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Aluminum chlorohydrate is often referred to as basic aluminum chloride. The pH value of ACH is about 4. How is basicity in this context defined?

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Aluminum chlorohydrate (ACH) is a group of specific aluminum salts having the general formula $\ce{Al_nCl_{(3n-m)}(OH)_m}$ (Wikipedia). It is used in cosmetics as an antiperspirant and as a coagulant in water purification. Specifically in wastewater treatment processes, ACH is used as a coagulant to remove dissolved organic matter and colloidal particles present in suspension. In his paper on Water Treatment Coagulants, Peter Gebbie describes what coagulants do (Ref.1):

With aluminium-based coagulants, the metal ion is hydrolyzed to form aluminium hydroxide floc as well as hydrogen ions. The hydrogen ions will react with the alkalinity of the water and in the process, decrease the pH of the water as can be seen from Equation [...]. Similarly for ACH, which is described as being a pre-hydrolyzed coagulant, the following reaction takes place: $$\ce{Al2(OH)5Cl -> Al2(OH)5+ + Cl- + H2O -> 2Al(OH)3 + H+ + Cl- }$$ Less hydrogen ions are produced with ACH, reflecting the hydroxylated nature of this compound.

There it says it all: ACH is more basic than other aluminum salts because it is pre-hydroxylated.

Aluminum chlorohydrate is one of the most common active ingredients in commercial antiperspirants. The variation most commonly used in deodorants and antiperspirants is $\ce{Al2Cl(OH)5}$, similar to the one used in water purification.

Note: Your statement of "The pH value of ACH is about 4" is evidently not correct. However, W. R. Grace & Company (producer of solution form of ACH) listed solubility of ACH as $\pu{1000 g/L}$ at $\pu{20 ^\circ C}$ and $\mathrm{pH} = 3.3$, the optimum condition.

Reference:

  1. Peter Gebbie, “An Operator’s Guide to Water Treatment Coagulants,” 31st Annual Water Industry Workshop – Operations Skills; University Central Queensland Campus: Rockhampton, Australia, 4 to 6 July, 2006, pp. 14–20 (http://wioa.org.au/conference_papers/06_qld/documents/PeterGebbie.pdf).
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  • $\begingroup$ Thank you very much! So is it correct that aluminum chlorohydrate is still acidic, but less than other aluminum chloride compounds due to hydroxylation? Did I understand you correctly that acidity is not defined by the pH value? The statement that the pH value of ACH is about 4 can be found here: books.google.de/… (page 506) $\endgroup$ – jona173 Jun 19 at 0:00
  • $\begingroup$ It says "buffered" ACH has pH around 4. Not ACH itself. $\ce{Al2(OH)5+}$ ion should be basic since $\ce{Cl-}$ is neutral. $\endgroup$ – Mathew Mahindaratne Jun 19 at 0:25
  • $\begingroup$ That makes sense, thank you! $\endgroup$ – jona173 Jun 19 at 0:26
  • $\begingroup$ @MathewMahindaratne Commonly we say that pKa = optimal pH of buffer, so that would make ACH acidic not basic. This is consistent with the equation in your answer, no? $\endgroup$ – Buck Thorn Jun 19 at 6:00
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    $\begingroup$ @Buck Thorn: You are probably correct on that argument. Yet, I think it must be fairly close to neutral to be able to use as an ingredient in antiperspirants because original $\ce{AlCl3}$ based antiperspirants had side effects due to acidity. $\endgroup$ – Mathew Mahindaratne Jun 19 at 6:31

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