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UV Spectroscopy Gauges Sea Salt

Photonics Spectra
Feb 1998
Dr. Flavio Fontana

PADOVA, Italy -- Ultraviolet absorption measurements provide a precise gauge of sea water salinity, an important gauge of ecological health, according to marine researchers here.
Measuring sea salinity is important in assessing the movements of water masses, which influence the marine habitats and the distribution of pollutants. Salinity changes have helped spread some species of algae, whose uncontrolled growth is becoming a severe ecological problem in parts of the Mediterranean Sea.
In a collaboration between the Università di Padova and an Italian research institute devoted to the study of marine environment, Professors Vito Di Noto and Mauro Mecozzi recently unveiled an optical method for salinity measurements that could lead to real-time monitoring of this parameter.
Their approach has been based on a thorough analysis of the ultraviolet absorption spectra from 190 to 250 nm of the various salts (KBr, NaCl, etc.) in sea water, identifying the characteristic absorption patterns of each species.
Their analysis has addressed the problems of interaction between ions of the same species and mutual influence of different ions. At high saline concentrations, the departure from the ideal solution behavior causes a remarkable nonlinearity of the optical absorption curve; clustering of water molecules around the ions modifies the ultraviolet absorption. This effect was previously a limitation for optical methods based on linear algorithms.
In the new approach, the researchers have correlated the effect so that their calculations are reliable up to salinity levels higher than those encountered in the Mediterranean Sea. Di Noto said the new method attains a precision of 0.02 percent in salinity determination, sufficient for routine environmental control of sea water.
Although methods based on conductivity measurements can attain high precision, their use outside the laboratory is rather difficult because they need frequent calibration. The spectroscopic approach is amenable to full automation, and researchers can use a quartz fiber optic probe to monitor the sea water, allowing underwater measurements as well. The instrument can be reduced easily to a size acceptable for on-board use in small boats.
The Italian Ministry for Environment will exploit the technique in a campaign aimed at controlling pollution in the Adriatic Sea.

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