 4.7) and from the acid hydrolysis of silicates and carbonates. The total concentration of weak acid anions like HCOf in water is referred to as alkalinity. These anions are available to neutralize acidity (H+) in natural waters, consequently it is important to understand their chemical behaviour. 

This shows that at typical pH values for continental waters the HCOf anion is 200 times more abundant than the CO( anion. Repeating this exercise for a range of pH values results in the graphical relationship shown in Fig. 5.5. Note that when pH falls below 5 on Fig. 5.5, almost all of the weak acid anions (HCO, and C03 ) have disappeared and at pH of 4 only undissociated acid (H2C03) remains. This relationship is used as the basis for measuring alkalinity (Box 5.2). 

Common Weak Acids Anions of These Weak Acids ... 

The formation of metal-ligand complexes, because protons compete with metal ions to bond with weak-acid anions, and OH" competes with other ligands that would form complexes. 

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

The data show that the relative retentions of the weak-acid anions are almost independent of the size of the alkyl groups. However, as the size of the R groups increase, large changes occur with the more polarizable anions such as nitrate, iodide, chlorate and BF4 ions. 

Anions of strong acids may be separated in acidic solution (as in Fig. 6.10) or at a basic pH. Weak acid anions require a basic solution to exist in the anionic form. Separation of borate, silicate, sulfide, cyanide as well as the anions of two stronger acids is shown in Fig. 6.11. An alkaline solution of sodium benzoate was used as the eluent. Carbonate may also be separated under alkaline conditions. Separation of these anions by suppressed IC is usually not attempted because they arc converted to the non-conducting molecular form by the acidic suppressor. 

Figure 6.11. Separation of weak acid anions. Conditions Alltcch Anion/R column, sodium hydroxide/ sodium benzoate eluent, 1.5 mL/min, conductivity detection. Hydroxide is the primary eluent for this separation and the weak acid anions are detected with indirect conductivity. A small amount of benzoate is added to speed the elution of the anions (Courtesy Alltcch).

Tanaka et al. devised a method for determination of the ions in acid rain in a single run [18]. The column used contained a polyacrylate weak-acid cation exchange resin (TOSOH TSK gel OA-PAK, 300 x 7.8 mm, 5 pm particle size). Water alone as the mobile phase only separated weak acid anions from strong acid anions as a group cat-... 

Detection of weak acid anions is best by indirect conductivity detection or post-column reaction detection because the suppressed conductivity detection will not perform. Indirect conductivity detection is often used because the high pH used to separate the anions will also facilitate indirect conductivity detection of these anions. Chapter 4 describes a method of combining suppressed conductivity detection and nonsuppressed detection. 

Conductivity detection is the most popular for ion chromatography. Although UV detection is often overlooked, it can be quite powerful. Amperometric detection, for example, offers selectivity and sensitivity, in many cases unsurpassed. The optimum eluent separation pH may not be the optimum pH for detection. An anion may be separated but not detected. This is especially true for some weak acid anions and suppressed conductivity detection. Chapter 4 discusses the use of different detectors for IC. 

Ion exclusion is nseful for the separation of weak acid anions. The decision to use ion exclusion over ion exchange frequently depends on the matrix of the sample. If a mixture of weak acid anion and strong acids anions is to be analyzed, then ion exchange is a separation tool that will be the most effective. However, if weak acid anions are to be analyzed in the presence of large concentrations of strong acid anions,... 

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