Ama-buffer: indlela i-asidi ebuthaka nesisekelo sayo se-conjugate ezigcina ngayo i-pH izinzile

Abalingani ababili ngamunye ovimba usongo olulodwa

Thatha ingxube ye-acetic acid, CH₃COOH, ne-sodium acetate, enikeza i-CH₃COO⁻. Uma kwengezwa i-asidi eqinile, i-ayoni ye-acetate ithatha ama-proton angeziwe: CH₃COO⁻ + H₃O⁺ → CH₃COOH + H₂O. Uma kwengezwa isisekelo esiqinile, i-acetic acid iyasenza singathathi hlangothi: CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O.

Kokubili ukusabela kucishe kuphele ngokuphelele, ngakho i-asidi eqinile noma isisekelo esiqinile esengeziwe siguqulwa sibe ngumlingani obuthaka owengeziwe. I-pH isahamba, ngoba isilinganiso se-asidi nesisekelo siyashintsha, kodwa kancane kuphela. Isixazululo se-asidi ebuthaka sisodwa asikwazi ukwenza lokhu, ngoba cishe asinaso isisekelo se-conjugate sokumunca i-asidi eyengeziwe.

Amabhangqa e-buffer avamile ahlanganisa i-acetic acid ne-acetate, i-ammonium ne-ammonia, i-dihydrogen phosphate ne-hydrogen phosphate, ne-carbonic acid ne-hydrogen carbonate. Ibhangqa lokugcina lisiza ukugcina igazi lomuntu eduze ne-pH 7.4, njengoba isigaba 14.6 se-OpenStax sichaza.

Ukuthola i-Henderson–Hasselbalch esishweni sika-Ka

Qala ku-Ka = [H₃O⁺][A⁻]/[HA] bese uxazululela i-hydronium: [H₃O⁺] = Ka × [HA]/[A⁻]. Thatha u-−log wezinhlangothi zombili ukuze uthole pH = pKa + log([A⁻]/[HA]). Lesi yisibalo se-Henderson–Hasselbalch.

Ku-buffer, kokubili u-HA no-A⁻ bakhona ngamanani amakhulu, futhi ngamunye ucindezela ukusabela komunye namanzi. Ngakho ukugxila kwabo kokulingana kucishe kulingane namanani owaxubile. Yingakho ungafaka ukugxila okulungisiwe ngqo esibalweni ngaphandle kwetafula le-ICE.

Ngoba bobabili abalingani bahlala kuvolumu efanayo, isilinganiso sokugxila silingana nesilinganiso sama-mole. Ungasebenzisa ama-mole ngqo, okonga isinyathelo ngemva kokwengeza i-asidi noma isisekelo.

• Lapho [A⁻] = [HA], ingxenye ye-log iyi-zero futhi pH = pKa.
• Ukukhuphuka ngakunye okuphindwe kayishumi esilinganisweni sesisekelo ne-asidi kukhuphula i-pH ngeyunithi eyodwa.
• Isisekelo se-conjugate esiningi kune-asidi sinikeza i-pH engaphezu kuka-pKa; i-asidi eningi inikeza i-pH engaphansi kwayo.

Isibonelo esisebenzile: i-pH ye-buffer ye-acetate

I-buffer iqukethe i-CH₃COOH engu-0.300 M ne-CH₃COONa engu-0.250 M. USithasiselo H unikeza u-Ka = 1.8 × 10⁻⁵, ngakho pKa = 4.745. Bese pH = 4.745 + log(0.250/0.300) = 4.745 − 0.079 = 4.67.

Phatha u-pKa ongayisiwe kuze kube sekugcineni. Ukumyisa kuqala ku-4.74 kunikeza u-4.66. Umehluko ungaphakathi kokungaqiniseki kuka-Ka onezinombolo ezibalulekile ezimbili, kodwa ukuyisa kanye ekugcineni kugcina izimpendulo zakho zivumelana nesixhumanisi sezimpendulo. I-pH ingaphansi kancane kuka-pKa, njengoba bekulindelekile, ngoba kune-asidi eningi kancane kunesisekelo.

Isibonelo esisebenzile: ukwengeza i-asidi eqinile, bese isisekelo esiqinile

Engeza u-0.0200 mol we-HCl ku-1.00 L we-buffer engenhla. Yenza ukwenza kungathathi hlangothi kuqala, njengesinyathelo se-stoichiometry: i-H₃O⁺ iguqula u-0.0200 mol we-acetate ube yi-acetic acid. I-acetic acid ikhuphuka isuka ku-0.300 iye ku-0.320 mol kanti i-acetate yehla isuka ku-0.250 iye ku-0.230 mol. Bese pH = 4.745 + log(0.230/0.320) = 4.60. I-pH yehla isuka ku-4.67 yaya ku-4.60, ngaphansi kweshumi leyunithi.

Ukwengeza u-0.0200 mol we-HCl ofanayo ku-1.00 L wamanzi ahlanzekile bekuzonikeza u-[H₃O⁺] = 0.0200 M ne-pH engu-1.70, ukwehla kwamayunithi angaphezu kwamahlanu kusuka ekungathathini hlangothi.

