ACID-BASE BASICS Stephen Wright, Ph.D. Dept of Physiology

Transcription

1 Foundtions Lerning Objectives for the self-lerning module on ACID-BASE Bsics 1. Define ph. 2. Clculte the hydrogen ion concentrtion from ph nd vice vers. 3. List the mjor body buffer systems. ACID-BASE BASICS Stephen Wright, Ph.D. Dept of Physiology 4. Define buffer solution nd explin how it works to limit chnges in ph upon the ddition of cid or bse. 5. Given the pk of wek cid (or bse) nd the relevnt concentrtions of ionized nd unionized species in solution, clculte the ph of tht solution using the Henderson-Hsselblch eqution. Use this eqution to clculte the concentrtion of wek cid (or bse) in solution given the ph of the solution nd the pk of the wek cid (or bse). An understnding of cid-bse chemistry is essentil to pprecite the properties of biologicl molecules. Mny of the low moleculr weight metbolites nd the mcromoleculr (e.g., protein) components of living cells re cids nd/or bses nd, thus, hve the potentil to ionize. The electricl chrges on these molecules influence the stbility nd conformtion of proteins, the interction of mcromolecules with ech other nd with their substrtes, nd, therefore, cn profoundly ffect the rte of enzyme-ctlyzed rections. So let s begin this discussion of cids, bses nd ioniztion with review of. wter nd ph! Wter An cid is molecule tht dontes protons (hydrogen ions) nd bse is molecule tht cn ccept protons. We typiclly tlk bout cid-bse rections in queous-phse environments -- tht is, in the presence of wter. Consequently, the most fundmentl cid-bse rection is the dissocition of wter: H 2 O H + + OH - In fct, hydrogen ions (proton) in queous environments exist s H 3 O +, or hydronium ions: 2H 2 O H 3 O + + OH - For ll prcticl purposes, H 3 O + nd H + men the sme thing. The more typicl convention for hydrogen ion is H +, nd we will use them interchngebly. In pure wter, we cn define specil equilibrium constnt (K w ) s follows: 1

2 K w = [H + ][OH - ] = 1.00 x Where K w is the equilibrium constnt for wter, [H + ] is the molr concentrtion of hydrogen (hydronium), nd [OH - ] is the molr concentrtion of hydroxide. Note, n equilibrium constnt less thn one (1) indictes tht the rection prefers to sty on the side of the rectnts -- in this cse, wter tends to sty s wter. (As n side, becuse wter hrdly ionizes, it is very poor conductor of electricity!) ph Wter ctully behves both like n cid nd bse. The cidity or bsicity of substnce is defined most typiclly by the ph vlue, defined s below: ph = -log[h + ] At equilibrium, the concentrtion of H + in pure wter is 10-7, therefore the ph is: ph = -log[h + ]= -log[10-7 ] = 7 Solutions with ph of 7 re sid to be neutrl, while those with ph vlues below 7 re defined s cidic nd those bove ph of 7 s being bsic. poh gives us nother wy to mesure the cidity of solution. It is just the opposite of ph, so high poh mens the solution is cidic, wheres low poh mens the solution is bsic. poh = -log[oh - ] ph + poh = The Brønsted Concept of Conjugte Acid-Bse Pirs As noted erlier, n cid is molecule tht dontes protons (hydrogen ions) nd bse is molecule tht cn ccept protons. This is generlly referred to s the Brønsted concept (fter the Dnish chemist Johnnes Nicolus Brønsted, for those who cre ). The originl cid nd resulting bse re referred to s conjugte cid-conjugte bse pir. The molecule tht ccepts the proton is different Brønsted bse; by ccepting the proton, nother Brønsted cid is produced. Thus, in every ioniztion of n cid or bse, two conjugte cid-bse pirs re involved. HA + B - A - + HB [conjugte cid] 1 [conjugte bse] 2 [conjugte bse] 1 [conjugte cid] 2 A strong cid is substnce tht undergoes lmost 100% ioniztion in queous solution. For exmple, HCl is solution is essentilly 100% ionized to H 3 O + nd Cl - :

