[PDF] Calculation of the Buffering Capacity of Bicarbonate in the Rumen PDF

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Calculation of the Buffering Capacity of Bicarbonate in the Rumen

The pH under different pressures at equilibrium and the buffering capacity are easily calculated for in vitro and in vivo systems. Key Words: Rumen pH

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[PDF] Calculation of the Buffering Capacity of Bicarbonate in the Rumen

The addition of NaHCO3 to buffer solutions and ruminal fluid under high pressure of CO2 increased pH as predicted The buffering capacity of ruminal fluid

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Unfortunately however derivation and systematic calculation of buffer capacity is a topic that seems to be neglected in the undergraduate analytical

1702
1

A contribution from the Maryland Agric. Exp. Sta.

Received June 24, 1997.

Accepted February 16, 1998.

Calculation of the Buffering Capacity of Bicarbonate in the Rumen and In Vitro

R. A. Kohn and T. F. Dunlap

Department of Animal and Avian Sciences, University of Maryland, College Park 20742

ABSTRACT:We describe a model to calculate the

buffering capacity of bicarbonate in the rumen. The addition of NaHCO 3 results in the release of CO 2from solution and eventually from the rumen via eructa- tion. This process directly neutralizes ruminal acidity. The degree to which the process continues depends on the partial pressure of CO 2 in the gas phase, the pH, and a constant(7.74),according to the Henderson-

Hasselbalch equation: pH = 7.74 +log([HCO

3- pressure of CO 2 in atmospheres). Theaddition of NaHCO 3 to buffer solutions and ruminal fluid under high pressure of CO2 increased pH as predicted. Thebuffering capacity of ruminal fluid under CO 2 was greater at low pH than was previously determined by titration in air. In contrast, in vitro systems in which CO 2 is not permitted to escape may result in reduced buffering capacity. In vitro systems in which excess CO 2 may escape(under N 2 gas pressure) mayresult in uncontrolled pH elevation. Dilution of ruminal fluid under constant pressure of CO 2 decreased ruminal pH as predicted by the model. The pH under different pressures at equilibrium and the buffering capacity are easily calculated for in vitro and in vivo systems.

Introduction

The prediction of ruminal pH has been a major

concern of ruminant nutritionists for many years, and HCO 3- is thought to be an important buffer of ruminal pH (Erdman,1988) and ofmost in vitro media used for fermentation studies(Goering and VanSoest,

1970). Themechanism by which HCO3-

buffers the rumen and in vitro media is often misunderstood.

Because the bicarbonate system is ubiquitous in

nature, physical chemists have systematically deve- loped calculations for predicting buffering capacity as affected by the medium's pH, ionic strength, and temperature(Fogg andGerrard,1985).These calcu- lations are applicable to ruminal fluid and to in vitro media used for fermentation studies.

This article describes how HCO

3-buffers the

rumen, and it describes the calculation of the buffer- ing capacity of HCO 3- in vitro and in vivo. This understanding is a prerequisite for the development of a mechanistic mathematical model to predict ruminal pH. This article addresses issues related to the function of added NaHCO 3 in the diet and the impact of increased salivation, which increases NaHCO 3 flowto the rumen. In addition, the buffering capacity is calculated for in vitro methods that use NaHCO 3 at different pH levels and with different pressures of CO2 (i.e.,continuous perfusion of CO 2 , perfusion of N 2 ,or systems with high CO 2 pressures).

Background and Equations

Buffering capacity refers to the number of moles of H that must be added to 1 L of solution to decrease the pH by 1 unit(Segel, 1976).This value depends on the buffer system and on the pH. Weak acids and bases provide better buffering than strong acids and bases because of the establishment of equilibria between the acid and conjugate base. For example, consider the weak acid, HA, and its base, A- HA H +A If the forward reaction is first order with respect to acid concentration, the rate is expressed as forward rate = k f [HA], where k f represents the fractional rate constant and [HA] represents the concentration of acid. If the reverse reaction is first order with respect to products,

BUFFERING CAPACITY OF BICARBONATE1703

the rate is expressed as reverse rate = k r [A ][H where k r represents the fractional rate constant for the reverse reaction and [A ] and [H ] represent the concentrations of A and H in moles per liter, respectively. If the system comes into equilibrium, the forward reaction rate equals the reverse rate, k f [HA] = k r [A ][H

The equilibrium constant (k

eq ) for the reaction is determined as k eq =k f /k r =[A ][H ]/[HA] Because this is the constant for acid dissociation, it is also referred to as theacid constant(K a ). The negative log 10 of the K a is referred to as the pK a pK a =-log K a pK a =-log([A ][H ]/[HA]) pK a =-log([A ]/[HA])-log[H pK a =-log([A ]/[HA]) + pH Rearranging provides for the Henderson-Hasselbalch equation, pH = pK a + log([A ]/[HA])

The pK

a is therefore the pH at which the acid is half- dissociated when in equilibrium. At > 1 unit of pH below the pK a , > 90% of the buffer would be in the acid form at equilibrium, and at > 1 unit of pH above the pK a , > 90% of the buffer would exist as the conjugate base at equilibrium.

Major buffers that exist in the rumen(Counotte et

al.,1979)include HCO 3- (pK a = 3.80),carbonate (pK a = 10.25),phosphate (pK a = 2.12, 7.21, and

12.32),acetate (pK

a = 4.76),propionate (pK a = 4.87), butyrate (pK a = 4.82), andlactate (pK a = 3.86).Most of these weak acids and bases have pK a that are outside the normal pH range of the rumen. If the ruminal pH is > 6.0, most of the VFA would be dissociated. As the pH drops to < 6.0, the rumen may be buffered by the protonization of fatty acids. Under normal conditions, these acids would provide for little buffering. The ability of phosphate in the rumen to buffer pH would decline as the pH decreases from neutrality.

The Bicarbonate System

The most prevalent ruminal buffer is HCO

3- (Counotte etal., 1979; Erdman,1988). Thebicar- bonate system includes two major ionic forms: HCO 3- and CO 32-
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