Hi y'all. One nice cheat is to get a groovy little web server to do the work for you:
http://www.liv.ac.uk/buffers/ Enter your requirements and you'll get a nice little recipe given back. Dave 2008/7/22 Nadir T. Mrabet <[EMAIL PROTECTED]>: > I bet it is more difficult to adjust a pH-meter than to use the > Henderson-Hasselbalch equation > and still get the expected pH with a pretty good accuracy especially if your > work near the pKa. > > There are actually two ways to prepare this 25 mM buffer, pH 4.5. > > The pKa of acetate is 4.76 at 25 °C (with dpKa/° C = +0.0002, so don't worry > too much about this). > Reference is "Buffers for pH and Metal Ion Control", Perrin & Dempsey, > Chapman & Hall, NY, ISBN 0 412 21890 9. > > High-grade glacial acetic acid (99-100%) is 18 N. > Make a stock solution of 250 mM (eg 3.472 mL for 1.0 L final). Keep is a > dark, tightly closed bottle. > > Make a stock solution of 250 mM sodium acetate (if you use FW, not MW, to > calculate mass to use, then no worry about anhydrous or not since water is > also taken into account if present) > > or > > make a stock solution of 5N NaOH. Keep is a dark, tightly closed bottle. > > Use then the Henderson-Hasselbalch equation (HH), pH = pKa + log > ([A-]/[AH]). > > In the first case, you write it : 4.5 = 4.76 + log ([sodium acetate]/[acetic > acid]) > Second equation is [sodium acetate] + [acetic acid] = 25 mM > which gives [sodium acetate] = 8.886 mM and [acetic acid] = 16.134 mM. > For 1.0 L buffer, mix adequate volumes of stock solutions of sodium acetate > and acetic acid and complete with water (add acid after un first fill with > water to ~ 800 mL). > > In the second case, the HH is written 4.5 = 4.76 + log([NaOH])/(25 - > [NaOH]), > which gives [NaOH] = 8.886 mM (same result as above for sodium acetate which > was then the base). > > The added advantage of using HH and stock solutions is that even if your pH > is not exactly 4.5, say 4.55, if you make a new buffer the next day or even > the next month, > your buffer will have the same pH value. I don't expect you can ever achieve > such a repeatability using a pH-meter. > > HTH, > > Nadir Mrabet > > -- > > Pr. Nadir T. Mrabet > Cellular & Molecular Biochemistry > INSERM U-724 > Nancy University, School of Medicine > 9, Avenue de la Foret de Haye, BP 184 > 54505 Vandoeuvre-les-Nancy Cedex > France > Phone: +33 (0)3.83.68.32.73 > Fax: +33 (0)3.83.68.32.79 > E-mail: [EMAIL PROTECTED] > > > > > William G. Scott wrote: >> >> So what, then, will be the concentration of the acetate ion in your stock >> solution when you have finished? >> >> (Disclaimer: I get to teach this stuff periodically in remedial chemistry >> as a punishment for deployment of excessive sarcasm during faculty >> meetings.) >> >> On Jul 22, 2008, at 6:10 AM, Santosh wrote: >> >>> Hi, >>> Make a 1M Na-Acetate do not make up to the 1 Ltr volume. Leave some >>> extra >>> volume and now start adding Acetic acid till you get pH 4.5 (Glacial >>> Acetic >>> Acid). >>> Now make up the volume to 1ltr or how much ever you are deciding to make >>> the >>> 50X stock solution. >>> Best, >>> Santosh >>> >>> On Mon, Jul 21, 2008 at 11:20 PM, William G. Scott < >>> [EMAIL PROTECTED]> wrote: >>> >>>> This is a job for the trusty Henderson-Hasselbalch equation: >>>> >>>> http://en.wikipedia.org/wiki/Henderson-Hasselbalch_equation >>>> >>>> >>>> >>>> On Jul 21, 2008, at 8:12 PM, Meg wrote: >>>> >>>> Dear All, >>>>> >>>>> I want to prepare 25 mM sodium acetate buffer pH 4.5. can anyone give >>>>> the >>>>> exact composition of how to prepare it. we prepare it using sodium >>>>> acetate >>>>> and acetic acid combination. i am not able to arrive at the >>>>> calculatation >>>>> correctly, so if anyone can explain me with the above buffer how to >>>>> calculate. and what sodium acetate [Anhydrous / trihydrate] and acetic >>>>> acid >>>>> [glacial/ plain] to use. >>>>> >>>>> thanks n regards >>>>> >>>>> Meg goyal, >>>>> M.SC Biotechnology [Research] >>>>> Institute of science, >>>>> Fort >>>>> Mumbai, INDIA >>>>> >>>> >> >> > -- ============================ David C. Briggs PhD Father & Crystallographer http://www.dbriggs.talktalk.net AIM ID: dbassophile ============================