Manje engeza u-0.0500 mol we-NaOH ku-1.00 L omusha we-buffer yokuqala. I-hydroxide iguqula u-0.0500 mol we-acetic acid ube yi-acetate, kusale u-0.250 mol we-asidi no-0.300 mol wesisekelo. pH = 4.745 + log(0.300/0.250) = 4.82, ukukhuphuka kusuka ku-4.67 okungaphansi kuka-0.2. Emanzini ahlanzekile i-NaOH efanayo ibizonikeza i-pH 12.70.

Umqondo ofanayo ukuvumela ukuthi wenze i-buffer ngokwenza kungathathi hlangothi ngokwengxenye. Ukuxuba u-0.0400 mol we-acetic acid no-0.0150 mol we-NaOH kushiya u-0.0250 mol we-asidi no-0.0150 mol we-acetate, ngakho pH = 4.745 + log(0.0150/0.0250) = 4.52, noma ngabe ivolumu yokugcina ingakanani.

Ukuklama i-buffer ye-pH ehlosiwe

Okokuqala khetha ibhangqa le-conjugate u-pKa walo ongaphakathi kweyunithi eyodwa ye-pH oyifunayo. Bese usebenzisa i-Henderson–Hasselbalch uhlehla ukuze uthole isilinganiso, ne-stoichiometry ukuguqula isilinganiso sibe amanani.

Isibonelo esisebenzile. Lungisa i-buffer ye-pH 9.50 kusuka ku-500.0 mL we-ammonia engu-0.200 M ngokwengeza i-ammonium chloride eqinile. Umlingani we-asidi yi-NH₄⁺. Kusuka kuSithasiselo I, Kb(NH₃) = 1.8 × 10⁻⁵, ngakho pKb = 4.745 no-pKa(NH₄⁺) = 14.00 − 4.745 = 9.255. Isilinganiso esidingekayo singu-[NH₃]/[NH₄⁺] = 10 ephakanyiswe ku-(9.50 − 9.255) = 1.76.

Ifulaski iphethe u-0.5000 L × 0.200 mol/L = 0.100 mol we-NH₃, ngakho idinga u-0.100 / 1.76 = 0.0569 mol we-NH₄⁺. Ngo-M(NH₄Cl) = 53.49 g/mol, lokho kucishe kube ngu-3.0 g we-ammonium chloride, kubikwe ezinombolweni ezibalulekile ezimbili ngoba u-Kb unezimbili. Ngokwenza, ubuzobe usuhlola i-pH ngemitha elungisiwe bese uyalungisa.

Umthamo we-buffer nebanga le-pH eliwusizo

Ibanga le-buffer yibanga le-pH lapho i-buffer isebenza kahle khona. Isilinganiso esingu-10:1 noma 1:10 sihambisana no-pH = pKa ± 1, futhi ngaphandle kwalelo banga omunye umlingani muncane kakhulu ukuthi angamunca i-asidi noma isisekelo esiningi. Ama-buffer e-acetate awusizo kusuka cishe ku-pH 3.7 kuya ku-5.7, kanti ama-buffer e-ammonia kusuka cishe ku-8.3 kuya ku-10.3.

Umthamo we-buffer yinani le-asidi noma lesisekelo i-buffer engalimunca ngaphambi kokuthi i-pH yayo ishintshe kakhulu. Uncike ekutheni mngakanani umlingani ngamunye okhona, hhayi nje isilinganiso sabo. Qhathanisa ama-buffer amabili e-acetate, ngayinye ingu-1.00 L ne-asidi nesisekelo esilinganayo. Zombili ziqala ku-pH = pKa = 4.74. Ngo-0.100 M wakho kokubili, ukwengeza u-0.00500 mol we-HCl kwehlisa i-pH ngo-0.04, kuye ku-4.70. Ngo-0.0100 M wakho kokubili, ukwengeza okufanayo kuyehlisa cishe ngengxenye yeyunithi, kuye ku-4.27. Isilinganiso besifana, kodwa i-buffer ehlanjululiwe ibinengxenye eyodwa kweziyishumi yomthamo.

I-buffer iyaphela lapho i-asidi noma isisekelo esengeziwe sisebenzisa wonke omunye wabalingani. Ngemva kwalokho, i-pH inqunywa yi-asidi eqinile noma isisekelo esiqinile esingaphezulu, futhi ishintsha kakhulu.

Amaphutha enza impendulo ye-buffer ingalungi

Izinkinga ze-buffer zixuba izinhlobo ezimbili zokucabanga: i-stoichiometry yokwenza kungathathi hlangothi nokulingana kwe-pH. Zigcine ngalokho kulandelana.

• Ukubhala isilinganiso sihlanekezelwe. Yisisekelo phezu kwe-asidi: log([A⁻]/[HA]).
• Ukufaka i-HCl noma i-NaOH eyengeziwe esibalweni se-Henderson–Hasselbalch ngaphambi kokwenza isinyathelo sokwenza kungathathi hlangothi.
• Ukusebenzisa u-pKb esikhundleni sika-pKa ku-buffer ye-ammonia. Guqula ngo-pKa + pKb = 14.00 ku-25 °C.
• Ukuxuba ama-mole omlingani oyedwa nokugxila komunye lapho besemavolumu ahlukene.
• Ukucabanga ukuthi i-buffer isasebenza ngemva kokuba i-asidi noma isisekelo esengeziwe sidlule inani lomunye wabalingani.
• Ukuthatha i-asidi ebuthaka iyodwa, noma i-asidi ebuthaka ne-asidi eqinile, njenge-buffer.