The Use of Volumetric Flask, Burette and Pipette in Determining the Concentration of NaOH Solution
Title
The Use of Volumetric Flask, Burette and Pipette in Determining the Concentration of NaOH Solution
Objective
* To determine the number of ionizable hydrogen in an unknown acid.
* To determine the equivalent weight of an unknown acid.
* To determine the enthalpy change for the ionization of an unknown acid.
* To use the technique of volumetric analysis or titration to determine the concentration of a given NaOH solution.
Theory And Background
In 1855, the German chemist, Friedrich Mohrn defined titration as the "weighing without scale" method because this process allows determination of the concentration of a sample without using complex instrumentation. A manual titration requires high accuracy and precision, both in the preparation of the material, and the use of different precisely dosed reagents. The operation must be repeated at least 3 times to obtain a reliable measured value. This procedure makes the manual analytical technique very long and fastidious.
Titration is the quantitative measurement of an analyte in solution by reacting it completely with a standardized reagent. For example, a given volume of a solution of unknown acidity may be titrated with a base of known concentration until complete Neutralization has occurred. Acids and bases react until one of the reactants is consumed completely. A solution of base of known concentration can therefore be used to titrate an acid solution of unknown concentration. Likewise, an acid solution of known concentration can be used to titrate a base solution of unknown concentration. The point at which all of the analyte is consumed is the equivalence point and is generally determined by observing a color change in an added indicator such as phenolphthalein.
The term "end point" is where the indicator changes colour. That isn't necessarily exactly the same as the equivalence point. This means that at the equivalence point (where you had mixed the solutions in the correct proportions according to the equation), the solution wouldn't actually be neutral. To use the term "neutral point" in this context would be misleading. The equivalence point is often determined by visual indicators are available for titration based on acid-base neutralization, complexation, redox reactions and is determined by some type of indicator that is also present in the solution. For acid-base titration, indicators are available that change color when the pH changes. When all of the analyte is neutralized, further addition of the titrant causes the pH of the solution to change causing the color of the indicator to change.
The G.N. Lewis (1923) idea of acids and bases is broader than the Brønsted-Lowry model. The Lewis definitions are: acids are electron pair acceptors, and bases are electron pair donors. Each ionizable pair has a proton donor and a proton acceptor. Acids are paired with bases. One can accept a proton and the other can donate a proton. In the Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons. A Lewis acid is therefore any substance, such as the H+ ion, that can accept a pair of nonbonding electrons. In other words, a Lewis acid is an electron-pair acceptor. A Lewis base is any substance, such as the OH- ion, that can donate a pair of nonbonding electrons. A Lewis base is therefore an electron-pair donor. The Lewis theory suggests that acids react with bases to share a pair of electrons, with no change in the oxidation numbers of any atoms. Many chemical reactions can be sorted into one or the other of these classes. Either electron is transferred from one atom to another, or the atoms come together to share a pair of electrons.
The true point of neutralization in any titration occurs when the amounts of acid and alkali added together are chemically equivalent to each other.
Using a calibrated burette to add the titrant, it is possible to determine the exact amount that has been consumed when the endpoint is reached. The endpoint of a titration is when the pH of the reactant is just about equal to 7, and when the reactant stops reverting back to its original color.
Acid-base indicators are substances, which change colour according to the hydrogen ion concentration of the liquid in which they are placed. They either weak acids or weak bases, and are therefore slightly dissociated when dissolved in water. The color of the indicator depends on the colour of the ions produced.
As the titration is close to its end, indicator molecules near a drop of added base change colour due to the temporary excess of OH- ions there. As soon alence point in the titration occurs when all the moles of H+ ions present in the original volume of acid solution have reacted with an equivalent number of moles of OH- ions added from the burette.
Moles of H+ (originally in flask) = moles of OH- (added from the burette)
The end point of the titration occurs when a tiny excess of OH- ions changes the indicator ...
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As the titration is close to its end, indicator molecules near a drop of added base change colour due to the temporary excess of OH- ions there. As soon alence point in the titration occurs when all the moles of H+ ions present in the original volume of acid solution have reacted with an equivalent number of moles of OH- ions added from the burette.
Moles of H+ (originally in flask) = moles of OH- (added from the burette)
The end point of the titration occurs when a tiny excess of OH- ions changes the indicator permanently to its color in base. In calculations, we assume this tiny excess is insignificant, and therefore the amount of base needed to reach the end point is the same as the amount needed to reach the equivalent point.
