In common usage, the word sadaqah, unless specified, refers to voluntary charity, and zakah applies to  the obligatory one. Linguistically, however, sadaqah applies to both charities: obligatory as well as voluntary. In the Qur’an, and hadith, however, the word has been interchangeably used for both. Therefore, we have no recourse but to specify the precise meaning from the context. The following Qur’anic verse is an example of the use of sadaqah for the obligatory charity:

“The alms (the word used here is: sadaqaat) are only for the poor and the needy, and those who collect them, and those whose hearts are to be reconciled, and to free the captives and the debtors, and for the cause of Allah, and (for) the wayfarer; a duty imposed by Allah. Allah is Knower, Wise.” (Qur’an: 9: 60).

The word sadaqaat in the above verse, according to scholars, refers to zakah or obligatory charity.

While zakah is obligatory, sadaqah (other than zakah) is only recommended or optional. This, however, should not be taken to mean that as Muslims we should deprive ourselves of the infinite blessings and rewards of voluntary charities. It should help us to know that charities serve as expiations for our various sins of omission and commission; they also protect us against calamities; they also help us receive answers for our prayers, etc.