1. Introduction: The color of water in the swimming pool varies from sky blue to green due to varying of refractive index with the contents of soluble calcium ion in the water.
The more the soluble calcium is in the water, the more the tendency of water to be blue color. In the spectrum of light (figure 1) the green color is adjacent to blue color. Since water is losing calcium ion gradually by carbon dioxide from the air dissolved into the water, thus, the color of water will be gradually changed from sky blue to green when the water is exposed to the air or before and after the rain when the atmospheric pressure was increased, making more carbon dioxide dissolved into the water. The pH and alkalinity of water drops down and the water becomes more opaque due to calcium bicarbonate salt suspended in the water. When the water which turns green was left untreated for about two weeks its color will be changed to cloudy pale green because of calcium bicarbonate and calcium carbonate salts suspended in the water which is misunderstood as algae.
Violet: Indigo: Blue: Green:Yellow:Orange: Red
Figure 1. In the spectrum of light, blue and green are adjacent. The color of water in the swimming pool will gradually change from blue to green when the soluble calcium ion in the water drops down because of carbon dioxide in the air dissolved into the water.
Another cause of changing of color of water in the swimming pool from sky blue to green relates to the iron ions and rust in the make up water. Iron ions and rust in the make up water have yellow color by nature. Thus, when the make up water containing iron ions or rust is added into the swimming pool water to replace overflow and evaporation, the sky blue color of water in the swimming pool will be changed to green (Figure 2).
Figure 2. Sky blue water when contaminated with Yellow iron ions or rust gets Green water.Iron ions in the make up water have two forms. The first is ferrous ion (Fe+2) which is colorless and the second is ferric ion (Fe+3) which is yellow color. The ferrous ion when contacted with oxygen or residual chlorine in the swimming pool water becomes ferric ion. Thus, even with colorless make up water, the color of swimming pool water may be eventually turns green because of ferrous ion in the make up water will gradually becomes ferric ion when contact with oxygen and residual chlorine. Ferrous ion comes mainly from deep well water which is usually mixed with tap water during dry seasons when the supply of surface water is short, while ferric ion comes mainly from surface water or rust in the pipe lines, pumps or storage tanks. The concentration of iron ions in the water which makes the sky blue color of water turned green is very small, i.e., in the parts per billion (ppb) order of magnitude which is out of the range of common test kits.
However, we can check if the tap water contains iron ions or rust or not by investigating the white color bathtub filled with tap water or concrete storage tank laid with white color tiles and the inside of storage tank of the toilet. If the color of water in the bathtub or storage tank is pale green then the tap water surely contains iron ions or rust. If it is sky blue then the tap water has no iron ions at that investigation period. The storage tank of toilet is too small to observer the color of water inside it. But it keeps the history of tap water over a long period. So, if the color of the wetted part inside the storage tank is brown with soil on the surface then the tap water surely contains iron ions or rust over a long period. Meaning, although there are no iron ions or rust in the tap water during investigation period, they existed in other periods of the year.
Although very small amount of ferric salts in the water may not cause the color of water to turn green as seen from the side of the pool explicitly, it makes a big difference to some divers. Some divers noticed that water in some swimming pool is so clear that when they dived on one side of the pool and looked through the water to the far side of the pool they could see the tiles on that far side clearly while in other swimming pools they could not. As a matter of fact, most of the above mentioned calcium salts and ferric salts once occurred cannot be filtered out by sand filters or diatomaceous earth filters used in the swimming pools. In section 3, we will talk about how to make such “superclear” water for swimming pool. See full paper in www.sahatchaiw.com
Tags: , Swimming pool
