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 and Equipment

Sodium tetraphenylborate powder
LaMotte 6364-C $35.80
6% sodium hydroxide solution
LaMotte5709-E $7.95
Potassium standard, 1000 ppmLabchemLC201007$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:


Potassium Concentration


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. 

Sample measurements 

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 preparation

Method 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

Method B

  • 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
  • Pour 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
(HI 93750-01)
Powdered and liquid reagents $100+ for 100 tests
Red Sea Potassium Pro Reagent Refill KitReagent solutions A and B  $25 for 40 tests

Joanne Long,
Apr 11, 2013, 4:35 PM
Joanne Long,
Apr 11, 2013, 4:35 PM
Joanne Long,
Apr 11, 2013, 4:35 PM