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MCAT
Kurs: MCAT > Rozdział 9
Lekcja 2: Titrations- Titration questions
- Miareczkowanie kwasów i zasad
- Przykład krok po kroku: Wyznaczanie stężenia na podstawie miareczkowania kwasowo-zasadowego
- Miareczkowanie mocnego kwasu mocną zasadą
- Miareczkowanie mocnego kwasu mocną zasadą (ciąg dalszy)
- Miareczkowanie słabego kwasu mocną zasadą
- Miareczkowanie słabego kwasu mocną zasadą (ciąg dalszy)
- Miareczkowanie słabej zasady mocnym kwasem
- Miareczkowanie słabej zasady mocnym kwasem (ciąg dalszy)
- Acid-base titration curves
- Krzywe miareczkowania i wskaźniki pH
- Miareczkowanie redoksymetryczne
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Acid-base titration curves
Before we start discussing about titration and titration curves, we should quickly refresh the concept of a weak/strong acid and weak/strong base.
A strong acid dissociates (or ionizes) completely in aqueous solution to form hydronium ions (Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript)
A weak acid does not dissociate completely in aqueous solution to form hydronium ions (Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript)
A strong base dissociates completely in aqueous solution to form hydroxide ions (OHstart superscript, start text, negative, end text, end superscript)
A weak base does not dissociate completely in aqueous solution to form hydroxide ions (OHstart superscript, start text, negative, end text, end superscript)
Examples of weak/strong acids and bases
| Type | Examples |
|---|---|
| Strong Acids | hydrochloric acid (HCl), sulfuric acid (Hstart subscript, 2, end subscriptSOstart subscript, 4, end subscript), nitric acid (HNOstart subscript, 3, end subscript) |
| Weak Acids | acetic acid (CHstart subscript, 3, end subscriptCOOH), hydrofluoric acid (HF), oxalic acid (COOH)start subscript, 2, end subscript |
| Strong Bases | sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH) |
| Weak Bases | ammonium hydroxide (NHstart subscript, 4, end subscriptOH), ammonia (NHstart subscript, 3, end subscript) |
Weak acids and weak bases always exist as conjugate acid-base pairs in an aqueous solution as represented below
Here, HA is the acid and Astart superscript, start text, negative, end text, end superscript is termed as the conjugate base of HA
In the above reaction, Astart superscript, start text, negative, end text, end superscript is a base and HA is the conjugate acid of Astart superscript, start text, negative, end text, end superscript
Rule of thumb is: Weak acids have strong conjugate bases, while weak bases have strong conjugate acids. As shown in the above two reactions, if HA is a weak acid, then its conjugate base Astart superscript, start text, negative, end text, end superscript will be a strong base. Similarly, if Astart superscript, start text, negative, end text, end superscript is a weak base, then its conjugate acid HA will be a strong acid.
How do we define ‘titration’?
Titration is a technique to determine the concentration of an unknown solution. As illustrated in the titration setup above, a solution of known concentration (titrant) is used to determine the concentration of an unknown solution (titrand or analyte).
Typically, the titrant (the solution of known concentration) is added through a burette to a known volume of the analyte (the solution of unknown concentration) until the reaction is complete. Knowing the volume of titrant added allows us to determine the concentration of the unknown analyte. Often, an indicator is used to signal the end of the reaction, the endpoint. Titrant and analyte is a pair of acid and base. Acid-base titrations are monitored by the change of pH as titration progresses.
Let us be clear about some terminologies before we get into the discussion of titration curves.
- Titrant: solution of a known concentration, which is added to another solution whose concentration has to be determined.
- Titrand or analyte: the solution whose concentration has to be determined.
- Equivalence point: point in titration at which the amount of titrant added is just enough to completely neutralize the analyte solution. At the equivalence point in an acid-base titration, moles of base = moles of acid and the solution only contains salt and water.
