The separation of NH4+ from Nat, K+, Mg2+ and Ca2+ on silica gel columns was investigated using natural water samples and lithium oxalate as a retention modifier. The pore size of silica gels was found to be the most important factor affecting the retention of the cations. When a 0.0025 mol dm-3 lithium oxalate solution of pH 7 was used as the mobile phase these cations could be completely separated on a short silica-gel column (Develosil 30-3, 35 mmX4.0 mm id.) within about 20 min.NHa+ contents in river (0.01-0.27 ppm) and rain (1-1.8 ppm) water samples were determined successfully.
KeywordsSilica gel column, ion chromatography, sodium, river water, rain water potassium, ammonium, magnesium, calcium, oxalic acid, Silica gel is known to act as an inorganic ion exchanger.l However, the application of silica gel to the chromatographic separation of ionic species is not as popular as those of resin-based ion exchangers.2,3 Recently, Smith and Pietrzyk4 have revealed that many inorganic cations are separable on an unmodified silica gel column by ion chromatography (IC). We have also shown some fruitful results regarding the chromatographic separation of alkali and alkaline earth metal cations (M+ and M2+, respectively) on unmodified silica gel.5,6 However, nobody has yet succeeded in obtaining a complete separation of NH4+ from K± on silica gel.We found that silica gel with a small pore size (30 A, 1 A=0.1 nm) makes it possible to effectively separate NH4+ from K+. This paper describes how NH4+ in river and rain water samples could be successfully determined on a silica gel-oxalate IC system. Experimental manner: to a 1-1(11=1 dm3) flask, 0.0025 mol of oxalic acid (C2H2O4.2H2O, Tokyo Kasei) was transferred; about 950 ml of water was added, and then titrated with a concentrated LiOH solution to give pH 7. The resulting solution was adjusted to 11 in a measuring flask. The mobile phase, thus obtained, was filtered through a membrane filter (pore size: 0.22 µm, Millipore) before use.
ColumnsSeveral types of spherical silica gels , Nomura Kagaku) were tested as ion exchangers. The former and latter figures refer to the pore size (A) and particle size (µm), respectively. The silica gels were made into slurries using a slurry mix (NP type, Chemco) under irradiation of ultrasonic waves and then pumped into stainless-steel columns of different size (35, 50 and 75 mmX4.0 mm i.d.) using a slurry packer (Model 124, Chemco).
ReagentsReagent-grade alkali metal (Merck) and alkaline earth metal chlorides (Wako) were used as received. Deionized water was prepared by the successive use of an Auto Still Yamato) and an all-quartzmade, non-boiling type distilling apparatus Fujiwara). The test analyte solution was prepared to give concentrations of 0.0025 mol dm 3 for M+ and 0.01 mol dm-3 for M2+. The concentrations of M2+ were determined by a well-known EDTA titration method. The mobile phase was prepared in the following Apparatus and procedureThe apparatus used was an assembly of a solvent delivery pump (LC6A, Shimadzu), a syring...