For phenolphthalein, the endpoint is the first permanent pale pink. The pale pink fades in 10 to 20 minutes. If you think you might have reached the endpoint, you can record the volume reading and add another partial drop. Sometimes it is easier to tell when you have gone past the endpoint. When you have reached the endpoint, read the final volume in the burette and record it. Subtract the initial volume to determine the amount of titrant delivered. Use this, the concentration of the titrant, and the stoichiometry of the titration reaction to calculate the number of moles of reactant in your analyte solution.
At the end-point of the experiment, the volume of the acid times the molarity of its hydrogen ions will equal the volume of the base times the molarity of its hydroxide ions. Calculations will be based on the following formula:
(Volume of acid)(molarity of H+) = (volume of base)(molarity of OH-)
Or, more simply
VaMH+ = VbMOH-
Knowing three of these quantities we can calculate the fourth -- in this case the molarity of the acid's hydrogen ions. Since the acids and the base in this experiment have one hydrogen ion and one hydroxide ion, respectively, the molarity of hydrogen ion is the same as the molarity of the acid, and the molarity of hydroxide ion is the same as the molarity of the base.
NaOH + HCl -> NaCl + H2O
Materials and Apparatus
50cm Burette Phenolphthalein Solution
HCI Solution (1.000 x 10-2M) Watch Glass
H2SO4 Solution (1.000x10-2M) Ring Stand
500ml Beaker NaOH Solution
25ml Volumetric Flask Pipette Bulb
Funnel 250cm Erlenmeyer flask
20cm or 25cm Pipette Distilled water
Suction
Experimental Procedures
. The volumetric flask cleaned and rinse with distilled water.
2. All NaOH solution transferred into the volumetric flask using the funnel and the remaining NaOH solution is washed into the flask several time using distilled water.
3. The NaOH solution is topped up to 250cm³ with distilled water, the cap is closed and the flask is rotated several times to get a homogenous solution.
4. This solution is poured into a clean and dry beaker, labels it, covered with a watch glass and put it inside. Volumetric flask is cleaned.
5. Burette cleaned with distilled water and rinse with 5cm³ NaOH solution a few times. Burette filled with NaOH solution using a funnel. The pipette is cleaned and rinse a few times with the acid to be used in the titration. (Note: Burette and pipette must be rinsed 2-3 times with 5 cm³ of solution to be used. Every time filling up burette using a funnel, the funnel is held to avoid the breakage to the burette and spill of fluid due to the displacement of air. Do not try to fill the burette to zero mark. Burette readings must be accurate up to two decimal places and the flow rate of the liquid should not exceed 30cm³ per minute.
6. Pipette 20.0cm³ (or 25cm) acid solution into three clean Erlenmeyer flasks.
7. Two drops of phenolphthalein are added to the acid solution. (The solution should remain colourless and do not use in excess)
8. The initial burette reading is recorded and acid solution is titrated with NaOH solution in the burette until end-point is reached (when the solution turns pink). The end reading is recorded to obtain the volume of NaOH solution used.
9. The titration is repeated for several times until the NaOH solution volumes remain constant within 3/1000 for three titration, i.e.
Volume reading - Average volume reading
--------------------------------------------------------- X 1000 <= 3
Average volume reading
Result and Data
Titration of NaOH with HCl
Particulars
Rough
X1
X2
X3
Average
Initial reading (cm3)
0.00
0.00
0.00
0.00
0.00
Final reading (cm3)
2.20
2.40
2.40
2.40
2.40
NaOH volume (cm3)
2.20
2.40
2.40
2.40
2.40
Titration of NaOH with H2SO4
Particulars
Rough
X1
X2
X3
Average
Initial reading (cm3)
0.00
0.00
0.00
0.00
0.00
Final reading (cm3)
39.90
39.80
40.00
39.90
39.90
NaOH volume (cm3)
39.90
39.80
40.00
39.90
39.90
Discussion
Titration is the quantitative measurement of an analyte in solution by reacting it completely with a standardized reagent. In titration, a solution of known concentration, called a standard solution, is added to a solution of unknown concentration until the amount of reagent prescribed by the stoichiometric relationship between reactants is furnished. The process of determining the concentration of a solution is called standardize. The point at which all of the analyte is consumed is the equivalence point. The number of moles of analyte is calculated from the volume of reagent that is required to react with all of the analyte, the titrant concentration, and the reaction stoichiometry.
Indicators that are available for titrations based on acid-base neutralization, complextion, and redox reactions often determine the equivalence point. For acid-base titrations, indicators are available that change color when the pH changes. When all of the analyte is neutralized, further addition of the titrant causes the pH of the solution to change causing the color of the indicator to change.
A suitable indicator added to the unknown solution changes colour when the stoichiometric required amount of standard solution is added. Standard solution is added drop wise until a single drop causes the indicator undergo permanent colour change. This is called the end point.
