Alginic acid and its salts (calcium, magnesium and sodium) are naturally present in the cell walls of brown seaweed. The most used species for the production of alginic acid and its salts are Macrocystis, Ascophyllum, Durvillaea, Ecklonia, Laminaria, Lessonia, and Sargassum. (1)

Generally, sodium alginate is made by the reaction of sodium carbonate with alginic acid. The brief manufacturing process are as follows (2):

Washing seaweed with water and acid to move water-soluble substances, such as sodium chloride, potassium chloride, colors, mannitol and soluble proteins.
Convert insoluble ingredients (alginic acid and calcium alginate) to soluble sodium alginate by adding sodium carbonate, and then remove cellulose (insoluble) by filtration.
Convert sodium alginate to alginic acid by adding acid or calcium chloride to form calcium alginate first and then wash with acid.
Neutralization with sodium carbonate, and followed up by the process of drying, granulation or sieving.
Specification
Other names   
Algin gum
Alginic acid, sodium salt
CAS number    9005-38-3
Chemical formula    (C6H7NaO6)n
Molecular weight    10 000 -600 000 (typical average)
Properties
Appearance
White to yellowish brown filamentous, powder or granular, nearly odorless and tasteless.

Structure
Sodium alginate chemical structure
Image Source

Sodium alginate is a polymer of polysaccharide mainly made of α-L-guluronic acid (G unit) and β-D-mannuronic acid (M unit) connected by α ( 1→4) glycosidic bonds.

The ratio of M/G (were related to the species of seaweed and the harvested season) will affect the viscosity, gelling capability and gel strength.

Solubility
Dissolve in hot or cold water to produce a viscous colloidal solution while calcium and magnesium alginate does not soluble insoluble in ethanol.

Viscosity
Generally, the viscosity depends on:

Degree of polymerization
The molecular weight of the sodium alginate polymer
Temperature
Concentration
Presence of polyvalent metal cations
This property can be used as a thickener and the viscosity (1% solution) of its food grade can vary from 4 to 1000 mPa·s.

PH
Ph ranges from 4.0-10.0 in its food applications.

Gelling
A heat-stable gel would be formed with acids or multivalent cations, e.g. calcium, copper, and lead ions.

A calcium salt is commonly used to react with a sodium alginate solution to form a gel in which polymers are cross-linked by calcium ions, such as in the production of jelly.

The following image is the gel before and after heating, and we can see the alginate gel is heat stable than the gels of gelatin, carrageenan and agar.

sodium alginate gels before and after healting
Image Source

Gel synergy: a firmer gel occurs by the combination of sodium alginate with pectin. Such gels are thermo-reversible compared with thermally stable sodium alginate (with calcium ions) gels.