Can I make a buffer from two different weak acids or bases?

Can I make a buffer using two different acids/bases? For example, adding 1 mole of acetic acid and 1 mole of ammonia?


The only information I can ever find is that I can make a buffer using *only* an acid (or base) and its *conjugate base* (or conjugate acid). So, carbonic acid and carbonate, $\ce{H2CO3}$ and $\ce{HCO3-}$.


Why do I need a conjugate acid-base pair and not two random weak acid-base pairs to make a buffer?




Yes, having multiple weak acids and/or bases having different pKa values would give buffering capacity in more than one pH range, or over a greater pK range. 


There is a term "[universal buffer](http://en.wikipedia.org/wiki/Britton-Robinson_buffer)" which refers to a buffer having multiple pKas spaced thoughout the pH ~2-12 range to give some buffering capacity over the enter pH range. 


As explained in [A Neutral Buffered Standard for Hydrogen Ion Work and Accurate
Titrations Which Can be Prepared in One Minute](http://pubs.acs.org/doi/abs/10.1021/ja01344a025) J. Am. Chem. Soc., 1932, 54, pp 1911–1912, ammonium aceate has the property that if it is added to pure water it produces a buffered solution of pH 7.00, accurate to a hundreth of a pH unit. 


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In theory you can, but it depends on the way you prepare the buffer.


*A buffer solution is prepared by mixing a weak acid (or base) with a strong salt of that acid*


For example you can prepare the following buffer in two ways:


$\ce{HOAc + NaOAc}$


**1:**
Mixing directly a solution of the salt with the acid


**2:**
Adding (let's say dropwise) a solution of $\ce{NaOH}$ to the acid. In this case $\ce{NaOAc}$ will be formed *in situ*.


### What you cannot do!


Prepare a buffer by mixing $\ce{HOAc}$ with $\ce{NaCl}$ for example


On the other hand you can mix two different buffers to get a new one with a different pH range like in the case of universal buffers.


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