Regine D. Venzon Student number: 2009-12028-MN-1
NASC 2115 Biochemistry: July 06, 2011
Experiment Number 1 Ph Determination I.
Introduction
Objective: The goal of this lab is to determine the pH level of three different liquids. One of these liquids was a Lemon juice; another was pineapple juice, saliva and NaOH.
To use the pH meters properly and obtain the pH measurements of the
solution. pH is defined as the negative logarithm of the hydrogen ion concentration concentration . The letters ph stand for the power of the hydrogen ion [H +]. To indicate the concentration of H + ions in a solution, scientists have devised a scale based on the slight degree of spontaneous ionization of water. This scale is called pH scale. The ph values of this scale generally range from 0 to 14. Any substance that dissociates to form H+ ions when it is dissolved in water is called an acid. The more hydrogen ions an acid produces, the stronger an acid it is. Any substance that combines with H+ ions, as OH- ions do, is said to be base. Any increase in the concentration of a base lowers the H+ ion concentration. Bases therefore have pH values higher than water¶s neutral value of 7. When acids and bases react together, they form compounds collectively known as salts .Almost every biological process is pH-dependent; a small change in pH produces a large change in the rate of the process. Normally cells and organism have the ability
to maintain a specific and constant pH, keeping biomolecules in optimal ionic state. But in certain medical conditions like as severe uncontrolled diabetes, the pH-regulating mechanism fails; this may lead to permanent cell damage and death. The organism¶s first line of defense against changes in internal pH is provided by buffer systems, e.g. phosphate and bicarbonate systems. The phosphate buffer systems acts in the cytoplasm while the bicarbonate system in the blood plasma (1). The pH concept was developed in 1990 by S.P.L Sorensen, a Danish biochemist. It is a convenient method of expressing hydrogen ion concentration. By definition, ph= -log[H +]. To determined a pH of a solution experimentally we used indicatators. An indicator is an acid-base system in which the weak acid (conjugate acid) has one color, and its conjugate base has another color. An example is methyl orange which in its HIn form is red, and as In in yellow. Indicators are used in small quantities. They are used for estimation of ph or detecting large changes in pH. There are known natural indicators extracted from red cabbage, violets, roses, camote tops, red apple skin, beets, cherries, red onion, yellow onion, radish skin, tomato and turnip skin (2). II.
Methodology Before we started this laboratory experiment our professor advised us to wear
safety protection against harmful chemicals and advised us to follow all lab safety instructions. The first step in this lab was to prepare the liquid samples for testing. To do this, we used a graduated cylinder to measure 2 ml of the following samples in separate test tubes: lemon juice, pineapple juice, 0.1 M NaOH and saliva. We
determined the ph of these samples using a ph paper or ph meter. We repeated the procedure and placed 2 ml of lemon juice, pineapple juice, 0.1 M NaOH and saliva in four (4) test tubes. The liquid samples were now properly prepared and ready for testing, so we began the color test. Before starting this test, it was necessary to wash out the test tubes and wash them with fresh plain tap water. This was done as a precaution to prevent any contaminants from the color wheel test from altering the results of this test. First, we poured 2 ml. into each of 4 test tubes. Next, the prescribed amount of solution phenolphthalein was added to each of the test tubes, and we waited for the color change to take place and recorded it. This procedure is repeated for the indicators methyl red and bromothymol blue. These indicators were also added separately to each of the sample liquids in test tube, and we waited for the color change to take place and recorded it. The final method we used to determine the pH levels of our samples was the ph paper and the lithmus paper To perform the ph paper test, we submerged each of them in a different liquid sample. We kept the paper submerged in the liquid for a few seconds to be sure that the color change would be clear enough to read. When we removed the ph paper from the liquid, we were able to determine the pH level of each sample by matching the color at the wet end of each ph paper with one of the colors printed on the box. We recorded our results in the lab data. We also repeated this procedure for the lithmus paper test.
We used a graduated cylinder to measure 2 ml of the following samples in separate test tubes: lemon juice, pineapple juice, 0.1 M NaOH and saliva. We determined the ph of these samples using a ph paper or ph meter. We repeated the procedure and placed 2 ml of lemon juice, pineapple juice, 0.1 M NaOH and saliva in four (4) test tubes. We added one drop of phenolphthalein indicator and recorded the color. After that we placed 2 ml of lemon juice, pineapple juice, 0.1 M NaOH and saliva in four test tubes and added one drop of methyl red and recorded the color.c III.
