Question

"Peyton’s and Ryan’s swimming pool is turning green. They will fix the chlorine generator again, but they need to add shock to clear up the algae in the pool. Commercially available shocks are typically powders containing calcium hypochlorite. The hypochlorite ion forms the active hypochlorous acid when the product is added to water. Household bleach contains sodium hypochlorite, which Peyton and Ryan think should serve the same purpose. Will one gallon of bleach from the dollar store be as effective at generating hypochlorous acid as a one pound bag of pool shock?

Describe an experiment that would provide strong evidence for evaluating this claim. Strong essays are generally 300-500 words in length and include several references."

Answer 1

Hi! I hope you are doing great!

• Commercially available shocks (Powders): calcium hypochlorite  Ca(ClO)₂
• Household bleach: sodium hypochlorite NaOCl

The reactions with water are the followings:

(1) Ca(CLO)₂ + 2 H₂O <======> 2 HCLO + Ca(OH)₂ (Double replacement reaction)

(2) NAOCl + H₂O <=======> HOCl + NAOH (Double replacement reaction)

Generally there is a difference between both Chlorine forms, the Sodium hypochlorite is commonly called “liquid bleach” and the Calcium hypochlorite is general referred to as “powder chlorine”, this last can be used by missing a solution of water and calcium hypochlorite, decanting the solution into a tank and using a small chemical feed pump. When comparing elemental chlorine as an oxidizing agent to powder (calcium hypochlorite) or solution (sodium hypochlorite), the oxidizing effect is the same. That is, all three produce hypochlorous acid, the oxidizing agent present for disinfection in water and waste water application. When Sodium hypochlorite is added to water, it reacts to form hypochlorous acid and hydroxide:

NAOCl + H2O        <=======> HOCl + NAOH

When Calcium hypochlorite is addes to water, the same occurs.

The slight contribution of alkali when a hypochlorite is added or the slight contribution of acid when chlorine is added, generally, makes no difference in the final pH of the water. The amount of chlorine added in either form is very small compared to the buffering power of most waters.

The pH of the water is important relative to the varying proportions of the hypochlorous acid and hypochlorite ions. In a solution with pH rising over 6.0 the proportion of hypochlorous acid declines from virtually 100% down to almost 0 at pH 9.0. The activity of hypochlorous acid as a bactericide is greatly superior to that of the hypochlorite, being almost 80 times more powerful. It may be concluded that in free residual chlorination, the higher the pH value the less active is the residual because of its lower proportion of hypochlorous acid. The graph below shows how the equilibrium changes with pH. (Fig. 1) [1]

Fig. 1: Relationship Between HOCl and OCl¯ at Various pH Values

Experiment:

If the type of swimming pool is 1000 galloons you need the following:

• 10 oz. Sodium hypochlorite
• 2 oz. Calcium hypochlorite
• Phmeter
• Acid or a base
• One pump
• One filter
• Scrubber
• Pool water test kit

Steps:

1. Test both the chlorine and PH levels of the water using the pool water test kit. Chlroine levels should be between 1.0 and 3.0 ppm. These are safe levels for swimming. As for PH levels, 7.0 to 7.6 is the ideal range.

2. Balance pH levels Adding acid or a base to the pool until levels are at 7.8 or 8.0. And turn on the pump. This will allow the chemicals to circulate around the water. In order to balance the PH levels, increase PH with Sodium Carbonate or decrease it with Sodium Bisulfate.

3. Wait until the late afternoon or dusk to brush/scrub the entire pool. Paying close attention to hard-to-reach places like under the ladders and creases.

4. Turn on your filter and keep it running 24/7. Always make sure the filter is working properly before adding any types of chemicals to the water to kill algae. If it is necessary, backwash the filter. Have the filter run for 24 hrs per day so you will be able to get rid of the algae during the cleaning process.

5. Shock your pool with the correct amount of Sodium hypochlorite as mentioned earlier. Pre-dissolve each pound of shock in a bucket of pool water before broadcasting it throughout your pool. Make sure you cover the entire pool with shock as best you can. Use the Calcium hypochlorite in another experiment with another swimming pool with the same condition as this.

6. Let the swimming pool run overnight. Shocking at night helps the shock to do a more efficient job. Sunlight can eat up 1ppm (part per million) of unstabilized chlorine (like shock) every hour.

7. Keep brushing/scrubbing the swiming pool for the next couple of days and brush often. Make sure you keep a good level of pH, alkalinity, and chlorine during this time. You can use the pool water test kit.

After a couple of days, the algae isn´t going to appear anymore. You have to keep brushing/scrubbing for good measure and keep a very close eye on the chemical levels to ensure the algae doesn’t make a return.

This experiment applied to both Sodium and Calcium hypochlorite is going to make the algae disappear, so both types of pool shocks have the same effectiveness and purpose. So the answer to the question made is YES, one gallon of bleach from the dollar store will be as effective at generating hypochlorous acid as one pound bag of pool schock [2].

References:

[1] Farming, Perls. "Basic Chemistry of chlorination". Daily Chemistry. Ed. 2012.

[2] Breva, Ana. "Swimming Pool Algae: Death by Chlorine". Poolcenter.com 22 October 2017

http://blog.poolcenter.com/article.aspx?articleid=6280

I hope this essay to be helpful for your request. Regards.