Experiment 9 22 March 2018 Designer Buffers

Transcription

1 Experiment 9 22 March 2018 Designer Buffers h"p://cellgro.com/products/microbiology- and- molecular- biology/ buffers- buffers/phosphate- buffered- saline- 1x.html Welcome to the World of Designer Buffers where specialty buffers for research and medical applica;ons are sold for big money. I m dressed for big money!

2 Objec>ve: To prepare a buffer of a specific assigned ph using Recipe 2 or 3. Last week we used Recipe 1 to make a buffer of assigned ph. This week we will use either Recipe 2 or 3 to make a specific buffer. Overview: As it turns out, Recipes 2 and 3 are far more commonly used than Recipe 1 when making buffers. 1. Review making buffers and the Henderson Hasselbalch Buffer Equa>on 2. Choosing the best weak acid + conjugate base pair 3. The math for Recipe 2: Star>ng with weak acid and adding hydroxide to make the buffer 4. The math for Recipe 3: wb + sa 5. What we are doing in lab today 6. Your lab report When do we get to the big money part? 2

3 1. Review making buffers and the Henderson Hasselbalch Buffer Equa>on There are three recipes for making buffers as you recall. This week we will be using either Recipe 2 or Recipe 3. We can t use Recipe 1 unless we have both the weak acid and its conjugate base. The 3 recipes for making a buffer: So how does this work, anyway? 1. Mix a weak acid with its conjugate weak base 2. React a weak acid with OH - as the limi>ng reagent 3. React a weak base with H 3 O + as the limi>ng reagent 3

4 1. Review making buffers and the Henderson Hasselbalch Buffer Equa>on n ph = pk a + log( wb ) n wa So here is Henderson again the equa;on we use whenever we have a buffer situa;on. With Recipes 2 and 3, we must work in moles. With Recipe 1, we could use moles or molarity, even though working in moles is always beter. Go Moles! n = MV Info for IntroducFon 4

5 2. Choosing the best weak acid + conjugate base pair. n ph = pk a + log( wb ) n wa So the strategy is to pick a weak acid with a pk a similar to the desired ph HA pk a = 4.75 HB pk a = 5.15 HC pk a = 6.05 Info for IntroducFon HD pk a = 7.45 Even if we end up star;ng with a weak base (Recipe 3), we Last week, in the example we shop for the weak acid worked in the pre- lab material, with a pk a closest to we picked HC because we were assigned a ph = the desired ph. 5

6 2. Choosing the best weak acid + conjugate base pair. But this week, we will all be working with a buffer that consists of H 2 PO 4- (aq) as the weak acid and HPO 4 2- (aq) as the conjugate weak base. Some groups will start with the weak acid, H 2 PO 4-, in the form of NaH 2 PO 4 (s), MM = g/mol and do Recipe 2. Info for IntroducFon The weak acid H 2 PO 4- (aq) has a pk a = This number has 3 significant figures the 198 part. The 7 represents an exponent. The corresponding K a is 6.38 x We can only make buffers with ph values around 7 with this wa + wb combo. While other groups will start with the weak base, HPO 4 2-, in the form of Na 2 HPO 4. 7H 2 O(s), MM = g/mol and do Recipe 3. 6

7 3. The math for Recipe 2: Star>ng with weak acid and adding hydroxide to make the buffer Henderson for Recipe 2: So why is it that we have n OH - in the top and botom? ph = pk a + log( 0 mol wb + n OH - ) n wa - n OH - Info for IntroducFon Recipe 2 is used if you have the weak acid available, but not its conjugate weak base. This is the reac;on that takes place in Recipe 2: HA + OH - à H 2 O + A - We can make the weak base from the weak acid by adding strong base. Hydroxide will convert some moles of the weak acid into weak base as shown in the equa;on in the white box. 7

8 3. The math for Recipe 2: Star>ng with weak acid and adding hydroxide to make the buffer The Henderson for Recipe 2 can be simplified to this. 0 mol wb + n ph = pk a + log( OH - ) n wa - n OH - n ph = pk a + log( OH - ) n wa - n OH - Info for IntroducFon Just like last week, we are supposed to make 50 ml of a M buffer. This is taken to mean that we want the molarity of wa plus wb to add up to M. n = MV n = mol wa + wb 8

9 3. The math for Recipe 2: Star>ng with weak acid and adding hydroxide to make the buffer We subs;tute in n wa = mol because we are making our buffer from weak acid. Suppose the assigned buffer was to have a ph = The rest is math. n ph = pk a + log( OH - ) n wa - n OH - n ph = pk a + log( OH - ) mol - n OH - n 7.00 = log( OH - ) mol - n OH - Info for IntroducFon n = log( OH - ) mol - n OH - 9

