7.1: Thermodynamics of Borax - Experiment

Part A: Standardize the HCl Solution

A standardized HCl solution is prepared using primary standard anhydrous sodium carbonate (Na₂CO₃). Samples of known mass are titrated to a methyl orange endpoint (pH 3.1–4.4) with hydrochloric acid:

\[ \mathrm{CO_3^{2-}(aq) + 2\,H_3O^+(aq) \rightarrow 3\,H_2O(l) + CO_2(g)} \label{carbonatetitration} \]

Near the endpoint, the solution is boiled to expel dissolved CO₂ gas, which shifts the carbonic acid equilibrium to ensure sharp color changes.

1. Obtain approximately 1 g of anhydrous Na₂CO₃ from the desiccator and 200 mL of ~0.20 M HCl.
2. Weigh approximately 0.180 g of Na₂CO₃ into a clean 125-mL Erlenmeyer flask (record mass to ±0.001 g). Estimate volume of 0.20 M HCl required for neutralization. This will give you an approximate end point to make your titrations go faster. DO NOT add this volume directly to the flask, you will add the acid during the titration below.
3. Clean a 50-mL buret, rinse with 3–5 mL portions of the HCl solution, and fill with HCl. Read and record the initial buret volume to ±0.02 mL.
4. Dissolve each Na₂CO₃ sample in 50 mL of deionized water and add a few drops of methyl orange indicator until a clear yellow solution is observed (see the figure below). Titrate with HCl to the first faint orange color. Heat the solution to a gentle boil for 1 minute to expel CO₂ (color will revert to yellow). Cool to room temperature and complete the titration dropwise to a permanent pink-orange endpoint.

   

5. Calculate the exact molar concentration of HCl for each trial. The average of three trials agreeing within ±2% will be used for Part C.

Part B: Prepare the Borax Test Solutions

6. Calibrate six test tubes (labeled 1–6) by pipetting exactly 5.00 mL of water, marking the bottom of the meniscus with a permanent marker, and pouring out the water.
7. Prepare a saturated borax stock solution by mixing 30–35 g of borax with 100 mL of deionized water in an Erlenmeyer flask. Stopper and agitate vigorously. Undissolved solid borax must remain present to ensure saturation.
8. Fill six additional clean test tubes half-full with the saturated borax supernatant solution containing solid crystals.
9. Place test tubes into temperature baths set to approximately 0 °C (ice bath), room temperature (~22 °C), 32 °C, 40 °C, 48 °C, and 56 °C (do not exceed 60 °C).
10. Equilibrate test tubes in their respective baths for 10–15 minutes, stirring gently every 2–3 minutes. Allow solids to settle for 1 minute prior to sampling.
11. Carefully decant exactly 5.00 mL of clear supernatant into the calibrated test tubes without transferring any solid crystals. Record the exact bath temperature at the moment of transfer.

    

Part C: Analyze the Borax Test Solutions

12. Transfer each 5.00 mL supernatant sample into a clean 125-mL Erlenmeyer flask. Rinse the calibrated sampling tube with warm deionized water and add the rinsings to the flask to ensure quantitative transfer.
13. Dilute the solution to approximately 25 mL with deionized water, add a few drops of bromocresol green indicator to obtain a blue-green solution (see the figure below), and titrate with your standardized HCl solution to a yellow endpoint.