Results and Discussions
Sample
pH
Red
Blue
Pape
Lithmu
Lithmu
r
s Paper s Paper
Pineappl e juice
4
red
red
Lemon juice
2
red
Saliva
12
red
Methyl Red
Bromothymo
Phenolphthalei
l Blue
n
Red/ Acidic
Acidic/Y
Acidic/C
red
Red/ Acidic
Acidic/Y
Acidic/C
blue
Yellow/Basi
Basic/B
Acidic/C
Basic/B
Basic/P
c NaOH
17
blue
blue
Yellow/Basi c
Final Assessment Sample
Pineapple juice
Acidic
Lemon juice
Acidic
Saliva
Basic but in Phenolphthalein acidic
NaOH
Basic
³Among the first to explain the behavior of acids and bases was Svante August Arrhenius. His theory is often called the Arrhenius ionic theory, defines an acid as a substance which gives hydrogen ions(H +) on dissociation in water and a base as a substance which gives hydroxyl ions (OH +)´(3) ³The Bronsted-Lowry definition is named for Johannes Bronsted and Thomas Lowry, who independently proposed it in 1923. A Bronsted-Lowry (BL) acid is defined as any substance that can donate a hydrogen ion (proton) and a Bronsted-Lowry base is any substance that can accept a hydrogen ion(proton). Bases are the opposite of acids. Bases are basic because they take or accept protons. Hydroxide ion, for example can accept a proton to form water. Brønsted and Lowry realized that not all bases had to have a hydroxide ion. As long as something can accept a proton it is a base´ (4).
³Just as the Arrenhius theory did not support observations of acid-base behavior in nonaqueous systems, the Bronsted-Lowry model excluded nonprotonated systems. Lewis suggested his theory in a 1923 book "Thermodynamics and the Free Energy of Chemical Substances" and fully developed the theory in 1938.Lewis said that acid is a substance that can accept a pair of electrons from another atom to form a new
bond while base is a substance that can donate a pair of electrons to another atom to form a new bond´ (5).
³Chemically, acids and bases may be considered opposites of each other. The concept of acids and bases is so important in chemistry that there are several useful definitions of "acid" and "base" that pertain to different chemical environments, Acids and bases are chemical compounds that have certain specific properties in aqueous solutions. In most chemical circumstances, acids are chemicals that produce positively-charged hydrogen ions, H +, in water, while bases are chemicals that produce negativelycharged hydroxide ions, OH -, in water. Bases are sometimes called alkalis. Acids and bases react with each other in a reaction called neutralization. In a neutralization reaction, the hydrogen ion and the hydroxide ion react to form a molecule of water:. Acids and bases have some general properties. Many acids have a sour taste. Citric acid, found in oranges and lemons, is one example where the sour taste is related to the fact that the chemical is an acid. Molecules that are bases usually have a bitter taste, like caffeine. Bases make solutions that are slippery. Many acids will react with metals to dissolve the metal and at the same time generate hydrogen gas, H2. Perhaps the most obvious behavior of acids and bases is their abilities to change colors of certain other chemicals. Litmus paper is still commonly used to indicate whether a compound is an acid or a base. Extracts made from red onions, red cabbage, and many otherfruits and vegetables change colors in the presence of acids and bases. Such materials are called indicators´
(6)
.
³ An acid-base indicator is a weak acid or a weak base. The undissociated form of the indicator is a different color than the biogenic form of the indicator. An Indicator does not change color from pure acid to pure alkaline at specific hydrogen ion concentration, but rather, color change occurs over a range of hydrogen ion concentrations. This range is termed the color change interval . It is expressed as a pH range. Several acid-base indicators are listed below, some more than once if they can be used over multiple pH ranges. Quantity of indicator in aqueous (aq.) or alcohol (alc.) solution is specified. Tried-and-true indicators include: thymol blue, tropeolin OO, methyl yellow, methyl orange, bromphenol blue, bromcresol green, methyl red, bromthymol blue, phenol red, neutral red, phenolphthalein, thymolphthalein, alizarin yellow, tropeolin O, nitramine, and trinitrobenzoic acid. Data in this table are for sodium salts of thymol blue, bromphenol blue, tetrabromphenol blue, bromcresol green, methyl red, bromthymol blue, phenol red, and cresol red´ (7) IV.
Conclusions Finally, it should be said that this lab was, as a whole, very instructive. It provided
a practical, hands-on experience with testing the pH of various liquids, and was a valuable teaching tool.
IV. V.
Conclusion References: 1. Prof. Carmela V. Dellova, ³Nutrional Biochemistry´, Lecture Guide 2008 ed.,pages 7-8
2. Mendoza, Estrella E. Religioso Teresita F., ³Chemistry´, Chapter 18, page309-313 3. Mendoza, Estrella E. Religioso Teresita F., ³Chemistry´, Chapter 18, page301-303) 4. Bauer Richard C., Birk, James P., Marks, Pamela S.,´A Conceptual Introduction to Chemistry´, Chapter 13, pages 490-492 5. Retrieved from site : http://web.fccj.org/~ethall/acidbase/acidbase.htm 6. http://science.jrank.org/pages/46/Acids-Bases.html 7. Lange's Handbook of Chemistry, 8th Edition, Handbook Publishers Inc., 1952.) VI.
Appendices(Calculations) or problems)
