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PURPOSE:
• To standardize a solution of sodium hydroxide by titration with a primary standard, potassium hydrogen phthalate (KHC8H4O4)
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Use ph electrode
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ANALYTICAL CHEMISTRY
• Determination of iron in a razor blade• Determination of acetic acid in vinegar• Determination of Alcohol in Blood
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Must do three things before You start
• 1. Clean the Buret• 2. Delivering Solution from the Buret• 3. Filing the Buret for a titration• 4. Preparing the KHP Sample• 5. Titrating the KHP• * Must use Your Lab book today!• *** This is one of the MOST complicated labs
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KHP
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Why use KHP?
• The hydrogen is slightly acidic, and it is often used as a primary standard for acid-base titrations because it is solid and air-stable, making it easy to weigh accurately. It is not hygroscopic. It is also used as a primary standard for calibrating pH meters
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as a buffering agent
• As a weak acid hydrogen phthalate reacts reversibly with water to give hydronium (H3O+) and phthalate ions.
HP- + H2O <->P2- + H3O+
• KHP can be used as a buffering agent(in combination with hydrochloric acid (HCl) or sodium hydroxide (NaOH) depending on which side of pH 4.0 the buffer is to be)
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What I would like . . .
• In this experiment you will determine the amount of acid needed by titration with the strong base NaOH.
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What is a standardization???
• Since it is hard to prepare a NaOH solution of accurately known concentration directly
from the solid, you will need to standardize your NaOH solution against a precisely weighed amount of standard acid.
• The acid used is the weak monoprotic acid, potassium hydrogen.
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Excellent to use!!!
Can actually obtain 1 mole of H+by using a balance
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A titration
• Knowing the volume of titrant added allows the determination of the concentration of the unknown. Often, an indicator is used to usually signal the end of the reaction, the endpoint
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acid-base indicator
• An acid-base indicator is itself a weak acid (or its conjugate base).
• An acid-base indicator is a weak acid having a different colour in aqueous solution from its conjugate base.
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phenolphthalein
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phenolphthalein
• Phenolphthalein
• Type: HIn + H2O <->In- + H3O+
pK: 9.5Approximate pH range for color change:
• 8.0-9.8Color of acid form: clearColor of base form: red-violet
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phenolphthalein
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Commonly used equipment
• Buret• Erlenmeyer Flask• Magnetic Stir Plate• Electronic Balance
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accuracy• The accuracy of the results of your titration
will be a reflection of the care you took while performing it. When done carefully, titrations give very accurate, precise results.
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accuracy
• SOMETIMES!!!!!!!
• Titrations of unknown solutions are done in two steps: a scout titration used to determine the approximate amount of titrant needed followed by the actual titration that you will use to make your calculations.
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What You will do . . .
• An 0.8167 gram sample of primary standard KHP (assay = 99.95%) required 38.25 mL of
NaOH to neutralize.
Calculate the molarity of the NaOH solution.
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Goto pdf. . . .
• Path: XXXX g sample -> g KHP -> mol KHP -> mol NaOH -> M NaOH
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path
0.8167 g sample x (99.95g KHP)/(100 gram) x (1molKHP)/(204.22g KHP)
x (1mol NaOH)/(1 mol KHP) x (1 )/(0.03825 L)=0.1040 M NaOH
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Using the pH Probe
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Using the pH Probe
• Using the pH Probe• Preparation of the pH Sensor• A pH sensor connected to a computer will be
used to measure pH. Plug the pH probe into channel 1 of the interface box. Plug the power cord on the interface box into an electrical outlet.
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Using the pH Probe
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Best site
• http://www.chemistry.mtu.edu/~djchesne/classes/ch2212/Tutorials/Vernier_pH_tutorial.htm
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Arrhenius acids.
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The net ionic equation
H + (aq) + H2O(l) <-> H3O + (aq)
• Pure water already contains small quantities of the H + ion (proton), H3O + (hydronium ion)
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On Exam
• Example: Calculate the weight of primary standard potassium hydrogen phthalate
• (assay = 99.95%) that would be required to standardize a 0.1 N NaOH solution,
• assuming a 40 mL titration.• PATH: L NaOH mol NaOH mol KHP g KHP g
sample• Note that 1 equiv = 1 mol for both KHP and NaOH so
N = M and molecular weight = equivalent weight.
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• Example: An 0.8167 gram sample of primary standard KHP (assay = 99.95%) required
• 38.25 mL of NaOH to neutralize. Calculate the molarity of the NaOH solution.
• PATH: g sample g KHP mol KHP mol NaOH M NaOH
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• Example: A 1.7734 gram sample of KHP required 40.11 mL of 0.1036 N for titration.
• Calculate the assay of the KHP and report with a relative error of 1 part per 1000.