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PURPOSE:

• To standardize a solution of sodium hydroxide by titration with a primary standard, potassium hydrogen phthalate (KHC8H4O4)

Use ph electrode

ANALYTICAL CHEMISTRY

• Determination of iron in a razor blade• Determination of acetic acid in vinegar• Determination of Alcohol in Blood

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

KHP

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

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)

What I would like . . .

• In this experiment you will determine the amount of acid needed by titration with the strong base NaOH.

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.

Excellent to use!!!

Can actually obtain 1 mole of H+by using a balance

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

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.

phenolphthalein

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

phenolphthalein

Commonly used equipment

• Buret• Erlenmeyer Flask• Magnetic Stir Plate• Electronic Balance

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.

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.

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.

Goto pdf. . . .

• Path: XXXX g sample -> g KHP -> mol KHP -> mol NaOH -> M NaOH

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

Using the pH Probe

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.

Using the pH Probe

Best site

• http://www.chemistry.mtu.edu/~djchesne/classes/ch2212/Tutorials/Vernier_pH_tutorial.htm

Arrhenius acids.

The net ionic equation

H + (aq) + H2O(l) <-> H3O + (aq)

• Pure water already contains small quantities of the H + ion (proton), H3O + (hydronium ion)

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.

• 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

• 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.