Acid-base titrations are monitored by the change of pH as titration progresses
Indicator: For the purposes of this tutorial, it’s good enough to know that an indicator is a weak acid or base that is added to the analyte solution, and it changes color when the equivalence point is reached i.e. the point at which the amount of titrant added is just enough to completely neutralize the analyte solution. The point at which the indicator changes color is called the endpoint. So the addition of an indicator to the analyte solution helps us to visually spot the equivalence point in an acid-base titration.
Endpoint: refers to the point at which the indicator changes color in an acid-base titration.
What is a titration curve?
A titration curve is the plot of the pH of the analyte solution versus the volume of the titrant added as the titration progresses.
Let’s attempt to draw some titration curves now.
1) Titration of a strong acid with a strong base
Suppose our analyte is hydrochloric acid HCl (strong acid) and the titrant is sodium hydroxide NaOH (strong base). If we start plotting the pH of the analyte against the volume of NaOH that we are adding from the burette, we will get a titration curve as shown below.
Point 1: No NaOH added yet, so the pH of the analyte is low (it predominantly contains Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript from dissociation of HCl).
As NaOH is added dropwise, Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript slowly starts getting consumed by OHstart superscript, start text, negative, end text, end superscript produced by dissociation of NaOH. Analyte is still acidic due to predominance of Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript ions.
Point 2: This is the pH recorded at a time point just before complete neutralization takes place.
Point 3: This is the equivalence point (halfway up the steep curve). At this point, moles of NaOH added = moles of HCl in the analyte. At this point, Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript ions are completely neutralized by OHstart superscript, start text, negative, end text, end superscript ions. The solution only has salt (NaCl) and water and therefore the pH is neutral i.e. pH = 7.
Point 4: Addition of NaOH continues, pH starts becoming basic because HCl has been completely neutralized and now excess of OHstart superscript, start text, negative, end text, end superscript ions are present in the solution (from dissociation of NaOH).
2) Titration of a weak acid with a strong base
Let’s assume our analyte is acetic acid CHstart subscript, 3, end subscriptCOOH (weak acid) and the titrant is sodium hydroxide NaOH (strong base). If we start plotting the pH of the analyte against the volume of NaOH that we are adding from the burette, we will get a titration curve as shown below.
Point 1: No NaOH added yet, so the pH of the analyte is low (it predominantly contains Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript from dissociation of CHstart subscript, 3, end subscriptCOOH). But acetic acid is a weak acid, so the starting pH is higher than what we noticed in case 1 where we had a strong acid (HCl).
As NaOH is added dropwise, Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript slowly starts getting consumed by OHstart superscript, start text, negative, end text, end superscript (produced by dissociation of NaOH). But analyte is still acidic due to predominance of Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript ions.
Point 2: This is the pH recorded at a time point just before complete neutralization takes place.
Point 3: This is the equivalence point (halfway up the steep curve). At this point, moles of NaOH added = moles of CHstart subscript, 3, end subscriptCOOH in the analyte. The Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript ions are completely neutralized by OHstart superscript, start text, negative, end text, end superscript ions. The solution contains only CHstart subscript, 3, end subscriptCOONa salt and Hstart subscript, 2, end subscriptO.
Let me pause here for a second - can you spot a difference here as compared to case 1 (strong acid versus strong base titration)??? In the case of a weak acid versus a strong base, the pH is not neutral at the equivalence point. The solution is basic (pH ~ 9) at the equivalence point. Let’s reason this out.
As you can see from the above equation, at the equivalence point the solution contains CHstart subscript, 3, end subscriptCOONa salt. This dissociates into acetate ions CHstart subscript, 3, end subscriptCOOstart superscript, start text, negative, end text, end superscript and sodium ions Nastart superscript, start text, plus, end text, end superscript. As you will recall from the discussion of strong/ weak acids in the beginning of this tutorial, CHstart subscript, 3, end subscriptCOOstart superscript, start text, negative, end text, end superscript is the conjugate base of the weak acid CHstart subscript, 3, end subscriptCOOH. So, CHstart subscript, 3, end subscriptCOOstart superscript, start text, negative, end text, end superscript is relatively a strong base (weak acid CHstart subscript, 3, end subscriptCOOH has a strong conjugate base), and will thus react with Hstart subscript, 2, end subscriptO to produce hydroxide ions (OHstart superscript, start text, negative, end text, end superscript) thus increasing the pH to ~ 9 at the equivalence point.
