Salinity

Salinity is the saltiness or dissolved inorganic salt content of a body of water. Substances that are dissolved in water are usually called solutes. The typical seawater has a salinity of 35 ppt or 35‰. The average density of seawater at the surface is 1.025g/ml. Seawater is denser than freshwater because of the added weight of the salts. Natrium and sodium make up the majority of the salts dissolved in seawater. The other major ions are magnesium, sulfate, calcium and potassium.

Introduction

All organisms have to maintain a balance between water and solutes in their bodies to keep their cells alive. This is called homeostasis. To regulate this, they can import and export solutes through their membranes. These membranes are permeable to the substances that are dissolved or suspended in water. The random movement of dissolved substances across a permeable membrane is called diffusion. Diffusion occurs until the concentrations are equal on both sides of the membrane. When a solute cannot move through the membranes, the balance on both sides is maintained by water movement. The movement of water through a membrane in response to differences in solute concentration is called osmosis. This is vital to the survival of cells. If cells lose too much water, it will become dehydrated and die. If cells take in too much water, the cells start to swell and possibly burst. For this reason, organisms invest time and energy in maintaining the homeostasis. If the salt content differs on both sides of the membrane, pure water will move across the membrane in the direction of higher salt content.

Measure methods

Originally, salinity was calculated from the Cl- content (chlorinity). The assumption for this method is that the major ions remain in constant proportions to each other. The chlorinity is measured as the mass in g of halides that can be precipitated from 1,000 g of seawater by Ag+ using a standard AgNO3 solution. The reaction is: 3 Ag

===+=== + Cl- + Br- + I- AgCl + AgBr + AgI