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

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:

Tube	Potassium Concentration (ppm)	Volume of 50 ppm potassium (mL)	Volume of distilled water (mL)
1	0.00	0.00	5.0
2	6.25	0.625	4.375
3	12.5	1.25	3.75
4	18.75	1.875	3.125
5	25	2.5	2.50
6	31.25	3.125	1.875
7	37.5	3.75	1.25
8	43.75	4.375	0.625
9	50	5.0	0.0

• 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:

Results

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

• 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. 

Additional Commercial Test Kits (not currently tested)