Cation Separations with Complexing Eluents

Several inorganic acids exhibit a complexing effect for metal ions. The complexing acids include HF, HCI, HBr, HI, HSCN, and H2SO4. The complexed metal ions are converted into neutral or anionic complexes and are rapidly eluted, while the other cations remain on the cation-exchange column. 

The data for hydrochloric acid [9] indicate selective complexing between metal cations and the chloride ion. For example, cadmium(II) has a distribution coefficient of 

Calcium(II), which shows no appreciable complexing, has a distribution coefficient of 147 in 0.5 M perchloric acid and 191 in 0.5 M hydrochloric acid. Strelow. Rethc-meyer, and Bothnia [10] also reported data for nitric and sulfuric acids that showed complexation in some cases. Mercury(II), bismuth(III), cadmium(II), zinc(II), and lead(II) form bromide complexes and are eluted in the order given in 0.1 to 0.6 M hydrobromic acid [11]. Most other metal cations remain on the column. Aluminu-m(III), molybdenum(VI), niobium(V), tin(IV), tantalum(V), uranium(VI), tung-sten(VI), and zirconium(IV) form anionic fluoride complexes and are quickly eluted from a hydrogen-form cation-exchange column with 0.1 to 0.2 M HF [12]. 

An eluent containing only 1 % hydrogen peroxide in dilute aqueous solution will form stable anionic complexes with several metal ions. Fritz and Abbink [13] were able to separate vanadium(IV) or (V) from 25 metal cations including the separation of vanadium (V) from 100 times as much iron(III). 

In a few cases an eluent containing an organic complexing reagent has been used successfully for the chromatographic separation of several metal ions. A notable example is the separation of individual rare earth ions with a solution of 2-hydroxyl-isobutyric acid as the eluent [17]. However, such separations necessitate careful equilibration of the column to maintain a desired pH. Sometimes gradient elution is used, and either the pH or the eluent concentration is changed. 

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In this type of separation the analyte cations compete with the eluent cation for ion-exchange sites and move down the eolumn at different rates. The ionic eluent selected depends on the cations to be separated, the type of separation column and on the detector. In many cases an aqueous solution of a strong acid such as hydrochloric, sulfuric or methanesulfonic acid is a satisfactory eluent. Sample cations commonly separated include the following alkali metal ions (Li, Na+, K", Rb, Cs ), ammonium, magnesium, alkaline earths (Ca, Sr +, Ba ), and various organic amine and alkano-lamine cations. Most other metal cations are separated with a weakly complexing eluent. 

Separations of metal cations with ionic eluents has been limited mostly to the alkali metals, ammonium, magnesium(II), calcium(Il), strontium(ll) and barium(ll). Separations of other metal cations are usually performed with eluents that complex the sample cations to varying degrees (see Section 7.4). Some organic cations have also been separated with ionic eluents, although this appears to be an under-utilized area of cation chromatography. 

Mixtures of alkaline-earth metals are separated on strongly acidic cation-exchangers. The cations are retained on a cationite column, and then they are eluted selectively with appropriate complexants, based on the differences in stability of complexes formed by the alkaline-earth metals with suitable complexing eluents. 

Considerable interest has been shown in a novel cation exchanger first developed by Schomburg et. al. [43]. The material consists of a silica substrate of very uniform particle size coated with a poly(butadiene-maleic acid) resin which serves as the cation-exchange moiety. This material, which is now conunercially available, gives good separations of both monovalent and divalent metal ions in a single run. Ordinary eluents such as hydrochloric or methanesulfonic acid, or complexing eluents may be used [44,45]. 

Crown ethers have undergone limited investigation as components in ion chromatographic detection systems. One example is the work of Jane and Shih, who coated a piezoelectric quartz crystal with dibenzo-16-crown-5-oxydodecanoic acid. The detector was used for cation and anion detection after separation on a diaza-18-crown-6-based separator column with nonionic eluents. The frequency response of this detector for both cations and anions, due to cation complexation and anion association with the resulting complex, was as reproducible and sensitive as standard conductimetric detection, but peak broadening resulted from a relatively large cell volume. 

Cation exchangers based on PBDMA-coated silica are offered by a number of companies. Metrohm (Herisau, Switzerland), for example, offers two columns under the trade names Metrosep Cl and C4. The 5 pm Metrosep Cl is considered to be the high-performance column for the simultaneous analysis of mono-and divalent cations with an analysis time of less than 20 min. It can also be used for the separation of various amines, which are typically eluted with a nitric acid eluent. Figure 4.41 shows an example chromatogram obtained under standard chromatographic conditions with an eluent mixture of tartaric acid and pyridine-2,6-dicarboxyiic acid (PDCA) (dipicohnic acid). Under these chromatographic conditions, calcium elutes ahead of magnesium, followed by strontium and barium. This unusual retention behavior can be attributed to the complex-ing properties of pyridine-2,6-dicarboxylic acid. With a pure tartaric acid eluent. 

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