The pk of a buffer must be experimentally determined and cannot be predicted. Buffers and hendersonhasselbalch video khan academy. From the hendersonhasselbalch equation, ph is determined by the ratio of bicarb and pco 2. Example problem applying the hendersonhasselbalch equation calculate the ph of a buffer solution made from 0. From a table of molar masses, such as a periodic table, we can calculate the molar mass of naf to be equal to 41. American proficiency institute 2014 2nd test event educational commentary arterial blood gases. Henderson hasselbalch equation definition thoughtco. In this case, we have the acid, and we are looking for the base.
To derive the equation a number of simplifying assumptions have to be made. The following alphabetical list provides access to the principle topics, words, and authors mentioned in this website. I would go back and look at computation part to figure out if you rounded off too much, too early. The henderson hasselbalch equation was developed independently by the american biological chemist l. In addition, he described blood gas transport and the general physiology of blood as physicochemical system 19201932. Use the hendersonhasselbalch equation to calculate the of the solution. Ba the concentration of the ionized form of this acid. The hendersonhasselbalch equation mathematically connects the measurable ph of a solution with the pk a which is equal to log k a of the acid. Over much of the titration range, the calculation of ph relies on the henderson hasselbalch. Karl albert hasselbalch later reexpressed that formula in logarithmic terms, resulting in the hendersonahasselbalch equation 1. The hendersonhasselbach equation is derived from the definition of the acid dissociation constant as follows.
Hasselbalch, for relating the ph to the bicarbonate buffer system of the blood see below. This is conventionally represented using two dimensional plots. The pk of a particular buffer is the ph of the solution where the ratio between the weak base b and weak acid hb forms of the buffer are equivalent. Plugging those numbers into the henderson hasselbalch equation gives an expected ph of 7. When we plug that into the hendersonhasselbalch equation. Lawrence joseph henderson wrote an equation, in 1908, describing the use of carbonic acid as a buffer solution. Let us consider ha, a weak acid that ionizes as follows. Hello, the video discusses the basics of drug transportation, ionization, henderson hasselbach equation, and ion trapping phenomenon. The hendersonhasselbalch equation relates the ph of a solution containing a mixture of the two components to the acid dissociation constant, k a, and the concentrations of the species in solution. Remember that the h 2 co 3 is regulated by the brain and lungs. First we need to determine the number of moles of ammonium we have.
Hendersonhasselbalch approximation chemistry stack exchange. For example say the total volume is 10 l, moles base10moles acid10 moles basemoles acid. The hendersonhasselbalch equation is, of course, the mass action expression cast in. When you mix a weak acid with its conjugate base, the acid ionization reaction shifts to the left so the % ionization drops. Hasselbalch equation and comments on its history, including the development of the ph scale. Without severe rounding the answer should come out to using the hendersonhasselbalch equation. The equation is also useful for estimating the ph of a buffer solution and finding the equilibrium ph in acidbase reactions it is widely used to calculate the isoelectric point of. In chemistry, the henderson hasselbalch equation describes the derivation of ph as.
If we use the hendersonhasselbalch equation, we do not need to worry about using the molar amounts of both the acid and the base. Hendersonhasselbalch equation definition of henderson. This law is one of the basic laws of chemical equilibrium and was crucial to the subsequent development of both physical and analytical chemistry. We can make the approximation when the concentrations of the acid and the base are very high. In order to use the equation, the acid dissociation constant must be known. To find the ph of a buffer composed of h2po4aq and hpo42aq, which pka value would you use in the hendersonhasselbalch equation. I will derive the hh equation using the generic weak acid ha.
The hendersonhasselbalch equation can be considered as the backbone of acid base physiology. Its history and limitations journal of chemical education. There was a solution of acetic acid and some strong base was added resulting in a solution of a weak acid and its salt. In the henderson hasselbach equation, you should put in the molarity, but since the total volume is the same for both the base and the acid they are sitting in the same solution, so they have the same total volume of solution, the volumes will cancel. Hasselbalch was using the formula to study metabolic acidosis. The hendersonhasselbalch equation can also be used when you know the ka and are working with a weak acid. The henderson hasselbalch equation is described and an example problem of how to calculate the ph of a buffer solution using the equation is given. In chemistry, the henderson hasselbalch equation describes the derivation of ph as a measure of acidity using pk a, the negative log of the acid dissociation constant in biological and chemical systems. In its general form, the henderson hasselbalch equation is a useful expression for buffer calculations. The hendersonhasselbalch equation simple book production.
