ph and buffer

Ph and buffers

Ph and BufferAcid Base Balance

Prakash Pokhrel

PH

It is the negative log of the hydrogen ion concentration. pH = -log [H+] Acid base balance

pH is a unit of measure which describes the degree of acidity or alkalinity (basic) of a solution.

It is measured on a scale of 0 to 14.

Low pH values correspond to high concentrations of H+ and high pH values correspond to low concentrations of H+.

Ph value The pH value of a substance is directly related to the ratio of the hydrogen ion and hydroxyl ion concentrations.

If the H+ concentration is higher than OH- the material is acidic.

If the OH- concentration is higher than H+ the material is basic.

7 is neutral, < is acidic, >7 is basic

The ph scale The pH scale corresponds to the concentration of hydrogen ions.For example pure water H+ ion concentration is 1 x 10^-7 M, therefore the pH would then be 7.

Acid Any compound which forms H ions in solution (proton donors)eg: Carbonic acid releases H ionsBaseAny compound which combines with H ions in solution (proton acceptors) eg:Bicarbonate(HCO3) accepts H+ ions

AcidBase BalanceNormal pH : 7.35-7.45Acidosis Physiological state resulting from abnormally low plasma pH AlkalosisPhysiological state resulting from abnormally high plasma pH Acidemia: plasma pH < 7.35 Alkalemia: plasma pH > 7.45

Measurement of ph

Some important indicators used in a Clinical Biochemistry Laboratory are listed below:

sr,. No. INDICATORPh rangeColour in acidic phColour in basic ph1Phenophthalein9.3-10.5colourlesspink2Methyl orange3.1-4.6redyellow3Bromophenol blue3.0-4.6yellowblue4Methyl red4.4-6.2Red yellow5Phenol red6.8 8.4yellowred6Litmus4.5-8.3redBlue

PH meter

The pH meter is a laboratory equipment which used to measure acidity or alkalinity of a solution

The pH meter measures the concentration of hydrogen ions [H+] using an ion-sensitive electrode.

It is the most reliable and convenient method for measuring ph.

BUFFERS

BUFFERA buffer solution is a solution which resists changes in pH when a small amount of acid or base is added.

Typically a mixture of a weak acid and a salt of its conjugate base or weak base and a salt of its conjugate acid.

Types of buffers

Two types :ACIDIC BUFFERS Solution of a mixture of a weak acid and a salt of this weak acid with a strong base.E.g. CH3COOH + CH3COONa ( weak acid ) ( Salt )

BASIC BUFFERS Solution of a mixture of a weak base and a salt of this weak base with a strong acid.e.g. NH4OH + NH4Cl ( Weak base) ( Salt)

how buffers work

Equilibrium between acid and base.

Example: ACETATE BUFFERCH3COOH CH3COO- + H+

If more H+ is added to this solution, it simply shifts the equilibrium to the left, absorbing H+, so the [H+] remains unchanged.

If H+ is removed (e.g. by adding OH-) then the equilibrium shifts to the right, releasing H+ to keep the pH constant

Handerson hasselbalch equation

Lawrence Joseph Hendersonwrote an equation, in 1908, describing the use ofcarbonic acidas a buffer solution.

Karl Albert Hasselbalchlater re-expressed that formula inlogarithmicterms, resulting in the HendersonHasselbalch equation.

Ka = [H+] [A-][HA]

take the -log on both sides The Henderson-Hasselbalch Equation derivation -log Ka = -log [H+] -log[A-][HA]

pH =pKa + log [A-][HA]

= pKa + log [Proton acceptor][Proton donor]

HA H+ + A-

pKa = pH -log[A-][HA]

apply p(x) = -log(x)and finally solve for pH

16Logxy = logx + logy

- The greater the buffer capacity the less the pH changes upon addition of H+ or OH-

Choose a buffer whose pKa is closest to the desired pH.

pH should be within pKa 1

Buffer system in body fluids

ACIDS

VOLATILE ACIDS: Produced by oxidative metabolism of CHO,Fat,ProteinAverage 15000-20000 mmol of CO per dayExcreted through LUNGS as CO gasFIXED ACIDS (1 mEq/kg/day)Acids that do not leave solution ,once produced they remain in body fluids Until eliminated by KIDNEYSEg: Sulfuric acid ,phosphoric acid , Organic acidsAre most important fixed acids in the bodyAre generated during catabolism of:amino acids(oxidation of sulfhydryl gps of cystine,methionine)Phospholipids(hydrolysis)nucleic acids

Response to ACID BASE challenge

Buffering Compensation

Buffers First line of defence (> 50 100 mEq/day)Two most common chemical buffer groupsBicarbonateNon bicarbonate (Hb,protein,phosphate)Blood buffer systems act instantaneouslyRegulate pH by binding or releasing H

Buffers are always present and can act fast to reduce amount of free H+ ions.

