Answer:
About the relationship between pH and disinfection
  It is hypochlorous acid (HClO) that has a high disinfection effect. As acid is added to sodium hypochlorite (NaClO) solution, the ratio of hypochlorite ion decreases and the ratio of hypochlorous acid increases. Thus, the effectiveness of disinfection increases as pH decreases; however, it is dangerous to decrease pH too much because chlorine gas is generated, as this questioner mentioned. So pH 5 to 6 is a good condition for disinfection.

  A pH meter should be used to adjust the pH of sodium hypochlorite solution. Acid (hydrochloric acid, citric acid, lactic acid, and so on) can be added slowly to adjust the pH until it becomes pH 6.

On-site usage of sodium hypochlorite solution
  Although the methods used to disinfect the product lines would be different in different types of industries, most industries use diluted sodium hypochlorite solution for disinfection. Common usage is to dilute sodium hypochlorite 300 to 600 times with water using a resinous bottle.

  Actually it is very dangerous on site to adjust the pH of sodium hypochlorite solution by adding acid manually. Therefore, it is hard to apply the theory to real situations, even though in theory hypochlorous acid has a high disinfection effect.

About temperature
  Regarding the temperature effect on disinfection by sodium hypochlorite solution, there is no big difference between high and low temperature. Room temperature is adequate, so it would not be necessary to warm it up. However, chlorine in sodium hypochlorite solution can be broken down by heat or light, so it is possible to decrease its concentration at high temperature. It would be recommended to refrigerate the stock sodium hypochlorite solution.


Acknowledgement
  We would like to thank an anonymous professor for his careful teaching and kind support for this answer.
 
 

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