The hendersonhasselbalch equation simple book production. To find the ph of a buffer composed of h2po4aq and hpo42aq, which pka value would you use in the hendersonhasselbalch equation. The numerical value of the acid dissociation constant, k a, of the acid is known or assumed. 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. With focus on this question, this article examines the evolution of the hendersonhasselbalch equation and presents a critical evaluation of its usefulness. Hasselbalch was using the formula to study metabolic acidosis. The ph is calculated for given values of the concentrations of.
The hendersonhasselbalch approximation gives us a method to approximate the ph of a buffer solution. The henderson hasselbalch equation was developed independently by the american biological chemist l. Hendersonhasselbalch equation project gutenberg self. He wrote the henderson equation in 1908 to describe the use of carbonic acid as a buffer solution. Henderson hasselbalch equation ion trapping ph and pka. World heritage encyclopedia, the aggregation of the largest online encyclopedias available, and the most definitive collection ever assembled. The hendersonhasselbalch equation was developed independently by the american biological chemist l. American proficiency institute 2014 2nd test event educational commentary arterial blood gases. The hendersonhasselbach equation is derived from the definition of the acid dissociation constant as follows. This is conventionally represented using two dimensional plots. In chemistry, the henderson hasselbalch equation describes the derivation of ph as. Ice vs hendersonhasselbach equation chemistry community.
Glossary of terms used in physical organic chemistry iupac recommendations 1994 on page 1120. Hendersonhasselbalch approximation chemistry stack exchange. The hendersonhasselbalch equation is, of course, the mass action expression cast in. This law is one of the basic laws of chemical equilibrium and was crucial to the subsequent development of both physical and analytical chemistry. Hasselbalch, for relating the ph to the bicarbonate buffer system of the blood see below. In order to use the equation, the acid dissociation constant must be known. In its general form, the henderson hasselbalch equation is a useful expression for buffer calculations. Plugging those numbers into the henderson hasselbalch equation gives an expected ph of 7. In its general form, the hendersonhasselbalch equation is a useful expression for buffer calculations. Its history and limitations journal of chemical education. Ba the concentration of the ionized form of this acid.
If we use the hendersonhasselbalch equation, we do not need to worry about using the molar amounts of both the acid and the base. 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 hendersonhasselbalch equation makes use of the approximations a baseinitial and ha acidinitial. Karl albert hasselbalch later expressed the equation in logarithmic terms, creating the hendersonhasselbalch equation. The following alphabetical list provides access to the principle topics, words, and authors mentioned in this website. Hasselbalch equation and comments on its history, including the development of the ph scale. Over much of the titration range, the calculation of ph relies on the henderson hasselbalch. We can make the approximation when the concentrations of the acid and the base are very high.
Hendersonhasselbalch equation definition of henderson. To derive the equation a number of simplifying assumptions have to be made. The history of assessing the acidbase equilibrium and associated disorders is intertwined with the evolution of the definition of an acid. First we need to determine the number of moles of ammonium we have. Buffers and hendersonhasselbalch video khan academy. When we plug that into the hendersonhasselbalch equation. 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 equation can also be used when you know the ka and are working with a weak acid. In this article we will discuss about the hendersonhasselbalch equations.
The hendersonhasselbalch equation can be considered as the backbone of acid base physiology. The pk of a buffer must be experimentally determined and cannot be predicted. 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. A buffer solution contains a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. In addition, he described blood gas transport and the general physiology of blood as physicochemical system 19201932. 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. 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.
From the hendersonhasselbalch equation, ph is determined by the ratio of bicarb and pco 2. 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. 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 this class, the units for the concentrations in the hendersonhasselbalch equation will always be in mm mmoll meqml. I will derive the hh equation using the generic weak acid ha. From a table of molar masses, such as a periodic table, we can calculate the molar mass of naf to be equal to 41. Respiratory compensation by the lungs is done either by hyperventilating or hypoventilating to alter the pco 2 and counteract a primary metabolic process. Equation \\ref8\ is called the hendersonhasselbalch equation and is often used by chemists and biologists to calculate the ph of a buffer. 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. 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.
For example say the total volume is 10 l, moles base10moles acid10 moles basemoles acid. Let us consider ha, a weak acid that ionizes as follows. We would get the same results with the table generated by the decimal digits rule. We know from our hendersonhasselbalch calculation that the ratio of our baseacid should be equal to 0. 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. Use the hendersonhasselbalch equation to calculate the of the solution. 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. At the half equivalence point, the conjugate base concentration is equal to that of the weak acid. Example problem applying the hendersonhasselbalch equation calculate the ph of a buffer solution made from 0. 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. 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.
Henderson hasselbalch equation definition thoughtco. Lawrence joseph henderson wrote an equation, in 1908, describing the use of carbonic acid as a buffer solution. The equation is also useful for estimating the ph of a buffer solution and finding the equilibrium ph in an acidbase reaction. 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. When you mix a weak acid with its conjugate base, the acid ionization reaction shifts to the left so the % ionization drops.
Henderson hasselbach definition of henderson hasselbach. We need to use the hendersonhasselbalch equation to determine how many moles of ammonia we need. I would go back and look at computation part to figure out if you rounded off too much, too early. Remember that the h 2 co 3 is regulated by the brain and lungs. In this case, we have the acid, and we are looking for the base. Hello, the video discusses the basics of drug transportation, ionization, henderson hasselbach equation, and ion trapping phenomenon. 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. 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.
192 613 787 201 262 386 567 947 718 1443 1207 1406 1271 1514 673 1257 519 1548 502 94 325 225 975 1267 690 1195 992 162 607 1060 437 393 1357