Discussion of Results and Scientific Explanation
To achieve the goal of determining an unknown acid or base, anion and cation test were performed. These tests allowed for the components of the solutions to be identified, therefore, achieve the second goal. The pH was also used in to help find the identity of the unknown solutions and it was used to draw conclusions about the solutions. Dilutions were used to help understand the different behaviors of different concentrations. Household items were used to help apply basic theories about acids and bases such as pH. The titration curves were evaluated to help determine which indicator needed to be used for the indicator titration.
Anions are ions that have more electrons than protons, therefore, are negatively charged¬¬1. During electrolysis they are attracted to the anode and they are nonmetals. Cations are ions that have more protons than electrons, therefore, are positively charged. They are attracted to the cathode during electrolysis and are metals. In order to determine which anions and cations were present in the solutions, anion and cation test were performed. First a pH test using litmus paper was performed. Litmus paper is an indicator used to determine whether a substance is acidic or basic when its environment is changed. When the environment is changed from acidic to basic, the molecule changes. The molecule will turn into an ionized salt which will result in a color change from red to blue. Litmus paper turns blue when it comes in contact with a base and it stays red when it comes in contact with an acid. The colors of the paper are results that represent wavelengths. Acids have a lower energy, therefore, they result in red colors because red has the lowest energy level. Bases have a higher energy, therefore, resulting in a blue color with a high energy level. Substances that give off hydrogen ions when in water are known as acids, which have a pH lower than seven. Bases have a pH higher than seven and they give off hydroxide ion when in water. The pH helped determine whether a solution had the possibility of contain hydrogen or hydroxide ion.
As indicated in Table 1, the first solution, formed a white precipitate during the chloride ion test because the chloride ion reacted with the AgNO¬3¬ and formed AgCl which is a solid2. The other two solutions did not form a precipitate because there was no chloride present to form the solid product. During a positive nitrate ion test a brown ring is supposed to form when there is a nitrate ion present because there will be a junction between the two liquids. A positive acetate ion test will produce a fruity smell because CH3COOCH2CH3, which has a fruity smell, will form.
To identify the cations present in a solution, the ammonium test and flame test are used as indicated in Table 2. A positive ammonium test will produce an ammonia smell because the ammonium will react with the hydroxide ions and form the gas NH3. The flame test uses the presence of colors to determine which ions are present. When an electron gets excited it will jump up to a higher energy level and then fall back down to a lower level. When the electron falls down to the lower level it emits energy in the form of light. The color of the flame represents the amount of energy released. Potassium emits a pale violet color flame because it as a shorter wavelength of about 380-450 nanometers while sodium emits an orange/yellow light because its wavelength is between 570-620 nanometers.
The identity of the solution was determined using the results from the anion and cation test as indicated in Table 3. If none of the anion tests were positive it was presumed that the solution contained hydroxide and if none of the cation test were positive it was presumed that the solution contained hydrogen.
When strong acids are placed in water, it gives away all its hydrogen ions, therefore, the solution is 100% ionized. A strong base also is 100% ionized in water because it away all of its hydroxide ions. Since the solution ionize completely, the M1V1 = M2V2 equation can be used. Therefore, since weak acids and bases do not dissociate completely that equation cannot be used. Instead the equation Ka=([H][A])/([HA]) must be used. For a weak base, the same equation is used except Ka is now Kb. Ka is the ionization constant for an acid while Kb is the ionization constant for a base3. The Henderson-Hasselbalch equation can be used in order to estimate the pH of a buffer and to find the equilibrium pH during an acid-base reaction4. The equation is as followed, pH = pKa + log ([A])/([HA]) where A and HA are the conjugate base and starting acid.
During this experiment, acid-base titrations were performed on each solution as indicated in Figures 1-3. The titration was performed so that the solutions could be neutralized. Titration curves can be used to evaluate the pH when a titrant is added. You can calculate the pH from the curve by first determining how much titrant will be added. You can then evaluate the intersection between that volume and the graph. This intersection will tell you the pH at that volume. All titration curves should begin with slow movement as the titrant is added. It should then increase in speed before and then it should slow down again. This format is seen because at the start of the titration the titrant is added which slowly causes the pH to change. Then when the solution reaches neutralization the curve quickly changes. This point of change is called the equivalence point and it is where the hydrogen ions is equal to the hydroxide ions. The point commonly occurs around the pH of seven. After the equivalence point is reached, the solution will then start to be concentrated with hydrogen or hydroxide ions depending on the starting and titrant solutions. If the titrant is acidic, the solution will decrease in pH after the equivalence point until it reaches the pH of the titrant. The same theory is followed when the titrant is basic, except the pH of the solution increases instead of decreasing5.
