We had an experiment in class: Soda Ash Titration using Double Indicator Method
How would you know the composition of the sample based on the comparison volume of titrant (HCl) used? i.e. if Vmo > Vph, the sample is made up of NaHCO3 and Na2CO3. why is it like that?
Na2CO3 + HCl ==> NaHCO3 + H2O so the Na2CO3 is titrated half-way (one H has been added as opposed two) at the phenolphthalein end point. So if it takes 20 mL to add one H, it follows that it will take 20 more (for a total of 40) to reach the methyl orange end point. Thus a PURE sample of Na2CO3 (with no NaHCO3) will have the same volume for each indicator BUT if the sample is made up of Na2CO3 as well as some NaHCO3, then the phenolphthalein end point tell you that one H has been added to the Na2CO3 (nothing happens to the NaHCO3 at that point)and the m.o. end point titrates the second H to the Na2CO3 PLUS the added H to the NaHCO3 there originally. Hope this helps.
To determine the composition of the sample based on the comparison of titrant volumes, you would need to understand the chemistry behind the Soda Ash Titration using the Double Indicator Method.
In this method, you titrate a mixture of sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3) with hydrochloric acid (HCl). The purpose of the double indicator is to detect the point at which both sodium bicarbonate and sodium carbonate have reacted.
When HCl reacts with sodium bicarbonate, the following reaction occurs:
NaHCO3 + HCl → NaCl + H2O + CO2
When HCl reacts with sodium carbonate, the following reaction occurs:
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
In both reactions, carbon dioxide is released as a gas. The double indicator used in this method changes color when carbon dioxide is formed. It typically involves a mixture of bromothymol blue and phenolphthalein indicators.
Now, let's consider the comparison of titrant volume:
1. If Vmo (volume of titrant required to reach the initial endpoint) is greater than Vph (volume of titrant required to reach the phenolphthalein endpoint), it suggests that the sample contains both sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3).
Explanation:
- Vmo represents the volume of HCl required to neutralize only sodium bicarbonate in the sample.
- Vph represents the volume of HCl required to neutralize both sodium bicarbonate and sodium carbonate in the sample.
- Since Vmo is greater than Vph, it indicates that there is more sodium carbonate present in the sample than sodium bicarbonate.
- This is because the reaction between HCl and sodium bicarbonate is less vigorous compared to the reaction between HCl and sodium carbonate.
- As a result, more HCl is needed to neutralize the sodium carbonate, leading to a larger volume of titrant (HCl) required to reach the phenolphthalein endpoint.
By analyzing the comparison of titrant volumes, you can deduce the composition of the sample and determine if it contains both sodium bicarbonate and sodium carbonate.
To determine the composition of the sample based on the comparison volume of titrant used in the Soda Ash Titration using the Double Indicator Method, you need to understand the principle behind it. The Double Indicator Method is commonly used to determine the percentage of sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3) in a mixture.
In this method, two indicators are used: methyl orange and phenolphthalein. Methyl orange changes color at a pH value of around 4.5, shifting from orange to pink. Phenolphthalein changes color at a pH value of around 8.2, transitioning from colorless to pink. By using these two indicators, we can detect the presence of both sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3) in the sample.
When performing the titration, here's how the reaction progresses:
1. At the beginning of the titration, the sodium bicarbonate (NaHCO3) reacts with the hydrochloric acid (HCl) to form water (H2O), carbon dioxide (CO2), and sodium chloride (NaCl) according to the equation:
NaHCO3 + HCl → H2O + CO2 + NaCl
2. As the reaction goes on, the sodium bicarbonate gets consumed, and the excess hydrochloric acid reacts with the remaining sodium carbonate (Na2CO3) to form water, carbon dioxide, and sodium chloride:
Na2CO3 + 2HCl → H2O + CO2 + 2NaCl
Now, let's examine the comparison volume of titrant (HCl) used and how it relates to the composition of the sample:
- If the volume of titrant (Vmo) is greater than the phenolphthalein endpoint volume (Vph), it indicates that all the sodium bicarbonate (NaHCO3) has been reacted. At this point, only sodium carbonate (Na2CO3) is left in the solution. This is because the reaction between HCl and NaHCO3 occurs before the reaction between HCl and Na2CO3. Therefore, if Vmo > Vph, it suggests that the sample is made up of Na2CO3 only.
- On the other hand, if the volume of titrant (Vmo) is less than or equal to the phenolphthalein endpoint volume (Vph), it means that there is still unreacted NaHCO3 present even after all the Na2CO3 has been reacted. Thus, in this case, the sample is composed of a mixture of NaHCO3 and Na2CO3.
So, by comparing Vmo and Vph, you can determine the composition of the sample. If Vmo > Vph, the sample contains only Na2CO3. If Vmo ≤ Vph, the sample is a mixture of NaHCO3 and Na2CO3.
It's important to note that careful titration technique and accurate measurement of volumes are key to obtaining reliable results.