10 3. The math for Recipe 2: Star>ng with weak acid and adding hydroxide to make the buffer Remember with log numbers, all places aler the decimal are significant figures, so has three sig figs. n = log( OH - ) mol - n OH = n OH mol - n OH - n OH - = mol OH - Math headache!!! x x Make sure you can get this number for moles of hydroxide 10

11 3. The math for Recipe 2: Star>ng with weak acid and adding hydroxide to make the buffer Aler we ve added moles hydroxide, we will have that many moles of weak base and the rest is unreacted weak acid. They add up to mol total. There will be no NaOH lel it s the LR! n A - = mol A - n HA = mol HA n total = mol Woo- hoo! We get to use the Mohr pipet! The NaOH is 1.0 M. Using n = MV, we calculate that we need to add 1.94 ml of 1.0 M NaOH. n OH - = MV mol = 1.0 mol L - 1 x V V = L = 1.94 ml 11

12 4. The math for Recipe 3: Star>ng with weak base and adding strong acid to make the buffer With Recipe 3 we start out with the weak base and make the conjugate weak acid. Analogous to Recipe 2, we will start out with mol of weak base. (In Recipe 2 we started out with mol of wa.) Henderson for Recipe 3 n wb - n H O + ph = pk a + log( 3 ) 0 mol wa + n H O + 3 Info for IntroducFon This is the reac;on that takes place in Recipe 3: A - + H 3 O + à H 2 O + HA 12

13 4. The math for Recipe 3: Star>ng with weak base and adding strong acid to make the buffer n ph = pk a + log( wb - n H O + 3 ) n H O + 3 And here is Henderson for Recipe mol wb - n H O + ph = pk 3 a + log( ) n H O mol wb - n H O = log( 3 ) n H O + 3 Info for IntroducFon See if you can solve for moles of strong acid. I got mol. And I m just a bird. 13

14 4. The math for Recipe 3: Star>ng with weak base and adding strong acid to make the buffer Aler we ve added moles HCl, we will have that many moles of weak acid and the rest is unreacted weak base. They add up to mol total. There will be no HCl lel it s the LR! n A - = mol n HA = mol n total = mol This is where I say Woo- hoo! We get to use the Mohr pipet! The HCl(aq) is 1.0 M. Using n = MV, we calculate that we need to add 3.06 ml of 1.0 M HCl. n H O + = mol HCl mol HCl = 1.0 mol L - 1 x V V = L = 3.06 ml HCl 14

15 5. What we are doing in lab today. At your sta;on, you will have either NaH 2 PO 4 (s), MM = g/mol and do Recipe 2, or you will have Na 2 HPO 4. 7H 2 O(s), MM = g/mol and do Recipe 3. With Recipe 3, we start out with mol of the weak base, Na 2 HPO 4. 7H 2 O(s), From the MM = g/mol we can calculate what mass we need. With Recipe 2, we start out with mol of the weak acid, NaH 2 PO 4 (s). From the MM = g/mol we can calculate what mass we need. You should get g You should get g Then just follow the calcula;ons provided here for your recipe. Make sure you understand everything. 15

16 5. What we are doing in lab today. The buffer probably won t work out like the math it ll be close. We can make adjustments quite easily, however. Remember this slide from last week? If the ph is too high, we can add drops of 1 M HCl(aq) un;l it s perfect. Hmmm What should I do if the ph is too low? Info for IntroducFon 16

17 5. What we are doing in lab today. 1 Wear your safety glasses. Dress for a mess. 2 Take ;me wri;ng your introduc;on in your own words. 3 Carefully record observa;ons and details. 4 Use the pocket balances at your sta;on. They are good to the hundredths place. 5 Make sure you are using the Mohr pipet correctly. 6 When your buffer is ready, ask your TA to check it for accuracy. She/he will ini;al the cover sheet if it s ok. Then clean up and turn in your lab report! 7 The cover sheet summarizes everything that you need to include with your report. 8 Be nice to your ph probe. Stuff to do today. 17

18 6. Your Lab Report 1 First, the cover page with TA ini>als. 2 Next, the trimmed copy pages from your lab notebook stapled together. 3 Staple together. There are no printed pages. 4 On- line results due at the end of class today. Late submissions are not graded see the syllabus. 5 Turn in lab report today. S>ck people inspired by xkcd cartoons by Randall Munroe ( Oh, one last thing before we turn out the lights Did you no;ce that both Recipes 2 and 3 result in making a buffer with mol HA and mol A -? (Check Slides 11 and 14.) PreTy cool, huh? 18

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20 Look Mom! I laid some eggs! Happy Easter! See you in two weeks!