Potassium standards

Turbidimetric Method

Potassium ions present in the sample react with Sodium Tetraphenylborate to produce an insoluble Potassium Tetraphenylborate resulting in a turbid suspension. There are a number of kits available for carrying out this test. Initially we selected the LaMotte potassium kit based on price and availability of separate refill reagents.

Reagents

  • Sodium tetraphenylborate powder (NaTPB), LaMotte, Part # 6364-C, $35.80

  • 6% sodium hydroxide solution (NaOH), LaMotte, Part # 5709-E, $7.95

  • Potassium standard, 1000 ppm, Labchem Part # LC201007, $18.10 for 125 mL

  • Distilled water

Equipment

List of vendors and prices: https://sites.google.com/iorodeo.com/colorimeter/equipment

  • Colorimeter with RGB board (use red for measurement)

  • Cuvettes

  • 1 x 50 mL capped tubes

  • 9 x 10 mL capped tubes

  • Pippete pump

  • 2 mL, 5 mL, 10 mL pipettes

Prepare calibration solutions

  • Prepare a 62.5 ppm potassium working stock: To a clean 50 mL tube add 1.25 mL of the 1,000 ppm potassium standard and 18.75 mL distilled water. Cap and mix.

  • Dilute the working stock as outlined in the Table below

Standard Curves

Method

  • To each 5 mL sample, add 33 uL of the 6% NaOH solution. Invert to mix.

  • For each sample, weigh out 0.05g of NaTPB into a fresh 5 mL tube (e.g. using a Horizon PRO-20B digital balance, Amazon approx. $20-30)

  • Add the contents of the first tube to the powder in the second tube, shake well and let stand. An image of all of the calibration solutions after adding the NaTPB powder is shown above

  • Transfer approx. 1mL of each calibration solution to a cuvette

  • In the colorimeter plotting program, select the red led and ppm as units. Calibrate the colorimeter with the 0 ppm solution and measure absorbance of all of the solutions in duplicate.

Sample calibration data

  • Sample calibration data: potassium_lamotte.txt

  • To view/use: Download the file and open in the colorimeter plotting software as described here

  • Data was collected from three separate calibration trials and compiled into one file. Data is shown in the graph below