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When testing for pH for common household cleaning products, such as dishwasher fluid, I noticed that majority of them tend to be basic? Why is that? Do acid make good cleaning solutions as well?

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  • 2
    $\begingroup$ As far as I know many household cleaning products contain anionic surfactants which are supposed to be mild on skin. I don't have any reference, though. $\endgroup$ – cgnieder May 11 '12 at 21:07
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Basic dishwashing agents?

  • Some dishwashing agents contain substantial amounts of NaOH, see below.

  • Sometimes NaClO solutions are used as disinfectant for dishwashing.
    $\require{mhchem}$ As $\ce{Cl- + ClO- + 2 H+ <=>> Cl2 ^ + H2O}$, they are kept basic (and $\ce{Cl-}$free, but $\ce{3 ClO- -> 2 Cl- + ClO3-}$ and $\ce{2 ClO^- -> 2 Cl^- + O2 ^}$) for stabilization.
    @Ashu asked whether I'm really sure, so here's an example: with 15 - 30 % KOH + 15 - 30 % NaOCl, pH 14 "Liquid intensive cleaner for dishwashing machines" (technical data, saftety sheet both German, though)

  • Soaps (chemical meaning) are salts of fatty acids. Na- and K-soaps are good for cleaning and as salts of weak acids and strong bases they are basic.

Dishwashing liquid chemistry?

If we approximate the food rests you want to clean away during the dishwashing as mixture of carbohydrate, proteins and lipids, then your dishwashing liquid needs that take care of those 3 substance classes in water.

  • carbohydrates are hydrophilic - no problem here.

  • amphiphils are needed to clean lipophilic substances like lipids, oils, greases as Ashu explained already. They can take care of quite a bit of protein as well.

  • basic solutions usually cause a faster hydrolysis of the amide bonds in proteins than acids. Usually quite a bit faster: NaOH-solutions immediately give a "soapy" feeling while e.g. HCl doesn't.

  • As far as I know here (Germany), basic liquids for household dishwashing machines are uncommon/seldom/not used at all (I'm not so sure about the tablets). The actually used solutions are less corrosive (for both the machine and your fingers if you more or less accidentally decide to use it for dishwashing by hand).

  • This page from a New Zealand poison centre talks about the "dishwasher [...] powder or tablets [...] are often highly alkaline"

  • For dishwashing liquids where you are thought/supposed to put your hands in, the pH is usually neutral to slightly acidic, so you need to scrub a bit more, but don't dissolve your skin.

Acids for cleaning?

Acids (usually acetic acid or citric acid, maybe HCl) are used in "household chemistry" to clean limescale (CaCO$_3$). But that usually isn't the problem of your dirty dishes.

Dishwashing machines use ion exchangers to soften the water (or phosphate in the dishwashing agent), so you don't get limescale or lime soaps inside the machine.

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  • $\begingroup$ Some dishwashing agents contain substantial amounts of NaOH .. are you sure? $\endgroup$ – Ashu May 14 '12 at 19:10
  • $\begingroup$ @Ashu: yes. I updated the answer with some technical and safety data for a gastronomic product (Germany) and a New Zealand Poison Central speaking about household dishwaser tablets and powder being "highly alkaline". If I remember (unlikely), I'll look at the ingredients of such tabs the next time I'm in the supermarket. $\endgroup$ – cbeleites supports Monica May 14 '12 at 21:36
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Dishwashing agents are usually anionic detergents—i.e. they are alkylbenzenesulfonates.
They consist of two parts:

  1. Sulfonate part which is hydrophilic(water loving i.e. dissolves in water)
  2. Alkylbenzene part which is lipophilic(fat loving i.e dissolves in fats and oils)

Thus, when in water, they dissociate to give $\ce{RSO3-}$ and $\ce{Na+}$ . $\ce{Na+}$ is from a strong base (making it a weak conjugate acid), while $\ce{RSO3-}$ is from a weak acid (making it a weak conjugate base), thus the resulting solution is basic.

The sulphonate ion bonds to water, while alkylbenzene bonds to oil, protein, or fat . Thus fats, oils, proteins, and grease dissolve in water

The following is the mechnism of a dishwashing agent:

enter image description here

These are the ingreadients mentioned on a dishwashing bottle in my house:

enter image description here

As you can see, sodium dodecylbenzenesulphonate (an alkylbenzenesulfonate) is one of the main ingredients.


Do acid make good cleaning solutions as well?

Yes they can make a good cleaning solution, but not as a dishwashing agent.

According to Wikipedia ,

Acidic washing agents are mainly used for removal of inorganic deposits like scaling. The active ingredients are normally strong mineral acids and chelants. Often, there are added surfactants and corrosion inhibitors.

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  • $\begingroup$ is that one correct: Sulfonate + Alkylbenzene = surfactant? $\endgroup$ – JinSnow Jan 12 '17 at 11:02

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