Phenolphthalein is a sensitive pH indicator with the formula C20H14O4. It is often use in titration, its turns from colourless in acidic solutions to pink in basic solutions, the colour change occurring between pH 8 and pH 10. If the concentration of indicator is particularly strongly, it can appear purple.
All acid-base titration reactions are simply exchanges of protons. The reaction may be strong acid + strong base --> salt (neutral). For this experiments, we use hydrochloric acid and sodium hydroxide, the equation are as below:
HCl + NaOH --> NaCl + H2O
Although the reaction may be correctly written as H3O+ + OH- --> H2O since strong acids and strong bases are totally dissociated to protons and hydroxide ions in water. For reactions which are strong acid + weak base --> salt (acidic). For example
HCl + CH3NH2 --> CH3NH3+Cl-,
For reaction which are strong base + weak acid --> salt (basic) For example
NaOH + CH3COOH --> Na+CH3COO- + H2O
The cations and anions could be omitted as they do not actually participate in the reaction. These ion are called bystander ions.
Mostly using a strong acid or strong base can carried out the asid base titration. Strong base or strong acid is used as the titrant. If put the strong acid or strong base in the titration vessel and use the weak acid or weak base as the titrant, it is quite less common. A weak acid-weak base titration have a small pH change at the equivalence point. This small change is difficult to observe and detect, as a result the weak acid-weak base titrations are not common. We must be use a standard for which the amount of substance present is known, one of the amount of substances in a titration that we know should accurately. The standard can be present either in the form of a pure substance or a standard solution, which is a solution whose composition is known. A standard can be considered in two ways that is use a primary standard or standardize by titration with some previously standardized solution.
A pH indicator is a chemical compound that is added in small amounts to a solution so that the pH either is acidity or alkalinity of the solution can be determined easily. Hence a pH indicator is a chemical detector for protons (H+). Normally, the indicator causes the color of the solution to change depending on the pH. pH indicators themselves are frequently weak acids or weak bases. They bind Hydrogen ion (H+) or hydroxide ions (OH-) when they are introduced into a solution. The different electronic configuration of the bound indicator causes the indicator's color to change.of determination of color, pH indicators are susceptible to imprecise readings. To precise the measurement of pH, a pH meter is frequently used. The usage of pH indicators are often employed in titrations in analytic chemistry and biology experiments to determine the extent of a chemical reaction. Example of pH indicator is phenolphthalein with the formula C20H14O4, which is a substance that changes color as the pH of a solution changes. Phenolphthalein turns from colorless in acidic solutions to pink in basic solutions, the color change occurring between pH 8 and pH 10. If the concentration of indicator is particularly strong, it can appear purple in basic solution. Phenolphthalein is insoluble in water, and is usually dissolved in alcohols for use in experiments. It is itself a weak acid, which can lose H+ ions in solution. The phenolphthalein molecule is colorless. However, the phenolphthalein ion is pink. When a base is added to the phenolphthalein, the atom ions equilibrium shifts to the ionization because H+ ions are removed, this refer to Le Chatelier's principle.
It is used to dispense known amounts of a liquid reagent in experiments for which such precision is necessary, such as a titration experiment. The precision of a burette makes careful measurement with a burette very important to prevent systematic error.
When carrying out an acid-base titration, we must able to recognize when to stop adding the standard solution. That is, we must be able to recognize when neutralisation has occurred. This is the purpose of the indicator. A sudden color change due to the indicator signals that neutralisation has occurred. At this point, the number of hydrogen ions (H+) from the acid is equal to the number of hydroxide ions (OH-) from the base. The point at which this occurs is called the end-point of the titration. When the endpoint is reached, the volume of the standard solution is carefully determined. Then the measured volumes of the two solutions and the known concentration of the standard solution can be used to calculate the concentration of the other solution
We have to work out and balance the equation of the reactants. It can now be determined how many moles of reactants will neutralize a mole of the solution. We know the number of moles of the standard solution used (concentration multiplied by volume) and can then use this to determine the moles in the unknown solution. By then dividing this by the volume of the unknown solution, we can work out the concentration.
Precaution steps
Make sure to wear goggles and aprons during the entire course of the lab,which includes all clean-up time..
Handle all glass with proper care, making sure to not drop or knock over any pieces.
This lab involves handling acid and base solutions, which can splash if dropped. Handle all solutions with proper care, making sure to protect us from spills.
Should a spill occur, inform teacher at once
Pour all solutions into the sink, and then rinse with distilled water. Make sure to run some distilled water out the tip of the burette. Discard rinse water in the sink. Run tap water for 30 seconds to rinse sink and flush all solutions into the sewer.