Bicarbonate active in both ICP and ECFPhosphate active in ICF

Protein buffers are largest source and are present in both intracellular and extracellular fluid Major protein buffers: hemoglobin, albumin, globulin21

Carbonic AcidBicarbonate Buffer System

Carbon DioxideMost body cells constantly generate carbon dioxide Most carbon dioxide is converted to carbonic acid, which dissociates into H+ and a bicarbonate ionPrevents changes in pH caused by organic acids and fixed acids in ECF

Cannot protect ECF from changes in pH that result from elevated or depressed levels of CO2Functions only when respiratory system and respiratory control centers are working normallyAbility to buffer acids is limited by availability of bicarbonate ions

AcidBase Balance

The Carbonic AcidBicarbonate Buffer System

The carbonic acid hydrogencarbonate buffer systemThe carbonic acid-hydrogen Bicarbonate ion buffer is the most important buffer system. Carbonic acid, H2CO3, acts as the weak acidHydrogen carbonate, HCO3-, acts as the conjugate baseIncrease in H+(aq) ions is removed by HCO3-(aq)The equilibrium shifts to the left and most of the H+(aq) ions are removed

The small concentration of H+(aq) ions reacts with the OH-(aq) ions

H2CO3 dissociates, shifting the equilibrium to the right, restoring most of the H+(aq) ions

Any increase in OH-(aq) ions is removed by H2CO3

The Hemoglobin Buffer System

CO2 diffuses across RBC membraneNo transport mechanism requiredAs carbonic acid dissociatesBicarbonate ions diffuse into plasmaIn exchange for chloride ions (chloride shift)Hydrogen ions are buffered by hemoglobin moleculesIs the only intracellular buffer system with an immediate effect on ECF pH Helps prevent major changes in pH when plasma PCO2 is rising or falling

Phosphate Buffer System

Consists of anion H2PO4- (a weak acid)(pKa-6.8)Works like the carbonic acidbicarbonate buffer systemIs important in buffering pH of ICFLimitations of Buffer Systems Provide only temporary solution to acidbase imbalanceDo not eliminate H+ ions Supply of buffer molecules is limited

Respiratory Acid-Base Control Mechanisms When chemical buffers alone cannot prevent changes in blood pH, the respiratory system is the second line of defense against changes.Eliminate or Retain COChange in pH are RAPIDOccuring within minutesPCO VCO/VA

28Sentence and phrases

29Phosphate buffer systemThe phosphate buffer system (HPO42-/H2PO4-) plays a role in plasma and erythrocytes.

H2PO4- + H2O H3O+ + HPO42-

Any acid reacts with monohydrogen phosphate to form dihydrogen phosphatedihydrogen phosphatemonohydrogen phosphate

H2PO4- + H2O HPO42- + H3O+ The base is neutralized by dihydrogen phosphatedihydrogen phosphate monohydrogen phosphateH2PO4- + OH- HPO42- + H3O+

Renal Acid-Base Control MechanismsThe kidneys are the third line of defence against wide changes in body fluid pH.movement of bicarbonateRetention/Excretion of acidsGenerating additional buffersLong term regulator of ACID BASE balanceMay take hours to days for correction

30S&P

Renal regulation of acid base balanceRole of kidneys is preservation of bodys bicarbonate stores. Accomplished by:Reabsorption of 99.9% of filtered bicarbonate Regeneration of titrated bicarbonate by excretion of:Titratable acidity (mainly phosphate)Ammonium salts

32Proteins as a bufferProteins contain COO- groups, which, like acetate ions (CH3COO-), can act as proton acceptors.

Proteins also contain NH3+ groups, which, like ammonium ions (NH4+), can donate protons.

