1997 saw the start of a study into the spatial and temporal changes in the chemical composition of the atmosphere over the littoral and offshore waters of the Baltic Sea. Collection of samples and their subsequent chemical analysis was preceded by meticulous laboratory experiments concerning the validation and adaptation of the analytical procedures. The colorimetric indophenol blue technique was used to analyse the aerosol samples for the concentration of ammonium ions and the air samples for their gaseous ammonia. The samples were collected with PTFE-filters and Whatman 41 filters coated with 5% orthophosphoric acid. This acid enhances the aerosol sampling efficacy but reduces the pH of the reaction in which indophenol is formed. The pH of the aerosol samples thus had to be raised, and this in turn required an alteration to the original procedure prior to ammonia determination. It was demonstrated that the addition of 0.1 N KOH to the filters coated with H<sub>3</sub>PO<sub>4</sub> increases the pH of the reaction medium to the required level of pH = 8-11.5 and does not substantially influence the precision of the determination; the error of the modified procedure was of the order of 5.2%. <br> Air samples for the determination of gaseous ammonia were collected with annular denuders. Oxalic acid and citric acid are the usual impregnating agents. In the present experiments oxalic acid was used for denuder impregnation; it turned out to be more effective than citric acid under the conditions of the southern Baltic Sea. <br> The detection limit of the indophenol blue method in these laboratory experiments was 0.045 mmol dm<sup>-3</sup>. The respective relative standard deviations (RSD) within the range of higher and lower concentrations were 0.64% and 4.53%
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