# change in dielectric constant of water

#### shreyas_bhat

Joined Jul 26, 2004
47
Hello Folks
If I added salt to sea water (which already has 3.5% by wt. of salt), would the dielectric constant of the sea water increase or decrease. Intuitively, dielectric constant is a measure of the insulating character of a material. However, literature search has it that the dielectric constant increases as the salt concetration in water increases. Somebody please explain. And how much do u think will be the increase/decrease ?

#### David Bridgen

Joined Feb 10, 2005
278
Originally posted by shreyas_bhat@Mar 16 2005, 04:18 AM
Hello Folks
If I added salt to sea water (which already has 3.5% by wt. of salt), would the dielectric constant of the sea water increase or decrease. Intuitively, dielectric constant is a measure of the insulating character of a material. However, literature search has it that the dielectric constant increases as the salt concetration in water increases. Somebody please explain. And how much do u think will be the increase/decrease ?
[post=6118]Quoted post[/post]​

Pure water is an insulator.

Many years ago I needed to load a high voltage source and draw a milliamps from it.

I started with a bucket of pure (laboratory reagent) water and inserted two bare wires from the supply. No current was observed.
I then added a very small pinch of household/table salt to the water and the current shot up spectacularly.

It was clearly not the route to take for my needs and I abandoned the idea but it does illustrate the point of pure vs "contaminated", or at least salty, water.

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

As David noticed, adding virtually any contaminant to water will alter its conductivity. That also ruins the diaelectric constant. Only pure water will have a measurable diaelectric constant. I would guess that it might reach a maximum at 4 deg. C, where water is maximially dense. Diaelectrics cannot be conductors.

#### Erin G.

Joined Mar 3, 2005
167
During my time in the Navy I had to maintain the "cathodic protection system" on board the ship. As it turns out, the ship's hull makes a very good sacrificial anode, and the sea water is an excellent electrolyte. The chemical reaction between the two not only causes rust, but unchecked, would cause "pits" to be eaten through the hull in various places. To counter this, zinc cathodes are placed at strategic points throughout the ships outer hull, below the water line. The cathodes are energized just enough to give an equal and opposite effect to the electrolytic action taking place between the hull and the sea. In other words, we were neutralizing the electrolytic action.

Here's the rub: Different areas of the world have different amounts of salinity in the sea water. We would have to make constant changes to the cathode voltages to counter the different salinties and specific gravities in the sea water, depending where we were on the map.