Point 4: Beyond the equivalence point (when sodium hydroxide is in excess) the curve is identical to HCl-NaOH titration curve (1) as shown below.
3) Titration of a strong acid with a weak base
Suppose our analyte is hydrochloric acid HCl (strong acid) and the titrant is ammonia NHstart subscript, 3, end subscript (weak base). If we start plotting the pH of the analyte against the volume of NHstart subscript, 3, end subscript that we are adding from the burette, we will get a titration curve as shown below.
Point 1: No NHstart subscript, 3, end subscript added yet, so the pH of the analyte is low (it predominantly contains Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript from dissociation of HCl).
As NHstart subscript, 3, end subscript is added dropwise, Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript slowly starts getting consumed by NHstart subscript, 3, end subscript. Analyte is still acidic due to predominance of Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript ions.
Point 2: This is the pH recorded at a time point just before complete neutralization takes place.
Point 3: This is the equivalence point (halfway up the steep curve). At this point, moles of NHstart subscript, 3, end subscript added = moles of HCl in the analyte. The Hstart subscript, 3, end subscriptOstart superscript, start text, plus, end text, end superscript ions are completely neutralized by NHstart subscript, 3, end subscript. But again do you spot a difference here??? In the case of a weak base versus a strong acid, the pH is not neutral at the equivalence point. The solution is in fact acidic (pH ~ 5.5) at the equivalence point. Let’s rationalize this.
At the equivalence point, the solution only has ammonium ions NHstart subscript, 4, end subscriptstart superscript, start text, plus, end text, end superscript and chloride ions Clstart superscript, start text, negative, end text, end superscript. But again if you recall, the ammonium ion NHstart subscript, 4, end subscriptstart superscript, start text, plus, end text, end superscript is the conjugate acid of the weak base NHstart subscript, 3, end subscript. So NHstart subscript, 4, end subscriptstart superscript, start text, plus, end text, end superscript is a relatively strong acid (weak base NHstart subscript, 3, end subscript has a strong conjugate acid), and thus NHstart subscript, 4, end subscriptstart superscript, start text, plus, end text, end superscript will react with Hstart subscript, 2, end subscriptO to produce hydronium ions making the solution acidic.
Point 4: After the equivalence point, NHstart subscript, 3, end subscript addition continues and is in excess, so the pH increases. NHstart subscript, 3, end subscript is a weak base so the pH is above 7, but is lower than what we saw with a strong base NaOH (case 1).
4) Titration of a weak base with a weak acid
Suppose our analyte is NHstart subscript, 3, end subscript (weak base) and the titrant is acetic acid CHstart subscript, 3, end subscriptCOOH (weak acid). If we start plotting the pH of the analyte against the volume of acetic acid that we are adding from the burette, we will get a titration curve as shown below.
If you notice there isn’t any steep bit in this plot. There is just what we call a ‘point of inflexion’ at the equivalence point. Lack of any steep change in pH throughout the titration renders titration of a weak base versus a weak acid difficult, and not much information can be extracted from such a curve.
To summarize
- In an acid-base titration, a known volume of either the acid or the base (of unknown concentration) is placed in a conical flask.
- The second reagent (of known concentration) is placed in a burette.
- The reagent from the burette is slowly added to the reagent in the conical flask.
- A titration curve is a plot showing the change in pH of the solution in the conical flask as the reagent is added from the burette.
- A titration curve can be used to determine:1) The equivalence point of an acid-base reaction (the point at which the amounts of acid and of base are just sufficient to cause complete neutralization).2) The pH of the solution at equivalence point is dependent on the strength of the acid and strength of the base used in the titration.-- For strong acid-strong base titration, pH = 7 at equivalence point
-- For weak acid-strong base titration, pH > 7 at equivalence point
-- For strong acid-weak base titration, pH < 7 at equivalence point
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