Titration for Potassium

• Determine the K content of a sample (for example, the green salt you synthesized)

• Titration
  • Unfortunately there are no useful titrants for K⁺ or Na⁺. The reason is fairly simple. K⁺ and Na⁺ ions are quite nonreactive-- they simply remain in solution as ions.
  • But we shouldn't give up hope. There is an indirect method that will handle K⁺ or Na⁺.

• We can pour our K⁺ solution through a column. The material in the column will swap H⁺ for K⁺. All the K⁺ will remain in the column. We can collect the H⁺ released and easily titrate it with NaOH. Measuring H⁺ is therefore equivalent to measuring K.

Background: Ion Exchange Resins

• Our resin begins as a polymer that is a long chain of carbon atoms; it is a solid that is completely insoluble in water. The simplest polymer is polyethylene. We will use another polymer called polystyrene. this contains six member carbon rings along a C-C backbone.
• The polymer is chemically modified to contain a number of anionic (negative) functional groups.
• Of course the solid must be electrically neutral, so there are an equal number of cations (positively charged ions.) For a typical cation exchange resin these are hydrogen ions (H+).
• Notice that the cations are not really bound to the resin (no chemical bonds.) However, they can't get very far away because of the electrical charges.
• In practice, the H+ ions are easily traded for other ions, including K+, Na, Ca²⁺, Ni²⁺, etc.
• The resin is manufactured as tiny beads that are easily packed into a column.
  • Ion exchange resins are widely used in water treatment (water softening, industrial waste treatment.)
  • They are also an important tool in purifying some metals, such as the Lanthanide series.

• A typical titration would use 30 ml of 0.1M NaOH (3.0 millimoles)
  • It takes about 2-3 grams of resin to produce that much H+; we'll use about 10 grams.
• We want to analyze our "green salt" K₃Fe(C₂O₄) 3 H₂O
  • molecular weight is about 500 (you should use the precise value)
  • to get 3.0 moles of K+ we want 1.0 mmoles of the compound.
  • 1.0 x 10^-3 moles weighs about 0.5 grams
• It is important that the solution pass through the resin slowly
  • allowing time for the ion swap
  • without forming channels
• We can reverse the process and recover the H+ resin
  • by passing 6M HCl through the column
  • the strong H+ solution will displace the bound K+
  • then rinsing well will eliminate the remaining HCl
  • (can test for HCl with Ag+, looking for AgCl precipitates)
• You might worry about the Fe³⁺ in our salt; will it bind to the resin?
  • No, because the Fe is part of the stable complex ion Fe(C₂O₄)₃2-.
  • This anion and it flows freely through the column.
  • Unfortunately, its yellow color makes the red phenolphthalein endpoint harder to observe

Experimental Directions

• Step 1-- filling the column
  • first, put a small glasswool plug into the bottom of the column and clamp it vertically
  • we have weighed out 10 grams of resin and allowed it to soak in water
  • pour this into a column, a little bit at a time
    • flush all the resin into the column
    • keep the water flowing, don't allow it to dry out
  • let about 10 ml of water flow through the prepared column
    • stop the flow with water about 1 cm above the resin.
    • place a clean 125 ml Erlenmeyer flask below the column
      • (the flask may be wet)

• Step 2-- Prepare the buret and the sample
  • fill a buret with 0.1M standardized NaOH
  • prepare a sample containing about 3-4 millimole of potassium
    • If you use the green salt, weigh out a sample of 0.25-0.35 grams, weigh to + 0.001 g and dissolve it in 80 ml of warm water.
    • for other samples, follow the directions provided.

• Step 3-- Slowly add the solution to the top of the column
  • open the valve so liquid flows, drop wise
  • keep adding liquid, maintaining the liquid level above the resin
  • be careful not to lose or spill any of the solution.
  • After the sample has all been added, slowly add an additional 25 ml of distilled water.
    • Now stop the flow (keep the column wet)
  • add 2-3 drops of phenolphthalein to the Erlenmeyer flask (or use a pH meter)
  • take an initial reading of the buret
  • titrate with NaOH
    • titrate until the solution is faint pink (pH >9) and remains so for 15 seconds.
    • record the final volume in the buret
• Repeat with a second sample of the same material; if time permits, do a third determination
  • you can use the same column without any additional treatment

• for the salt--
  • determine the number of moles of K per gram of salt
  • compare to the result expected from the formula
  • for other samples
    • report mg/liter of potassium

• Links
  • go to titration of oxalate, using permanganate
  • return to Chemistry 128 home page