Procedure as worded is ambiguous - acetate buffer means a buffer made by mixing acetic acid with some acetate (typically sodium), but what are concentrations of these and what is their ratio is in no way obvious. Acetate buffer typically means something between pH 4 and 5.5, exact value can be important, or it can be not, depends on the reaction details.
Buffer has to be selected carefully, so that it doesn't interfere with the detection reaction. Say, if you detect chlorides with Ag+, any ion reacting with Ag+ will give you a false positive. Acetate buffer is a common selection in such cases.