Standardization of a Base, NaOH Class Notes


Terms To Know

Molarity


Molarity =moles of solute/Liters of solution

Molarity is your concentration unit.


Titration


Titration is the procedure used to determine the concentration of some substance by the controlled addition of a solution into a reaction vessel (flask) from a buret.

By using titration, the volume of the solution delivered from the buret may be determined very precisely.


Indicator


An indicator is a substance used to signal when a titration reaches the point at which the reactants are stoichiometrically equal as defined by the balance reaction equation.


For example, in the acid-base titration between sodium hydroxide and hydrochloric acid,

NaOH(aq) + HCl(aq) ---> H2O(l) + NaCl(aq)


the indicator should tell when the number of moles of NaOH and HCl are exactly equal, matching the 1:1 ratio in the equation.


In the reaction

2NaOH(aq) +H,2SO4 (aq) ---> 2 H2O(l) + Na2SO4 (aq)


the indicator should tell when the number of moles of NaOH is exactly twice the number of moles of H2SO4, this time reflecting the 2:1 molar ratio between the reactants.


End Point


End point is the point at which the indicator changes color.

Our indicator, phenolphthalein changes from colorless to pink at the end point.

standardization results

Flask before endpoint , approximately at endpoint ... after endpoint

                                                (faint pink)                     (overshot)


Equivalence Point


The equivalence point is the point in the titration in which enough standard solution has been added to react exactly with the substance being determined. The reactants are mixed in exact molar proportions represented by the balanced equation.

You should pick an indicator that has an end point that is the same as the equivalence point.


Standard Solution


The standard solution is a solution for which the concentration (molarity) is accurately known.


Primary Solution


A primary standard is a highly purified chemical that is weighed accurately. In this experiment oxalic acid dihydrate is used to determine the concentration of a standard solution of NaOH (with approximate concentration) by making a primary standard solution of oxalic acid dihydrate and titrating it with the standard solution (NaOH) thus standardizing the NaOH solution.


A standard solution can be prepared in either of two ways:

1. A primary standard is carefully weighed, dissolved, and diluted accurately to a known volume. Its concentration can be calculated from this data.

2. A solution is made to an approximate concentration and then standardized by titrating an accurately weighed quantity of a primary standard.


Why not use NaOH as the primary standard?

Why can't you weigh solid NaOH on the balance, dissolve and dilute it accurately to a known volume and then calculate its concentration from the data? Solid NaOH has the property of absorbing water from the air so it is not possible to accurately weigh NaOH. Sodium hydroxide is unsuitable as a primary standard because of this property. Consequently, you must use method two for preparing a standard solution, using oxalic acid dihydrate, H2C2O4* 2 H2O


What we are doing in this lab

We will be using a solution of NaOH which has already been made up and the concentration is known approximately. This solution is standardized by titrating an accurately weighed quantity of the primary standard oxalic acid dihydrate.

The reaction is:


2 NaOH(aq) + H2C2O4 (aq) ---> 2 H2O(aq) + Na2C2O4 (aq)


You will perform titrations reproducibly, which means the concentrations determined for the NaOH solutions are the same for each separate titration.

Reproducibility Requirement

Reproducibility requirement is the standard of sameness.

Reproducibility requirement (standard of sameness) is met when the concentration (Molarity) calculated is the same in separate titrations. In our case 2 out of 3 calculated molarities are within 0.007 of each other.


Preparing the NaOH Solution and Calculating Approximate Molarity

To prepare you sodium hydroxide solution you measure about 100 mL of the 1M NaOH solution that is provided. This 100 mL is placed into a 600 mL beaker and the solution is diluted with distilled water until the final volume is 500 mL.

To calculate the approximate molarity of you NaOH solution you can use the following equation:


M1V1 = M2V2

where 1 is the starting molarity and volume and 2 is the final molarity and volume.

The starting molarity is 1M and the starting volume is 100 mL. The final molarity is what you are calculating and the final volume is 500 mL. You can use this equation anytime you are determining a dilution calculation. Be careful about the volumes. They must be the same units. If you are calculating final or initial volume and you want units as liters you must convert mL to liters.


Preparing the Buret for the Titration

The buret must be prepared before using. First, rinse the buret with distilled water. Close the stopcock and put 10 mL of NaOH in the buret. Swirl the NaOH so that all of the buret has been coated with the NaOH. Drain the NaOH through the tip. This first rinse is to coat the buret with the solution you will be placing into it. Close the stopcock and place another 10 mL NaOH in the buret. Swirl again and drain through the tip. The second rinse is to make certain that only your NaOH is on the inside of the buret and not water.


That Slippery Feeling

When you get NaOH on your skin you will feel a slippery feeling. That is the NaOH eating away the top layer of your skin. When this occurs, simply wash your skin with plenty of soap and water.


Filling the Buret

After preparing the buret, you will fill it so that the meniscus is above the 0.00 mL line. Drain some of the NaOH solution down so that the bottom of the meniscus is at 0.00 or lower. Make sure the tip of the buret is filled with NaOH and no air bubbles are in the tip or drops hanging out of the tip. Air bubbles can be removed from the tip by thumping the tip while the solution is flowing. Dry the tip of the buret and check to see if the buret is leaking.


In this experiment the buret ALWAYS contains NaOH.


Titrating

The reaction flask contains your oxalic acid mixed with water and a couple of drops of phenolphthalein solution. It should set on a white paper under the buret, with the tip of the buret just inside the flask's neck.

Slowly add the NaOH from the buret into the reaction flask. Swirl the flask continuously to make sure the solutions are mixed well. At first you will see a pink color appear where the NaOH hits which fades quickly as the flask is swirled.When you start to see pink that takes a longer time to disappear during swirling, slow the addition of the NaOH to a dropwise addition. Stop the titration and read the buret when the phenolphthalein indicator just turns pink with one drop of NaOH. The end point has been reached when the pink color stays for 30 seconds after stopping the tritration.


Reading the Buret




1. Make sure your eye is level with the bottom of the meniscus.

2. Read the volume to the nearest 0.01 mL by reading between the lines. The buret is marked off in 0.1 mL so you must estimate the uncertain digit.

3. Read the buret from top (0.00 mL) to bottom (50.00 mL)

4. Buret read 26.75 mL

5. Volume used = final - initial.






Calculating the Molarity of your NaOH Solution

Once your titration is complete, you can calculate the molarity of your NaOH solution using the following equation:



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last updated: March 17, 2014