BASIC THEORIES AND OBJECTIVES
A. Basic Theory
1. The theory of acids and bases
Acid (often represented by the general formula HA) is generally a chemical compound that when dissolved in water will produce a solution with a pH less than 7. In the modern definition, an acid is a substance that can give a proton (H + ions) to other substances (called bases), or may receive a free electron pair of the bases. An acid reacts with a base in a neutralization reaction to form a salt. Acids have a sour taste and are corrosive or damaging the metal.
General definition of the bases are chemical compounds that release hydroxide ions (OH -) when dissolved in water. Base is the opposite of acid, which is intended to elements / compounds that have a pH more than 7. Bases have a bitter taste and are caustic and feels slippery.
There are three theories about acids and bases.
a. Arrhenius Acids and Bases Theory
According to Arrhenius, acids are substances that when dissolved in water increase the concentration of ions H + (aq). Bases are substances that when dissolved in water can increase the concentration of ions OH-(aq). ion H+(aq) tidak berupa proton bebas tetapi terikat secara kimia pada molekul air, membentuk H3O+(aq). ion H + (aq) is not free but form protons chemically bound water molecules, forming H3O + (aq). This species is called the hydronium ion itself associated through hydrogen bonds with a number of water molecules. (Purba, 2009:172)
b. Bronsted-Lowry Acid and Base Theory
In 1923, John Bronsted and Thomas Lowry suggested that the reaction of acids and bases can be viewed as a proton transfer reaction, and acid-base can be defined in the form of proton transfer. According to acid-base theory Bronsted-Lowry, an acid is a species that gives (donors) protons, while bases are acting as the recipient (acceptor) protons in a proton transfer reaction.
c. Lewis Acids and Bases Theory
GN Lewis stated that the concept of acids and bases can be applied generally to include the reaction of the acid reaction of oxide and oxide base and a number of other reactions, including proton transfer reactions. According to this concept, a Lewis acid is a species that can form a covalent bond by accepting the free electron pair of another species (the acid as a free electron pair acceptor). A Lewis base is a species that can form covalent bonds with electron pairs to give other species. The concept of Lewis acid-base and the Bronsted-Lowry perspective differs according to the specific chemical reactions.
d. Indicator solution
Indicator solution is a substance that has a different color on acid, alkaline, and neutral. Akibatnya, larutan tersebut dapat digunakan untuk membedakan larutan asam, basa, dan netral. As a result, the solution can be used to distinguish acid, alkaline, and neutral. Some of the indicators commonly used in laboratories, among other fenolflatein solution (pp), red metal, and orange metal.
Many substances around us that can be used as an indicator of acid and alkali. For example, leaf, flower, turmeric, mangosteen skin, and purple cabbage. This is because the solution of these materials give a different color on acid and alkaline and neutral. To prove this, mangosteen skin that has been refined in a neutral state would be purple. However, if the added acid solution, the color purple will change to reddish brown. Conversely, if the alkaline solution is added, the color of the skin extract of mangosteen will change from purple to blue-black. With these color changes, skin extract of mangosteen can be used as an indicator of acid and alkali. (Purwanto dan Nugroho, 2008:18-19)
To find out how to distinguish between acid and base solution by using a natural indicator of acid and base.
B. Procedure / Step Work
Chinese Hibiscus as Acids and Bases indicators
1. 1. Separate the crown of flower from the another flower parts.
2. 2. Put the crown of flowers into the mortar.
3. 3. Pound and grind the crown of flowers until smooth by using the pestle, add water if necessary.
4. 4. Put aquades into the test tube labeled I, NaOH solution into the test tube labeled II, a solution of HCl in the test tube labeled III, the X solution to the tube labeled IV, and the Y solution to the tube labeled V.
5. 5. Put the extract of chinese hibiscus into each labeled test tube. Observe and record the color changes that occur.
Cassava leaves Trees as Indicators Acids and Bases
- Separate the leaves from the others parts of cassava leaves. If it is able, leaves chopped as small as possible to to make the grinding proccess easier.
- Put the cassava leaves in the mortar.
- Pound and grind until smooth leaves with a pestle, add water if necessary.
- Put aquades into the test tube labeled I, NaOH solution into the test tube labeled II, a solution of HCl in the test tube labeled III, the X solution to the tube labeled IV, and the Y solution to the tube labeled V.
