Titration of an Amino Acid

Prepare 50ml O.IM solution of each compound to be titrated (H3PO4, cysteine, histidine, glycine, and arginine). 

Place 20 ml water, 20ml O.IM solution of the substance to be titrated, and a small stirring bar in a 250 ml beaker. 

Turn the adjustable control of the stirring motor to its lowest position. 

Slowly increase the speed of the motor until a small vortex appears at the surface of the solution. 

Wait 5 seconds for the resulting solution to become homogeneous before measuring and recording the pH. The pH should be recorded to the nearest 0.05 pH unit. 

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Figure 2.21. Results of 96 titrations of an amino acid. (See Table 2.17.)...

FIGURE 3-10 Titration of an amino acid. Shown here is the titration... 

A student conducted the titration of an amino acid with a strong base. The point in the titration when 50% of the amino acid exists as a zwitterion is called ... 

To determine the true titration curve of any substance, you must measure how much acid or base is consumed in titrating the solvent (water) to each pH and then subtract this amount from the total amount of acid or base consumed in reaching that pH when titrating the sample (amino acid + water). The following example with the acid side of the titration of an amino acid illustrates the method for correcting for such acid or base dilution that you should use in correcting your data. 

Figure 1-12. Assembly of a pH meter electrode and stirring apparatus used to measure pH during the manual titration of an amino acid.

Titration of an amino acid can be used to determine both the pKa of its ionizable groups and its isoelectric point. (18.3)... 

Example 36 Four batches. A, B, C, and D, of an amino acid hydrochloride were investigated four different titration techniques were applied to every sample ... 

C18-0142. The amine group of an amino acid readily accepts a proton, and the protonated form of an amino acid can be viewed as a diprotic acid. The p Zg values for serine (H2 NCHRCO2 H, i = CH2 OH) are p ra(H3 N"") =9.1 and p (002 H) - 2.2. (a) What is the chemical formula of the species that forms when serine dissolves in pure water (b) If this species is titrated with strong acid, what reaction occurs (c) 10.00 mL of 1.00 M HCl is added to 200. mL of 0.0500 M serine solution. This mixture is then titrated with 0.500 M NaOH. Draw the titration curve, indicating the pH at various stages of this titration. 

Dissociation of the carboxyl group The titration curve of an amino acid can be analyzed in the same way as described for acetic acid. Consider alanine, for example, which contains both an a-carboxyl and an a-amino group. At a low (acidic) pH, both of these groups... 

Application of the Henderson-Hasselbalch equation The dissociation constant of the carboxyl group of an amino acid is called K1f rather than Ka, because the molecule contains a second titrat-able group. The Henderson-Hasselbalch equation can be used to analyze the dissociation of the carboxyl group of alanine in the same way as described for acetic acid. 

You will obtain a titration curve of an amino acid with a neutral side chain such as glycine, alanine, phenylalanine, leucine, or valine. If pH meters are available, you read the pH directly from the instrument after each addition of the base. If a pH meter is not available, you can obtain the pH with the aid of indicator papers. From the titration curve obtained, you can determine the pK values and the isoelectric point. 

In titrating the acidic form of an amino acid with NaOH solution, at which point in the titration curve does it become a zwitterion ... 

An alternative, simpler, procedure for improving the inflexion in the neutralization of an amino-acid is to add formaldehyde to the solution although this does not affect the acid-titration curve, the one for alkaline titration is changed, as seen in Fig. 107. The effect of the formaldehyde is to increase the strength of the ammonium ion acid which is being titrated, and so the pH inflexion at the equivalence-point becomes much more obvious. This is the basis of the formol titration of amino-acids discovered by Sorensen (1907) approximately 10 per cent of formaldehyde is added to the solution which is then titrated with standard alkali using phenolphthalein as indicator. In the presence of thii concentration of formaldehyde the pH-neutralization curve has a sharp inflexion in the region of pH 9, and so a satisfactory end-point is possible with the aforementioned indicator. 

Figure 3.62. Titration of the a-Carboxyl and a-Amino Groups of an Amino Acid.

When the zwitterion of an amino acid is titrated with strong acid, a buffer region is first established, consisting of the zwitterion (the salt ) and the conjugate acid. Halfway to the equivalence point, pH = Kai (just as with HC03 /... 

FICURE 3.62 Titration of the ct-carboxyl and ct-amino groups of an amino acid. 

Titration curves such as that for glycine permit us to determine p values for the ionizable groups of an amino acid. They also permit us to determine another important property the isoelectric point, pi—the pH at which most of the molecules of the amino acid in solution have a net charge of zero. (They are zwitterions.) By examining the titration curve, you can see that the isoelectric point for glycine falls halfway between the p Ta values for the carboxyl and amino groups ... 

Method. An amino-acid such as glycine, NHjCH,COOH, cannot be estimated by direct titration with standard alkali solution, owing to the opposing effects of the basic and the acidic groups. If, however, the amino-acid is first... 

There is also evidence for stable 3,4-adducts from the X-ray analysis of 2-amino-4-ethoxy-3,4-dihydropteridinium bromide, the nucleophilic addition product of 2-aminopteridine hydrobromide and ethanol (69JCS(B)489). The pH values obtained by potentiometric titration of (16) with acid and back-titration with alkali produces a hysteresis loop, indicating an equilibrium between various molecular species such as the anhydrous neutral form and the predominantly hydrated cation. Table 1 illustrates more aspects of this anomaly. 2-Aminop-teridine, paradoxically, is a stronger base than any of its methyl derivatives each dimethyl derivative is a weaker base than either of its parent monomethyl derivatives. Thus the base strengths decrease in the order in which they are expected to increase, with only the 2-amino-4,6,7-trimethylpteridine out of order, being more basic than the 4,7-dimethyl derivative. 

Typical values for pAlg are in the range of 9.0 to 9.8. At physiological pH, the a-carboxyl group of a simple amino acid (with no ionizable side chains) is completely dissociated, whereas the a-amino group has not really begun its dissociation. The titration curve for such an amino acid is shown in Figure 4.7. 

C18-0103. When a solution of leucine (an amino acid) in water is titrated with strong base, the pH before titration is 1.85, the pH at the midpoint of the titration is 2.36, and the pH at the stoichiometric point is 6.00. Determine the value of K. for leucine. 

It had been found previously [106] that cell adhesion on the surface of HA copolymers decreased with increasing ambient pH. With polyHEMA, on the other hand, no pH-dependency was observed. We considered the adhesion to be caused primarily by ionic interaction between the lymphocytes and the HA surfaces. The degree of protonation (a) of amino groups in HA copolymer was estimated by acid-base titration of an HCI solution of polyamine macromonomer with NaOH solution. In physiological conditions (pH 7.2-7.4) about 50% of the amino groups of the macromonomer are protonated. In this pH range, the polyamine macromonomer was found to be insoluble. 

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