By rearranging the last equation above we arrive at the hendersonhasselbalch equation it should be obvious now that the ph of a solution of any acid for which the equilibrium constant is known, and there are numerous tables with this information can be calculated knowing the concentration of the acid, ha, and its conjugate base a. The equilibrium between the weak acid and its conjugate base allows the solution to resist changes to ph when small amounts of strong acid or base are added. One writer, for instance, excels at a plan or a title page, another works away at the body of the book, and a third is a dab at the index. He wrote the henderson equation in 1908 to describe the use of carbonic acid as a buffer solution. A buffer solution contains a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. Many students of chemistry have wondered if putting the mass action expression in logarithmic format should have warranted immortalization of the names henderson and hasselbalch.
The hendersonhasselbalch equation makes use of the approximations a baseinitial and ha acidinitial. The history of assessing the acidbase equilibrium and associated disorders is intertwined with the evolution of the definition of an acid. We would get the same results with the table generated by the decimal digits rule. Its history and limitations in 1864 guldberg and waage first formulated the mass action law 1 and refined it to its final, present form in 1879 2. You should memorize the following normal values for the bicarbonate buffer system in a normal person and, in this course, always use a pka of 6. The ph is calculated for given values of the concentrations of. World heritage encyclopedia, the aggregation of the largest online encyclopedias available, and the most definitive collection ever assembled. As can be seen, when the value of b equals that of hb, the log term of the hendersonhasselbalch equation becomes zero, leaving ph pk. In the 1950s clinical chemists combined the hendersonhasselbalch equation and the bronstedlowry definition of an acid to produce the current bicarbonate ioncentred approach to metabolic acidbase disorders. The hendersonhasselbalch approximation gives us a method to approximate the ph of a buffer solution. Hendersonhasselbalch equation project gutenberg self.
Henderson hasselbalch equation ion trapping ph and pka. With focus on this question, this article examines the evolution of the hendersonhasselbalch equation and presents a critical evaluation of its usefulness. In this article we will discuss about the hendersonhasselbalch equations. The hendersonhasselbalch equation was developed independently by the american biological chemist l. Glossary of terms used in physical organic chemistry iupac recommendations 1994 on page 1120. Ice vs hendersonhasselbach equation chemistry community. The following equation, which relates the ph of an aqueous solution of an acid to the acid dissociation constant of the acid, is known as the hendersonhasselbach equation. The hendersonhasselbalch hh equation provides a recipe for making a buffer of a given ph. Respiratory compensation by the lungs is done either by hyperventilating or hypoventilating to alter the pco 2 and counteract a primary metabolic process. The henderson hasselbalch equation is an approximate equation that shows the relationship between the ph or poh of a solution and the pk a or pk b and the ratio of the concentrations of the dissociated chemical species. The numerical value of the acid dissociation constant, k a, of the acid is known or assumed. In this class, the units for the concentrations in the hendersonhasselbalch equation will always be in mm mmoll meqml.
In its general form, the hendersonhasselbalch equation is a useful expression for buffer calculations. We know from our hendersonhasselbalch calculation that the ratio of our baseacid should be equal to 0. At the half equivalence point, the conjugate base concentration is equal to that of the weak acid. Henderson hasselbach definition of henderson hasselbach. Equation \\ref8\ is called the hendersonhasselbalch equation and is often used by chemists and biologists to calculate the ph of a buffer. Karl albert hasselbalch later expressed the equation in logarithmic terms, creating the hendersonhasselbalch equation. Henderson equation we will discuss the limitations of the henderson hasselbalch equation focusing on the titration curve of a weak acid with a strong base. We need to use the hendersonhasselbalch equation to determine how many moles of ammonia we need. The equation is also useful for estimating the ph of a buffer solution and finding the equilibrium ph in an acidbase reaction.
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