Titration of weak acids Titration of amino acids Amino acids are weak polyprotic acids . Neutral amino acids are (gly, ala, threonine ) are treated as diprotic acids . acidic amino acids (glu, asp,) are treated as triprotic acids . Basic amino acids (lys , arg , his ) are treated as triprotic acids . pHm is the pH at which the maximum total number of charges present. Glycine can be obtained in three forms : a) Glycine hydrochloride c)Sodium glycinate
b) Isoelectric glycine Titration of amino acids Example: Calculate the pH of a 0.1M solution of a) Glycine hydrochloride , b) Isoelectric glycine , c)Sodium glycinate Ka1 = 4.57 x 10-3
a) Glycine hydrochloride is a diprotic acid , the carboxylic group is a much stronger acid than the amino group , the pH of the solution is dependent exclusively by the extent the carboxyl group ionizes . , assume y = [ AA+1] that ionizes . Thus, y = [H+] produced, and y = [AA] produced . AA+1 remaining at equilibrium = [ AA+1] original - [ AA+1] that ionizes = 0.1 y Thus, =4.57 x 10-3
Titration of amino acids Since amount of y (value of y) is stronger than that can be ignored , Thus 4.57 x 10-4 - 4.57 x 10-3 y = y y 2 2 + 4.57 x 10-3 y - 4.57 x 10-4 = 0.0
Y , where a = 1 , b = 4.57 x 10-3 , c = 4.57 x 10-4 Thus y = [H+ ] = 1.92 x 10-2 . pH = 1.72 . What is the degree of ionization of glycine hydrochloride in this 0.1M solution ? b) The pH of AA is the pI ( the pI is defined as that pH where the predominant ionic form is AA , so net charge on the amivo acid is zero .) So pH = = 5.97 c)Sodium glycinate is a diprotic base ,
Both the unionized amino group and the carboxylate ion can accept a proton from water , but since the amino group is a much stronger base than the carboxylate group , the pH of the solution depends almost exclusively on the extent to which amino group ionizes . Titration of amino acids For the amino group : Kb1 =x10-5 For carboxylate group ; Kb2 =x10-12 Kb1 =
[OH- ] [AA ] = (y) (y) Kb1 = Because the concentration of sodium glycinate is much larger than K b1 thus y can be neglected from the dominator . Titration of amino acids
3.98 x10 5 = , so y2 = 3.98 x 10 6 y= y = 1.99 x 10 3 M , [OH ] = 1.99 x 10 3 M [H+] = 5 x 10 12 M pH = -log[H+ ] = 11.3 . Titration Curve of Neutral Amino Acids Amino acids are titrated in exactly the same manner as diprotic and triprotic weak acids . Neutral amino acid
Titration Curve of Acidic amino Acid (Glutamate ) Titration Curves of Lysine Titration Curves of Amino Acids Information that can be obtained from a titration curve : 1- The number of ionizable groups in that amino acid , which can be detected from the number of titration stages in the curve , (or the number of pK a s or number of flat zones in the curve) . 2-Whether the triprotic amino acid is basic or acidic , that can be detected from the pKa 2 . If its value is closer to the value of pKa 1 (that of the carboxyl group ), then it is an acidic amino acid .
If the value of its pKa2 is closer to the value of pKa 3 (that of the aminogroup ), then it is basic amino acid . 3- The pKa values of the amino acid can be obtained from the curve which is equal to the pH value at the mid-point . 4- The isoelectric point , pI for each amino acid can be obtained from the curve by detecting the point where the amino acid is all in the zwitterion form (net charge = 0.0) the pH at that point is the pI . Or it can be obtained mathematically from ; pI = pKa1 + pKa2 ( in the case of a neutral amino acid ) . 2 In the case of triprotic amino acids , the pI is calculated from :
pI = pKa1 + pKa2 2 ( in the case of acidic amino acids ) . Titration Curves of Amino Acids pI = pKa2 + pKa3 in the case of basic amino acids. 2 5- You can also determine from the curve the pH values at which the amino acid can act as a
buffer . (the pH ranges 1 from the pH value of each midpoint). How to Obtain a Titration Curves of Amino Acids No of moles of weak acid or a.a Calculate the first moles of OH by A= Calculate the second moles of OH added B= No of moles of weak acid or a.a + A Calculate the third moles of OH added C= No of moles of weak acid or a.a + B
How to Obtain a Titration Curves of Amino Acids Sketch the pH curve for the titration of 100ml of 0.1M Glycine with KOH? Pka1=1.71, Pka2=9.6? No of moles of a.a = M*V = 0.1 * 0.1 =0.01 mole The first moles of OH by A= 0.01 / 1.71 = 0.005 The second moles of OH added B= 0.01 + 0.005 = 0.015 PI = (Pka1 + pka2)/ 2 =5.66 Plot the titration curve of Aspartic acid it has a volume of
100ml and0.1M When titrated with 0.1M KOH? Pka1=2.09, Pka2=3.86, Pka3=9.82? Plot the titration curve of Lysine which has a volume of 200ml and 0.3M When titrated with 0.1M NaOH? ? Pka1=2.18, Pka2=8.95, Pka3=10.35?
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