ChemLab - Chemistry 3/5 - Analysis of Seawater

Natural Salt Solutions 2: Analysis of Seawater

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Techniques

Procedure

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Techniques
In this experiment you will use volumetric glassware to perform a titration. An illustrated summary of titration techniques, quantitative transfer, and the use of pipets and burets can be found on the ChemLab website in the Techniques section, at http://www.dartmouth.edu/~chemlab/ techniques/buret.html

Use the website for learn or review these techniques and include them in your prelab procedure. You may also wish to print key web pages, since the website is your primary source for this information. You are responsible for knowing the following techniques: using a pipet and pipet bulb, using a buret, quantitative transfer, and titration. Your understanding of these techniques may be assessed with a prelab quiz.

Procedure
You will first carry out practice titrations to develop familiarity with titration technique. This will involve determining the molarity of the NaOH solution you obtain by titration against a primary standard potassium hydrogen phthalate solution. Following this, two ion exchange columns, prepared like last week, will be used to quantitatively replace each positive charge in seawater samples with a H⁺ ion. The resulting acid solutions will be titrated with standard NaOH to yield values of cation equivalents per liter for the seawater samples.

You will be given about 50 mL of the NaOH solution that you should place in a clean, dry, stoppered bottle or Erlenmeyer flask. Remember the caution about concentrations of standard solutions. In handling NaOH solutions, exposure to the atmosphere should be minimized since absorbed CO₂ will consume some of the hydroxide:

2 OH^- + CO₂

CO₃^2- + H₂O

Do not leave standard NaOH sitting around in open beakers, and pour it slowly to minimize aeration. Note the labeled concentration of the solution in your notebook.

Checking the standard NaOH solution
Carefully empty one vial of weighed KHP from last week into a clean 250 mL Erlenmeyer flask that has been rinsed with purified water. Rinse the vial three times with small amounts of purified water for quantitative transfer, adding the rinse to the solid in the large Erlenmeyer flask each time. The standard should be fully dissolved before proceeding. If gentle swirling for a minute doesn't suffice, add a few mL more water. Place a white sheet of paper beneath the flask. Prepare a buret for titration as described in the ChemLab website. Add two drops of phenolphthalein to the flask, swirl it, and titrate, using a magnetic stirrer, until a faint but permanent light pink color is achieved. Use the volume of NaOH consumed (mL) and the amount of standard in your sample (millimoles) to calculate the molarity of the standard NaOH solution. Comparison with the posted value will help you assess your titration technique. To repeat the analysis, simply rinse the flask thoroughly, (it need not be dried) refill the buret with the standard NaOH solution, and titrate another weighed sample. You should titrate three KHP samples.

Quantitative analysis of multi-cation solutions
You will analyze a 10 mL sample of sea water. Record the seawater source in your notebook, since several collection sites were used. Practice using a 2.00 mL volumetric pipet repeatedly with distilled water to learn how best to manipulate the suction bulb and obtain a clean delivery of 2.00 mL "aliquots". An aliquot is an accurately-measured portion of an analytical sample. Condition the pipet with seawater before delivering your sample.

Prepare two ion-exchange columns for receiving sea water samples. The columns should be fully converted to hydrogen-form and well-rinsed. Test the effluent to insure neutral pH. Refill your buret with the standard NaOH solution. Calculate the number of millimoles of H⁺ which should be released by 2.00 mL of your sea water sample assuming its cation concentration is 0.5 meq/mL. Is this less than the expected capacity of your column? If not, make a larger column and/or recheck your calculation.

Charge a column with 2.00 mL of sea water, then rinse all the released H⁺ ions off the column with at least 5 mL of pure, purified water added in two or more "slugs", collecting all the effluent in a clean 250 mL Erlenmeyer flask. Add 2 drops of phenolphthalein indicator solution. Forgetting this step has ruined many a titration, but you are unlikely to forget it if you are recording your procedure in your notebook as you work.

Next, use the assumptions discussed about to estimate how many milliliters of NaOH will be needed to reach the endpoint using the molarity you determined for the NaOH. This estimate will give you an idea of the likely position of the endpoint so you can save time in the early part of the titration. Finally, carry out the titration as instructed in the Techniques section, recording the results (and a statement of endpoint quality) in your notebook immediately.

After each analysis, the column must be regenerated before reuse by passing through 8 mL of 2 M HCl followed by two or more 3 mL purified water rinses until the effluent is neutral.

Carry out at least two analyses on sea water samples using each column at least once. One of your purposes here is to determine how reproducibly you can perform these analyses, since an analysis result has no value unless the analyst can also state the confidence with which the result is known. Thus a third or fourth try becomes necessary if something seemed awry about either earlier titration, or if the reproducibility seems to be less than is expected from the equipment (here (1% deviation from the mean is easily achievable).