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Buffer solutions .

Buffer solutions. ass. prof. I. R. Bekus. Plan Ionization of water . Acid-base theory . Buffer solutions. Buffer in blood. Water is а neutral molecule with а slight tendency to ionize. We usually express this ionization as: Н 2 О = Н + + ОН -.

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Buffer solutions .

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  1. Buffer solutions. ass. prof. I. R. Bekus

  2. Plan • Ionization of water. • Acid-base theory. • Buffer solutions. • Buffer in blood.

  3. Water is а neutral molecule with а slight tendency to ionize. We usually express this ionization as: Н2О = Н+ + ОН-

  4. There is actually no such thing as а free proton (Н+) in solution. Rather, the proton is associated with а water molecule as а hydronium ion, H3O+. The association of а proton with а cluster of water molecules also gives rise to structures with the formulas Н5О2+, Н7О3+, and so on. For simplicity, however, we collectively represent these ions by H+.

  5. Because the product of [Н+] and [ОН-] is а constant (10-14 М2), [Н+] and [ОН-] are reciprocally related. Solutions with relatively more Н+ are acidic (рН < 7), solutions with relatively more ОН- are basic (рН >7), and solutions in which [Н+] = [ОН-] = 10 -7 М are neutral (рН = 7). Note the logarithmic scale for ion concentration. K is the dissociation constant (ionization constant) Кw = [Н+][ОН-] =10 -14 M2 at 25 0C. [Н+] = [ОН-] = (Кw)1/2 = 10-7 М [Н+] = 10-7 М are said to be neutral [Н+] > 10-7 М are said to be acidic, [Н+] < 10-7 М are said to be basic. Most physiological solutions have hydrogen ion concentrations near neutrality.

  6. рН = - log[H+] The pH of pure water is 7.0, Acidic solutions have рН < 7.0 Basic solutions have рН > 7.0. 1 М NaOH -14 Household ammonia -12 Seawater – 8 Milk - 7 Blood -7.4 Saliva - 6.6 Tomato juice - 4.4 Vinegar - 3 Gastric juice - 1.5 1 М НСl - 0

  7. The relationship between the pH of а solution and the concentrations of an acid and its conjugate base is easily derived. [НА] [Н+]= K ---------- [А-] Taking the negative log of each term [А-] рН = - log К + log --------- [А-] This relationship known as the Henderson-Hasselbalch equation, that is often used to perform the calculations required in preparation of buffers for use in the laboratory, or other applications.

  8. BUFFER SOLUTIONS Buffers are solutions which can resist changes in pH by addition of acid or alkali.

  9. Buffers are mainly classified of two types: (а) mixtures of weak acids with their salt with а strong base (b) mixtures of weak bases with their salt with а strong acid. А few examples are given below:Н2СО3 / NаНСО3 (Bicarbonate buffer;carbonic acid and sodium bicarbonate)СН3СООН / СН3СОО Na (Acetate buffer; acetic acid and sodium acetate)Na2HPO4/ NaH2PO4(Phosphate buffer)

  10. Acid–Base Concept • The Arrhenius theory ACID - a substance that provides H+ ions in water BASE - a substance that provides OH- ions in water

  11. 2) The Brønsted-Lowry Theory All Brønsted–Lowry bases have one or more lone pairs of electrons:

  12. 3) The Lewis Acids and Base theory LEWIS ACID An electron-pair acceptor LEWIS BASE An electron-pair donor

  13. Acids There are many acids present in our everyday lives. Lemon juice contains citric acid, and vinegar contains ethanoic acid. Some strong acids are hydrochloric acid, sulphuric acid and nitric acid. Some weak acids are ethanoic acid, citric acid and carbonic acid.

  14. Alkalis Many everyday substances are alkalis. They feel soapy. They are corrosive. When the oxides of some metals dissolve in water they make an alkali solution. Alkalis react with acids and neutralisethem.

  15. Alkalis Alkalis are present in many cleaning substances in use in our homes. Kitchen cleaners are alkaline because they contain ammonia or sodium hydroxide, which attack grease. Calcium hydroxide and sodium hydroxide are strong alkalis. The most recognisable and common weak alkali is ammonia.

  16. BufferCapacity On the other hand, the buffer capacity is determined by the actual concentrations of salt and acid present, as well as by their ratio. Buffering capacity is the number of grams of strong acid or alkali which is necessary for а change in pH of one unit of one liter of buffer solution. The buffering capacity of а buffer is, definеdаs the ability of the buffer to resist changes in pH when an acid or base is added.

  17. Definition of buffer capacity A buffer absorbs strong acid and base through the two reactions shown on the left side of our diagram: A- + H3O+ => HA + H2O HA + OH- => A- + H2O The buffer will stop working when either one of its components (HA or A-) is exhausted, and therefore cannot neutralize any more strong acid or strong base. The most effective buffering solutions are those which have similar concentrations of HA and A- because then the buffer has the capacity to absorb both acid and base with the same effectiveness. Although the pH of a buffer depends only on the ratio [HA]/[A-], the ability of the buffer to absorb acid or base depends on the overall value of [HA] and [A-]. For instance, above we found a pH change of -0.02 units (from 7.20 to 7.18) when we added 0.010 moles of HCl to 1L of a buffer in which [HA] = [A-] = 0.50 M.

  18. Supposed that we had a buffer with [HA] = [A-] = 5.0 M. How much HCl would we need to add to get a pH change of -0.02 units? The answer is 10x as much as we found above, or 0.10 moles of HCl. This is summarized in this diagram: A ten-fold increase in the concentration of our buffering agents increased the ability to absorb acid, i.e. the buffer capacity, ten fold. The buffer capacity is directly proportional to the concentration of our buffering agents.

  19. Buffers Act • When hydrochloric acid is added to the acetate buffer, the salt reacts with the acid forming the weak acid, acetic acid and its salt. Similarly when а base is added, the acid reacts with it forming salt and water. Thus, changes in the pH are minimised. • СН3СООН + NaOH = СН3COONa + Н2О • СН3СООNа + HCI = СН3СООН + NaCI • The buffer capacity is determined by the absolute concentration of the salt and acid. But the рН of the buffer is dependent on the relative proportion of the salt and acid (see the Henderson - Hasselbalch's equation). When the ratio between salt and acid is 10:1, the pH will be one unit higher than the pKa. When the ratio between salt and acid is 1:10, the pH will be one unit lower than the pKa.

  20. 1.Buffers of body fluids, 2.Respiratory system, 3. Renal excretion. These mechanisms are interrelated. Acidic solutions have a high H+ concentration. Base solutions have a low H+ concentration. The pH scale is used to indicate the acidity or alkalinity of a solution. Pure water with an equal number of hydrogen and hydroxide ions has a pH of 7. Mechanisms for Regulation of pH

  21. Factors Affecting pH of а Buffer The pH of а buffer solution is determined by two factors: • 1. The value of pK: The lower the value of pK, the lower is the pH of the solution. • 2. The ratio of salt to acid concentrations: Actual concentrations of salt and acid in а buffer solution may be varied widely, with по change in рН, so long as the ratio of the concentrations remains the same.

  22. Thank you for attention

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