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 and Equipment
|Sodium tetraphenylborate powder|
|LaMotte ||6364-C|| $35.80|
|6% sodium hydroxide solution|
|Potassium standard, 1000 ppm||Labchem||LC201007||$18.10 for 125 mL|
| Distilled water|| || || |
Potassium-LaMotte Calibration Curve
- Dilute the 1,000 ppm potassium standard from LabChem (Cat # LC201007) to yield a 50 ppm potassium working stock.
- Dilute the working stock as outlined in the Table below:
Volume of 50 ppm potassium (mL)
Volume of distilled water (mL)
- To each 5 mL sample, add 33 uL of the 6% NaOH solution
- For each sample, weigh out 0.05g of NaTPB into a fresh 5 mL tube. I use a small Horizon PRO-20B digital balance as shown in the Image below which you can buy online from Amazon for $20-30.
- Add the contents of the tube to the powder, shake and let stand. Below is an image of all of the calibration points after adding the NaTPB powder.
- 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. Below is an image of the final data:
Data from three separate calibration trials are attached at the bottom of the page (lamotte_trial1.txt etc). To view this data, download the files and open them in the colorimeter plotting software as described here. An image of the data from all 3 trials is also shown below.
Select the method you wish to use from
the two options below. Select Option A if you have a micropipette and
analytical balance, Option B if you do not. Option B will use the
additional equipment in the LaMotte test kit (measurement spoon, 15 mL
sample tube and dropper bottle).
1. Sample preparationMethod A
- Fill an empty sample bottle to the 5 mL line with water sample to be tested
- Add 33 µL of 6% sodium hydroxide solution, cap and invert several times to mix
- Add 0.05g of sodium tetraphenylborate to the tube, cap and invert several times to mix
- Fill the LaMotte tube to the top line with water sample to be tested.
- Add 2 drops of 6% sodium hydroxide solution, cap and invert several times to mix
- Add 1 heaped spoon of sodium tetraphenylborate to the tube, cap and invert several times to mix
2. Sample measurement
(The measurement steps below are the same for both methods.)
- Launch the colorimeter Measurement program
- Select Potassium-LaMotte from the drop-down list. The Red LED should be selected.
- Place a cuvette with water into the colorimeter and click on the calibrate button
some of your treated sample from the 5 mL tube into a cuvette, place in
the colorimeter and click measure. The program will return the
potassium concentration in ppm.
Below are some sample potassium measurements from two aquariums:
Additional Commercial Test Kits (not currently tested)
|Powder pillows || $160+ for 100 tests|
|Powdered and liquid reagents|| $100+ for 100 tests|
|Red Sea Potassium Pro Reagent Refill Kit||Reagent solutions A and B || $25 for 40 tests|
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