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Turbidimetric Method
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
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
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 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
Method
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
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
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