Hydrogen peroxide test strips are typically used to confirm a dilution has been done correctly. This step also checks that the concentrate you used is at the strength you think it is. In this blog, we describe a method for finding the concentration of old or unspecified food-grade hydrogen peroxide. This need could arise for a variety of reasons. A label could be missing information, the peroxide could be very old, or it could have been stored incorrectly.
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The Reverse Dilution Process
The process is simple. Dilute 10mL of the unknown concentrate by a factor of 10 & test the concentration. Our high-level hydrogen peroxide concentration calculator does the rest.
**Disclaimer
Wear appropriate PPE, handle chemicals at your own risk & follow proper lab safety procedures.
The Indigo Reverse Dilution Tool Kit
Our Unknown Hydrogen Peroxide Dilution Kit consists of:
Glassware: 100mL & 10mL graduated cylinders are a convenient way to dilute by factors of 10. Measure 10mL of the unknown concentrate in the 10mL GC, pour it into the 100mL GC & top up it to 100mL with water. A 1mL medicine dropper is included to make topping up easier.
Or, do it by weight. 1 part peroxide mixed with 9 parts water.
Test Strips: A bottle of Indigo® 10% hydrogen peroxide test strips are included in the kit. You may also want a bottle of Indigo® 0-100ppm hydrogen peroxide test strips but more on than later.
How are PPM & Percentage Related?
PPM (or ppm) means parts per million & is how most test strips are calibrated. 1,000,000 parts per million (1,000,000 ppm) is the same as saying it is 100%.
Percentage typically describes concentrated liquids while ppm is used for diluted ones. This is mainly for convenience to avoid large numbers before or after the decimal.
For example, food-grade hydrogen peroxide ranges are simpler to read as 35% than as 350,000ppm or pharmaceutical grade as 3% instead of 30,000ppm. When hydrogen peroxide is diluted for pool or hot tub use, reading 50-400ppm is easier than 0.005-0.04%.
Hydrogen Peroxide Dilution Procedure
Step 1
Dip a 10% peroxide strip into your unknown, undiluted concentrate. It should turn black instantly if the peroxide is 10% or higher. If it doesn’t, skip Step 2.
Exposed 10% Strip
Step 2
Fill the 10mL graduated cylinder to near the 10mL mark with the undiluted concentrate. You can top up to the 10mL line with the medicine dropper. This degree of accuracy is not essential but we suggest it as good scientific procedure.
Pour the 10mL of concentrate into the 100mL graduated cylinder. Add water (distilled if you have it but tap water will do) to near the 100mL line & top up using the dropper. This level of accuracy is not essential & should not have any effect on the results.
Step 3
Test the diluted peroxide in the 100mL GC with a fresh 10% test strip. Write down the time it takes. Repeat this test 2 more times for a total of 3 trials.
The strip pad will turn black as in the picture above but because the diluted peroxide is <10%, this change will not be immediate.
Step 4
Using the Hydrogen Peroxide Calculator
Click on Unknown Hydrogen Peroxide Calculator & scroll down to enter the 3 results into the “Reaction Times” section as shown below & press Calculate.
Our 3 test results were averaged by the calculator to give us a concentration range of 2.6% to 2.9%. Since we diluted by a factor of 10 in Step 2, multiply by 10 to give us a range of 26 to 29% for the undiluted but now known solution.
Reaction times for different peroxide concentrations were plotted to create the graph below.
Results for these 3 were averaged by the calculator to give us a concentration range of 2.6% to 2.9%. Since we diluted by a factor of 10 in Step 2, multiply by 10 to give us a range of 26 to 29% for the undiluted but now known solution.
What About the Error?
We have not quantified the error but assume +/- 20%. We suggest using the lower calculated concentration in your applications. Statistically speaking, it means more bugs should get killed in a certain amount of time. Killing bugs faster than slower is simply erring on the side of safety.
A Backup Test
If you want a separate check on these results, you can use the 0-100ppm hydrogen peroxide test strips as shown below. You will need to dilute the starting solution by a factor of 10000 to end up at 26-29ppm. (Not exactly as shown below).
100ppm Peroxide test strips with exposed strip
Watch the Video
This video will show all the steps outlined above.
Test Strip Expiration Dates
All Indigo® Hydrogen Peroxide test strips have a guaranteed 3-year shelf life. However, our peroxide strips come in desiccant-lined bottles that can absorb air-borne moisture for 9 years or more based on typical ingress rates.
Storing the bottles in a Ziploc bag with silica gel should extend their life indefinitely. We found some buried in our warehouse with a batch date from 2015 that were still good. For more information, read our Blog: Test Strip Expiration Dates; Good Today, Dead Tomorrow?
Better yet. Since you are likely using 100 or 400ppm strips on an ongoing basis, you can use them to confirm that the 10% & 10K strips are still good. You can use our standard dilution calculator for any starting/ending concentration & volume. A virtuous circle indeed.