If acid comes into blood, hydronium ions can be neutralized by the COO- groups- COO- + H3O+ - COOH + H2O

If base is added, it can be neutralized by the NH3+ groups- NH3+ + OH- - NH2 + H2O

Titratable acidityOccurs when secreted H+ encounter & titrate phosphate in tubular fluidRefers to amount of strong base needed to titrate urine back to pH 7.440% (15-30 mEq) of daily fixed acid loadRelatively constant (not highly adaptable)

Renal reabsorption of bicarbonateProximal tubule: 70-90%Loop of Henle: 10-20%Distal tubule and collecting ducts: 4-7%

Factors affecting renal bicarbonate reabsorptionFiltered load of bicarbonateProlonged changes in pCO2Extracellular fluid volumePlasma chloride concentrationPlasma potassium concentrationHormones (e.g., mineralocorticoids, glucocorticoids)

If secreted H+ ions combine with filtered bicarbonate, bicarbonate is reabsorbedIf secreted H+ ions combine with phosphate or ammonia, net acid excretion and generation of new bicarbonate occur

NET ACID EXCRETIONHydrogen Ions Are secreted into tubular fluid alongProximal convoluted tubule (PCT)Distal convoluted tubule (DCT)Collecting system

Ammonium excretionOccurs when secreted H+ combine with NH3 and are trapped as NH4+ salts in tubular fluid60% (25-50 mEq) of daily fixed acid loadVery adaptable (via glutaminase induction)

Ammonium excretionLarge amounts of H+ can be excreted without extremely low urine pH because pKa of NH3/NH4+ system is very high (9.2)

AcidBase Balance Disturbances

Interactions among the Carbonic AcidBicarbonate Buffer System and Compensatory Mechanisms in the Regulation of Plasma pH.

Four Basic Types of ImbalanceMetabolic AcidosisMetabolic AlkalosisRespiratory AcidosisRespiratory Alkalosis

Acid Base DisordersDisorderpH[H+]Primary disturbanceSecondary responseMetabolic acidosis [HCO3-] pCO2Metabolic alkalosis [HCO3-] pCO2Respiratory acidosis pCO2 [HCO3-]Respiratory alkalosis pCO2 [HCO3-]

Metabolic AcidosisPrimary AB disorderHCO pH Gain of strong acidLoss of base(HCO)

CAUSES OF METABOLIC ACIDOSIS

LACTIC ACIDOSISKETOACIDOSISDiabeticAlcoholic Starvation RENAL FAILURE (acute and chronic)

TOXINSEthylene glycolMethanolSalicylatesPropylene glycol


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Responses to Metabolic Acidosis

Metabolic acidosisSymptoms are specific and a result of the underlying pathologyRespiratory effects:HyperventilationCVS: myocardial contractilitySympathetic over activityResistant to catecholaminesCNS:Lethargy, disorientation,stupor,muscle twitching, COMA, CN palsiesOthers : hyperkalemia

Metabolic Alkalosis pH due to HCO or acid Initiation process :in serum HCOExcessive secretion of net daily production of fixed acidsMaintenance:HCO excretion or HCO reclamationChloride depletionPottasium depletionECF volume depletionMagnesium depletion

CAUSES OF METABOLIC ALKALOSISI. Exogenous HCO3 loadsA. Acute alkali administrationB. Milk-alkali syndromeII. Gastrointestinal origin1. Vomiting2. Gastric aspirationIII. Renal origin1. Diuretics2. Posthypercapnic state3. Hypercalcemia/hypoparathyroidism4. Recovery from lactic acidosis or ketoacidosis5. Nonreabsorbable anions including penicillin, carbenicillin6. Mg2+ deficiency7. K+ depletion

Compensation for Metabolic AlkalosisRespiratory compensation: HYPOVENTILATIONPCO=0.6 mm pCO2 per 1.0 mEq/L HCO3-Maximal compensation: PCO 55 60 mmHgHypoventilation not always found due toHyperventilation due to paindue to pulmonary congestiondue to hypoxemia(PO < 50mmHg)


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Metabolic Alkalosis

Metabolic AlkalosisDecreased myocardial contractilityArrythmias

cerebral blood flowConfusionMental obtundationNeuromuscular excitability

Respiratory Acidosis PCO pHAcute(< 24 hours)Chronic(>24 hours)

Compensation in Respiratory AcidosisAcute resp.acidosis:Mainly due to intracellular buffering(Hb,Pr,PO)HCO = 1mmol for every 10 mmHg PCOMinimal increase in HCOpH change = 0.008 x (40 - PaCO)

Chronic resp.acidosisRenal compensation (acidification of urine & bicarbonate retention) comes into actionHCO = 3.5 mmol for every 10 mm Hg PCOpH change = 0.003 x (40 - PaCO)Maximal response : 3 - 4 days


Respiratory AcidBase Regulation.


Respiratory AcidBase Regulation.

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ph and buffer

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