3 HCl + H 2 O Cl - + H 3 O + By the wy, there re only 7 strong cids: HCl, HBr, HI, H 2 SO 4, HNO 3, HClO 3, HClO 4 A strong bse is substnce tht ionizes lmost completely in solution to yield OH -. Sodium nd potssium hydroxides re exmples of strong inorgnic bses. For exmple: KOH K + + OH - There re only 7 strong bses: LiOH, NOH, KOH, RbOH, C(OH) 2, Sr(OH) 2, B(OH) 2 Wek Acids nd Wek Bses Here we come to subject of extreme importnce to biologicl systems: wek cids nd bses. Its importnce comes from the fct tht the vst mjority of molecules found in biologicl systems re (or contin within their structures) wek cids nd/or bses. A wek cid is substnce tht ionizes to limited extent in queous solution: HA + H 2 O H 3 O + + A - [conjugte cid] 1 conjugte bse] 2 conjugte cid] 2 [conjugte bse] 1 The proton relesed from HA is ccepted by wter to form the hydronium ion. This reversible ioniztion rection is described by n equilibrium constnt, the K i : K i + [H3O ] = [H O] 2 Becuse the wter concentrtion is effectively constnt, we cn define new constnt, K, tht combines K i nd [H 2 O]. + [H O ] = 3 K Reclling tht [H 3 O + ] reflects the hydrogen ion concentrtion, [H + ], the K expression is usully written s: + [H ] K = eq. This is lso the reltionship tht results if we mke the (more typicl) ssumption tht the wek cid dissocites to yield H + nd A - : HA H + + A -

4 Ioniztion of wek bses follows the eqution: Wek Bse + H 2 O conjugte cid + OH - For exmple: NH 3 + H 2 O NH OH - Henderson-Hsselblch Eqution A useful expression relting K of wek cid, HA, nd the ph of solution of the wek cid rises from rerrnging eqution 1: + [ H ] = K Tking the logrithms of both sides of the eqution: + log[ H ] = log K + log Multiplying both sides by 1 results in: + log[ H ] = log K log or: ph = pk log cunning mnipultion results in: ph= pk + log This is the Henderson-Hsselblch eqution. 1 this reltionship is: A useful, conceptul form of ph = pk + [Bse] log [Acid] Note: when the concentrtion of A - equls the concentrtion of HA, i.e., when 50% of the wek cid is ionized, the ph of the resulting solution is equl to the pk (becuse the log of 1 is 0!). 1 From Wikipedi: Lwrence Joseph Henderson wrote n eqution in 1908 describing the use of crbonic cid s buffer solution. Krl Albert Hsselblch lter re-expressed tht formul in logrithmic terms, resulting in the Henderson-Hsselblch eqution.

5 Buffers A buffer is something tht resists chnge. In chemistry, ph buffer is substnce tht permits solutions to resist lrge chnges in ph following the ddition of smll mounts of H + or OH -. In other words, buffer helps mintin ner constnt ph upon ddition of smll mounts of cid or bse to solution. Common buffers consist of combintion of conjugte cid nd conjugte bse. An cidic buffer contins wek cid nd slt of the wek cid (i.e., the conjugte bse). A bsic buffer contins wek bse nd slt of the wek bse (the conjugte cid). Together, the combintion of conjugte cid nd conjugte bse prevent lrge chnges in ph by prtilly bsorbing dditions of H + or OH - ions to the solution. If H + ions re dded, they tend to rect with the conjugte bse (i.e., the slt of the wek cid) to form the conjugte cid, thereby reducing the number of free H + ions tht would otherwise be in solution (nd lowering its ph). In solution of fixed quntity, this lso results in decrese in the mount of the slt of the wek cid (i.e., the conjugte bse). Similrly, if OH - ions re dded, they tend to rect with the conjugte cid to form wter nd the conjugte bse. Importntly, buffered solutions do chnge in ph upon the ddition of H + or OH -. However, the chnge is much less tht tht which would occur if no buffer were present. The buffer cpcity is the mount of cid or bse the buffer cn neutrlize before the ph begins to chnge to n pproprite degree. High buffering cpcities reflect solutions with high (nd reltively equl) concentrtions of conjugte cid nd conjugte bse. Note: Buffers in most physiologicl solutions consist of wek cid nd the slt of the wek cid (e.g., the bicrbonte/crbonic cid system). The most importnt buffer systems in the body re: (i) the bicrbonte/crbonic cid system; (ii) orgnic phosphtes; nd (iii) proteins. The HCO 3 - /H 2 CO 3 system is the most importnt extrcellulr buffer, while orgnic phosphtes nd proteins re more importnt s intrcellulr buffers. References Chpter 2. Wter & ph. In: Hrper's Illustrted Biochemistry, 26th Edition, Eds., Robert K. Murry, Dryl K. Grnner, Peter A. Myes, nd Victor W. Rodwell (McGrw-Hill s Online Access Medicine) UNC-Chpel Hill Chemistry Fundmentls Progrm, Acid-Bse Chemistry ( Segel, I.H. Enzyme Kinetics, 2 nd Ed. Wiley, New York. 441 pp (Chpter 1, Acid- Bse Chemistry