- Put cassava leaf extract into each labeled test tube. Observe and record the color change occurs.
RESULTS OF OBSERVATIONS
The results of the test acids and bases with the crown indicators hibiscus extract and cassava leaves is presented in tables 1 and 2
Table 1. Indicator's Color ChangeAfter Mixed with Aquades, NaOH, and HCl.
|No|| Indicators ||Early Color ||Color After Mixing |
|Aquades ||NaOH ||HCl |
|1 ||Chinese Hibiscus ||Purple ||Purple ||Brown ||Red |
|2 ||Cassava Leaf Tree ||Bright Green ||Bright Green ||Dark Green || Yellow |
Table 2. Indicator's Color Change After Mixed with X and Y solution.
| No ||Indicators ||Early Color ||Color after mixing |
|X Solution || Y Solution |
|1 1 ||Chinese Hibiscus ||Purple ||Brown ||Dark Red |
|2 2 ||Cassava Leaf Tree ||Bright Green ||Dark green ||Greenish yellow|
Acid molecule would be ionized into ions H + and other constituent substances (such as SO 4 2 - etc.). The ions H + is involved in the nature of the acidity of a substance. In fact, the ions H + an acid dissolved in water are in a condition not a stand-alone proton, but it binds with water molecules to form a molecule that can form H 3 O + (aq), or H 9 O 4 + (aq ). (Purba Dkk,1990:116)
Base ionized molecules is OH - ions and the other constituent substances (such as Na + and others). these ions OH - that role in the base nature in a substance.
From the results of experiments carried out, both extracts of hibiscus flower and leaf extract of cassava showed color changes when the indicators of natural acids and bases are included in the test tube which had previously been included acid solution or alkaline solution. In NaOH which is the base, hibiscus extract change color from purple to brown and the extract of cassava leaves change color from light green to dark green. The same thing happens also in the X solution. Means, based on Bronsted-Lowry acid base theory (Purba Dkk, 1990) the proton from the hibiscus extract received by OH ions - thus causing color changes in the hibiscus extract from purple to brown and the protons of cassava leaf extract received by the OH - ions thus causing discoloration on cassava leaf extracts from light green to dark green. So, since this happens when the test of X solution , then X is also the a base. (Purba Dkk,1990:116). This is in accordance with the opinion of Kimball (1992:48-49) that the OH - ion from the base compound will be bound to the high side of electronegativity natural indicators.
While the acidic HCL, hibiscus extract changes color from purple to red and cassava leaf extract changed color from light green to yellow. The same thing happened to Y solution. Means, based on acid base theory Bronsted-Lowry (Ancient Dkk, 1990) H + is donated by HCl and accepted by hibiscus extract resulted in color changes from purple to red, and H + are donated by HCl and accepted by cassava leaf extract resulted in a change color from light green to yellow. So, since this also happens when the test of Y solution, then Y is also acid. As for, extracts from both acids and bases indicators natural does not have any color changes when the two indicators are included in the test tube containing neutral aquades. (Purba Dkk,1990:116). This is in accordance with the opinion Kimball (1992:48-49) that H + ions from acidic compounds will be bound with high electronegativity side of natural indicators.
CONCLUSION AND ADVICE
X Solution is base. This is because when a natural indicator, either extract of hibiscus or extract of cassava leaves put into a test tube containing an X solution, a natural indicator changes color immediately and became the same color with the color of natural indicators as it is put into the test tube containing the NaOH which is base.
Y is an acid.This is because when the natural indicators, either hibiscus extract or extract of cassava leaf are put into the test tube containing a Y solution, the natural indicator changes color quickly became the color of the exact color of a natural indicator when they are put into a test tube which is containing HCl acid .
This practice is recommended to use more natural indicator species. For example, from a variety of flowers, leaves and plants.
Kimball, John W. 1991. Biologi Jilid I. Erlangga. Jakarta
Purba, Michael. 2009. Kimia. Erlangga. Jakarta
Purba, Michael., Soetopo Hidayat., dan Sarwono Hadi. 1990. Ilmu Kimia. Erlangga. Jakarta
Purwanto, Budi dan Arinto Nugroho. 2008. Eksplorasi Ilmu Alam I. Tiga Serangkai. Solo