48

EXPERIMENT 2

NEUTRALIZATION TITRATIONS: THE DETERMINATION OF SODA ASH

A. INTRODUCTION

One of the most common types of titration involves an acid/base reaction. If the species

being titrated is a base, the buret (titrator or micropipette) contains a strong acid, usually HCl.

Likewise the titrant is a strong base (NaOH), if the analyte is acidic. In this experiment both

NaOH and HCl will be used as titrants.

The overall goal is the determination of the sodium carbonate content of a sample of soda

ash, which is the common name for impure Na2CO3. The carbonate anion is a weak diprotic

base, and it can be titrated with standard HCl. The titration curve exhibits two "break" regions,

corresponding to the equivalence points for the conversion of carbonate to bicarbonate and

bicarbonate to carbonic acid. Unfortunately, both endpoints are difficult to detect. The first

break region occurs in a good pH range for phenophthalein, but because the change in pH at the

break is small, the color change is not sharp. The magnitude of the pH change at the second

endpoint is also small, due to the accumulation of CO2 in the solution. The size of the second

pH break can be increased by boiling the solution to remove CO2. However, in this experiment

a mixed indicator, screened methyl orange, will be used. The combination of methyl orange

with xylene cyanole dyestuff produces an indicator which changes color over the narrow pH

range at the second endpoint.

The HCl titrant must be standardized, because a solution of accurately known

concentration cannot be prepared by dilution of the concentrated acid. It could be standardized

by titration of a primary standard solid, such as Na2CO3 or THAM, but, for instructional

purposes, a sodium hydroxide solution of known molarity will be used. The NaOH itself must

be standardized, because the solid reagent is not pure enough and cannot be massed accurately

because it is hydroscopic. Primary standard grade potassium hydrogen phthalate (KHP) will be

used for this purpose. Thus, there are actually three sets of titrations involved:

1) The molarity of NaOH must be determined by titration of KHP.

2) The molarity of HCl must be determined by titration vs the NaOH.

3) The weight % Na2CO3 must be determined by titration vs the HCl.

The actual order of these measurements is not important. Step 2 will be performed first. It is

easier to practice the proper techniques when solution aliquots are titrated. The more

time-consuming titrations of the weighed KHP samples should not be started until the

HCl/NaOH titrations can be performed with adequate precision.

49

B. PROCEDURE

Note: Before beginning work, be sure to review the material on weighing by difference

and the use of pipettes (Handout 5). You will also save time by watching the

introductory video “Laboratory Techniques.” We will not titrate using the

classical buret but instead will introduce aliquots of the titrant with the adjustable

volume micropipettes.

1) Place about 4 g of dried potassium hydrogen phthalate, KHC8H4O4, a.k.a. KHP, in

a clean, dry weighing bottle. The KHP will have already been dried by the

instructor at 110C for at least 2 hours.

2) Obtain an unknown soda ash sample from the instructor. It will already have

been dried by the instructor for at least 2 hours at 250C. This high drying

temperature is needed to ensure that any bicarbonate in the sample is completely

converted to carbonate.

3) Preparation of 0.1 M HCl: Add approximately 8.5 mL of concentrated HCl to

approximately one liter deionized water in a clean one liter glass bottle. Mix

well by shaking and inverting the bottle for at least 3 minutes.

4) Preparation of 0.1 M NaOH: Add approximately 10 mL of 10 M NaOH to

approximately one liter of deionized water in a clean one liter plastic bottle. Mix

for at least 3 minutes.

5) Titration of HCl with NaOH: Into three separate 250 mL Erlenmeyer flasks

pipet 10 mL of the 0.1 M HCl. Add about 100 mL of water to each. Add 3

drops of phenolphthalein indicator and titrate each with 0.1 M NaOH to a pink

color which persists for at least 20 seconds. Use the micropipette as you would a

buret, adding larger volumes initially and moving to smaller volumes near the

endpoint. You will want to use the smallest possible volume, 20 μL as you

approach the endpoint. The range of the three results should not be greater than

0.10 mL. If the precision is worse than this, it may be due to improper mixing of

the HCl and NaOH solutions or to sloppy pipetting and titrating techniques. Try

shaking the solutions for several minutes and repeating. If two more titrations

fail to give precise results, consult the instructor. Do not waste time trying over

and over without asking for help.

6) Standardization of 0.1 M NaOH: Accurately weigh by difference 0.2 to 0.3 g of

dry KHP into each of three Erlenmeyer flasks. Add about 100 mL of water and

3 drops of phenolphthalein to each flask and titrate with 0.1 M NaOH to the pink

endpoint described above. Calculate the molarity of NaOH for each titration, the

average molarity and the relative standard deviation (RSD). The RSD should be

no more than 3 ppt (3 parts per thousand = 0.3%). If it is greater, perform more

titrations until the RSD of three of them is within 3 ppt. (FWKHP = 204.22).

50

7) Determination of the unknown soda ash sample: Accurately weigh at least

three samples of about 0.15 g each into Erlenmeyer flasks and add about 50 mL of

deionized water to each. (From this point treat each sample separately until the

final endpoint has been reached.) Add 2 drops phenolphthalein and titrate with

standard 0.1 M HCl until the pink color just disappears. Record the volume of

HCl added. Then add 4 drops of screened methyl orange indicator. The

solution should now be olive green. Continue to titrate with standard 0.1 M

HCl. As the endpoint is approached, the solution will become gray-green and

eventually gray. Past the gray stage a purple tinge will appear. The gray color

immediately prior to the purple tinge is the end point. As described in Handout

5, section E, the best way to approach an uncertain endpoint is to record the

volumes added and the observed color for each addition. When the first sign of

purple appears against the gray background, the previous reading may be taken as

the endpoint.

C. CALCULATIONS

Report the results for each standardization titration (HCl and NaOH), the average MNaOH

and the average MHCl and the standard deviation and relative standard deviation (RSD) for each.

Report the % by weight of Na2CO3 for each soda ash sample, the average %Na2CO3, the

standard deviation, and the RSD. (FWNa2CO3 = 106.00.)

D. QUESTIONS

1) Explain the function of the NaOH solution in this experiment.

2) Give the required properties of a primary standard solid. Why does NaOH not

meet these requirements?

3) Define the term standard solution and describe the two general methods for

preparation. Explain how the two standard solutions were prepared in this

experiment.

4) Sodium hydroxide solutions are usually prepared by diluting a concentrated (50%

by weight) solution of NaOH. Explain why this method is preferred to

dissolving solid NaOH directly.

5) Write balanced net ionic equations for all titrations performed in this experiment.

Give the indicators and color changes for each endpoint.

6) Write the balanced net ionic equation for the conversion of bicarbonate to

carbonate when the unknown is dried in air at 250C (step 2 in the procedure).

7) A possible source of systematic error in this experiment is failure to dry the KHP.

If this occurred, would the final wt% Na2CO3 be falsely high, falsely low, or

unaffected? Give all reasoning to justify the prediction.