HTH is also a trade name of Arch Chemicals Incorporated for a common form of Calcium Hypochlorite or CaCl2O2. It is usually sold in granular or tablet form, and is familiar to anyone who owns or maintains a swimming pool. It is used to kill algae and bacteria, which can grow in swimming pools. In its granular form, it will quickly dispatch algae and other flora and fauna that conspire to foul your pool. In tablet form, it will dissolve slowly, providing a nice steady supply of chlorine to keep the water sparkling. Properly mixed with water, it can also be used as a substitute for chlorine bleach to whiten and sanitize laundry or food preparation surfaces, and industrial uses also include chlorination of drinking water.
Proper use of HTH in a Swimming Pool
I was fortunate to grow up in a house with a pool, and as a teenager it was my job to keep the pool clean and healthy. HTH does a wonderful job of killing algae and sanitizing the water in a swimming pool or hot tub, but using too little or too much can cause problems. Under most circumstances, a chlorine level of between 0.7 to 1.0 parts per million is adequate to keep bacteria and algae at bay. To determine if the pool needs more chlorine, a water test kit is used. The kit consists of 2 bottles of indicator solution, one for Chlorine, and the other contains phenolphthalein which measures the pH of the water. The kit also usually contains collection vials and a color coded scale. The water is tested by mixing the prescribed number of drops of solution in the testing vials and comparing the colors against the scale.
Dealing with problems
Water can get cloudy from bacterial or algae growth, or an imbalanced pH
. Pool water should be kept slightly alkaline
, with a pH of 7.2 to 7.6 being ideal. Cloudy water can occur after a large pool party introduces urea
and other potential nitrogen sources into the water, or the water can be dirty after a winter of neglect. The urea acts as a nutrient for algae which can grow suspended in the water, and also ties up available chlorine as it forms chloramines
. Acids and oils present in perspiration can also cloud the water. Ca(OH)2
Add enough HTH to raise the level of chlorine to 2 to 3 PPM, and this will kill any algae and most bacteria growing in your pool. The store will sell fairly expensive shock kits to do this, but this is just an expensive way to package HTH. If you are buying HTH in bulk, just add more. Chlorine released by mixing HTH with water reacts with organic matter forming chloramine
s, depleting the available chlorine.
Conserving on the use of HTH
A typical 18 by 36 foot pool can go through upwards of a hundred pounds or more of HTH a year, and at a cost of around a dollar a pound in bulk it makes sense to not use any more than necessary to keep the water clean and the liner free of algae. Adding HTH to pool water, as the chemical reaction shown in the last paragraph liberates chlorine gas. The dissolved chlorine gas does the job of sanitizing the water, but also can bleach bathing suits and hair (I had the nicest blonde hair each summer), and can irritate eyes and skin. The chlorine also evaporates from the water. Adding Undecylenic Acid
to the water reduces the evaporation rate of chlorine gas, and can reduce the amount of HTH needed to maintain an adequate chlorine level in the water. Covering the pool when not in use can reduce the amount of evaporation as well.
HTH is a powerful oxidizer
,and must be handled and stored with extreme care. It can cause severe chemical burn
s on your hands if it is left on wet hands full strength. When mixed with other household and lawncare products, it can spontaneously ignite. A common situation in which this can happen to a typical homeowner is this: The HTH is stored in a garden shed or garage where there also resides a car, a lawnmower
, and a rototiller
, along with insecticide
s, gas cans, and the like. While servicing one of these engines, some oil or fuel is spilled, or the collection pan is left with some oil or antifreeze
in it and stored on the floor near the HTH drum. Harry Homeowner needs to add some chlorine to the pool, so he gets a scoop of HTH from his 75 pound drum stored in his dark overcrowded shed. As he tries to make his way out of the shed, he trips on the mower and spills a couple of ounces of HTH right into that half a cup of oil that dribbled on the mower deck when he serviced his lawn tractor earlier that morning. He regains his balance, and seeing he only spilled a couple of ounces of HTH, he proceeds to the pool and adds it to the skimmer. As he makes his way back from the pool, he sees a cloud of white acrid smoke coming from his shed, and the fire has already spread over the mower deck, where a greasy mix of grass clippings and motor oil has caught the tractor on fire. As he grabs his garden hose to attempt to extinguish the fire as his shed starts to go up in flames along with his Harley and his John Deere tractor, he not only faces a sizable insurance claim
, but a potential HAZMAT
incident as well. Dry HTH generates a sizable amount of heat when it gets slightly wet, and while it will not burn by itself, the heat generated will speed up the decomposition reaction and feed any nearby combustion, and the steam created from the water and intense heat can cause the drum to rupture, releasing a large quantity of the powerful oxidizer at once. Water will also liberate large quantities of a toxic mix of Calcium Hydroxide, Hypochloric Acid and Chlorine gas directly into the air, causing respiratory distress to anyone caught in the toxic cloud.
While this scenario never happened to me, I saw the potential for this when I deliberately mixed small quantities of HTH with things such as antifreeze, gasoline, and motor oil when I was a kid. A quarter cup of HTH mixed with a few ounces of motor oil once resulted in a miniature fireball and a large cloud of white smoke! Again I urge extreme caution when handling this stuff! Anyone who handles this stuff should read the MSDS and CAS sheets for more information. Misuse can be explosive! HTH is also very corrosive, and should be stored in a building with plenty of ventilation, or it will very quickly rust and pit nearly anything made of uncoated steel or aluminum stored nearby.