The pH of these solutions was measured using a glass electrode pH probe. This devise uses a glass electrode and a reference electrode to measure pH6. The probe measured the voltage difference between the two electrodes. When there is a difference in pH between the solutions inside and outside the membrane of the electrode, the glass electrode generates an electromotive force. The glass in the probe is the membrane of the electrode. Inside the membrane, the liquid has a pH of seven so that a comparison can be made.
As shown in Figure 1, the titration curve for NH4Cl does not show a sharp increase or decrease near the equivalence point. This occurred because the solution was already close to neutral before the titration began, therefore, not much titrant was needed to reach the equivalence point. The titration curves for NaOH and NaC2H3O2 have various small jumps within the curves as seen in Figures 2-3. This could have occurred because the solutions could have contained impurities which could disrupt the format of the curves.
The indicator titrations were performed after the Pasco titrations because the indicator had to be determined from the titration curves. The appropriate indicated for each solution had to be determined from the equivalence point. Indicators are chemicals that undergo a color change when it is mixed with an acid or base. As seen in Table 4, cresol red was used as the indicator for NH4Cl and methyl red was used for NaOH and NaC2H3O2. Cresol red was used because its pH range was from 7.2-8.8 and the equivalence point for NH4Cl was in between those values. The pH range for methyl red was between 4.2-6.3 and the equivalence points for NaOH and NaC2H3O2 were in between those values. Once both of the titrations were performed, the concentration of all of the solutions could be calculated. As indicated in Table 4 the concentrations for the solutions were different for the Pasco and indicator titrations. The difference could have occurred because there could have been an issue with accuracy. The volume recorded in the indicator titration could have been wrong because the correct value was supposed to be determined exactly when the color changed. Therefore, if the stopcock value was not closed at the specific moment of color change, the volume could have been wrong. Another reason for a difference in concentrations could be the amount of indicator added. Some students had to add more than one drop to their solutions and this could have reacted differently causes a change in the amount of volume needed to change the color of the solution.
Molarity is defined as the moles of the solute divided by the liters of the solution. Molarity affects the pH of a solutions because the amount of hydrogen or hydroxide ions is changing7. When you dilute a solution you are decreasing the molarity because you are added more solvent without adding anymore solute, therefore you are changing the pH. When diluting an acid, you will increase the pH because the concentration of hydrogen ions is decreasing. Since the equation for pH is as follows, pH = -log [H+], the value for pH increases as the concentration of hydrogen ions decreases. When diluting a base, the pH will decrease because the concentration of hydroxide ions is decreasing. Since pH = 14 – pOH, the lower the concentration of hydroxide ions the lower the pH will be. When diluting a substance using one milliliter of solute and nine milliliters of water, the pH should change by one for each dilution. As indicated in Table 5 this did not occur. The reason why this did not occur could be because the water that was used for these dilutions was not at a neutral pH of 7. Instead it was a little acidic with a pH of 6.45.
Most household items and chemicals have acidic or basic products. As indicated in Table 6, lemon juice and fabric softener have a low pH, therefore, it is acidic. Coffee creamer is slightly basic but it close to neutral. Lemon juice is acidic because it contains citric acid. Fabric softener is acidic in order to help neutralize detergent and decrease the amount of static electricity. Coffee creamer is close to neutral in order to better neutralize coffee.
As indicated in Figure 4 there are very small disturbances in the titration curve for lemon juice. This could have occurred because the solution might not have been completely homogenous because it could have contain small pieces of lemons. As indicated in Figure 5 the titration curve for coffee creamer dropped very quickly. This occurred because the solution was already close to neutral before the titration began. As indicated by Figure 6 the beginning of the titration curve for fabric softener was completely flat. This could have occurred because there could have been an error in cleaning the burette. Some hydrochloric acid could have been left over in the burette when sodium hydroxide was added, therefore, causing an error in the beginning of the curve.
This project focused on certain properties of acids and bases such as pH and concentration. We evaluated the effects of dilutions on pH and molarity. Simple acid and basic properties were evaluated within household items in order to show how acids and bases are components of our everyday lives. This project was very similar to previous experiments I have done, therefore, I had a great understanding of the theories and tests involved in this project. Before starting the experiment, I already had a strong understanding on how to perform the anion and cation test. Also, I had experience on performing dilutions and titrations using Pasco. The only new part of this experiment was the titration using an indicator.