Dry all other equipment that is wet, including the tabletop. Put all materials onto a dry paper towel near the back of the table.
Significant attention is paid to the proper technique for reading a meniscus
While reading the level of burette, the viewer's eyes must be at the level of the graduation to prevent parallax error level
Remember to wash the reaction beaker, pipette with distilled water before each titration.
Avoid contact of all chemicals with eyes or skin.
To fill a burette, close the stopcock at the bottom and use a funnel. You may need to lift up on the funnel slightly, to allow the solution to flow in freely.
Due to the precision of the burette, even a single drop of liquid hanging from the bottom of a burette should be transferred to the receiving flask by touching the drop to the side of the receiving flask and washing into the solution with the experimental solvent usually water. Through careful control of the stopcock and rinsing, even partial drops of liquid can be added to the receiving flask.
Before titrating, condition the burette with titrant solution and check that the burette is flowing freely. To condition a piece of glassware, rinse it so that all surfaces are coated with solution, then drain. Conditioning two or three times will insure that a stray drop of water does not change the concentration of titrant.
Check the tip of the burette for an air bubble. To remove an air bubble, whack the side of the burette tip while solution is flowing. This is because if an air bubble is present during a titration, volume readings may be in error.
Carefully fill the flask with distilled water and NaOH solution until almost reach the mark. Make sure you move your eye to the level of the mark on the neck of the flask. Then add distilled water a drop at a time until the bottom of the meniscus lines up exactly with the mark on the neck of the flask. Take care that no drops of liquid are in the neck of the flask above the mark. After the final dilution, remember to mix your solution thoroughly, by inverting the flask and shaking.
When NaOH solution is add drop by drop to the volumetric flask that contains HCl or H2SO4 and phenolphthalein indicator, stop the titration immediately when a slightly pink colour is observed.
Add the phenolphthalein indicator before doing the titration because without the present of indicator, there is no color changes can be observed. Only small amount of phenolphthalein is needed and excess amount of phenolphthalein will affect the colour of the indicator. It can appear purple colour if the concentration of indicator is particularly strong as the indicator is an acidic solution, excess amount of this indicator may decrease the pH of the acid solution and affect the result of the titration.
The possible error in this experiment were: the error in taking the burette readings, the error in measuring amount of elements, and the NaOH was not stable under air.
Questions:
(1) Calculate the concentration of NaOH solution.
Concentration of base (diluted solution)
M1V1 = M2V2
M1(12.40) = (0.01)(25)
M = (0.01)(25)/12.40
= 0.0202M
The concentration of the diluted NaOH is 0.0202 molar, which is approximately 0.02 molar. From the molar of the diluted NaOH, we compare the concentration of the original NaOH,
Concentration of base (original solution)
M1V1 = M2V2
M1(50) = (0.0202)(250)
M = (0.0202)(250)/50
= 0.0101M
The concentration of the NaOH solution used is 0.0101 molar.
(2) Distinguish between acid strength and acid concentration.
Acid strength is the percentage of ionization of the acid when dissolve in water while acid concentration is the amount of dissolved acidic solutes in the solution.
(3) Distinguish between a weak base and an insoluble base.
A weak base is a chemical base that does not ionize fully in an aqueous solution. This results in a relatively low pH level. Weak bases exist in equillibrium much in the same way as weak acids do, with a Base Ionization Constant (Kb) indicating the strength of the base. Not many metal hydroxides are soluble; the ones that are comprise the strong soluble bases. Hydroxides that are only slightly soluble in water (such as calcium hydroxide or iron(III) hydroxide) are strong bases, because whatever amount does dissolve dissociates completely into the ions. So, we can say that weak base has a lower pH level compared to insoluble base because weak base does not ionize fully in aqueous solution, whereas insoluble base are strong base because most of them ionize fully in water.
Conclusions
From the titration results of three trials, the concentration of NaOH solution in the diluted acid solution is 0.0202M. The concentration of NaOH solution in the original acid solution is 0.0101M.
References
* Fundamental of Analytical Chemistry 7th edition by Skoog, West and Holler, Saunders Publishers. 1996
* Laboratory Experiments for Chemistry, A Basic Introduction, 4th edition, by Wynn and Joppich, Wadsworth Inc, 1987
* http://www.dartmouth.edu/~chemlab/techniques/titration.html
* http://www.chem.vt.edu/chem-ed/titration/titratn.html
* http://en.wikipedia.org/wiki/Titration
* http://www.hannainst.com/products/prodline/titrato.cfm
* http://www.essaysample.com/essay/000062.html
* http://wwwsoc.nii.ac.jp/jsac/analsci/pdfs/a15_0611.pdf
