Titrating for moisture doesn't have to be problematic.
Avoiding unstable volumetric titrations is a breeze if you follow these 10 best practices. You'll be able to start and end with a dry cell with ease and achieve accurate results.
#1. Make sure you start and end with a dry cell.
As we just talked about, Karl Fischer titrations are all about measuring water (moisture). If you start the titration with any extraneous moisture in your titration cell, or if you have a sample that cannot dry out (we'll get into side reactions later) then you will run into incorrect and inaccurate results. A number of things can contribute to the dryness of the titration cell. Age of the drying silica beads and o-rings, the grade of solvents (always use Karl Fischer grade), your sample introduction practices, and time for the cell to reach equilibrium and dryness. When using a Hanna Karl Fischer Volumetric Titrator HI933 an error 72 code (out of range) will pop up on the screen if there is too much moisture in the cell to go into the next phase after "Pre-Titration". Negating this is easy. Use a vacuum pump to empty the cell and refill the titration cell with fresh solvent to the minimum fill line. Carefully, lift the cell and give it a gentle swirl. This will allow the solvent to coat the sides of the titration cell and catch residual moisture. If the solvent doesn't turn yellow, manually introduce ~1 mL of titrant to the cell using the burette menu. Allow for approximately thirty minutes for the cell to come to dryness.
#2. Pick the right titrant for your sample.
To choose the proper titrant, you will need an idea of how much water could be in your sample. Composite 1 through 5 are different strengths of titrant, with the number indicating the number of milliliters of titrant needed to neutralize each milligram of water in the sample. Use the table below to help determine which strength is ideal.
|1 mg of H2O / mL of titrant (only 1 component)||For samples with less than 200 ppm H2O|
|2 mg of H2O / mL of titrant||For samples with less than 1,000 ppm H2O|
|5 mg of H2O / mL of titrant||For samples with 1,000 ppm to 100% H2O|
#3. Standardize your titrant.
Just like with potentiometric titrations, it is important to verify what your titrant strength is. This will help you to remove a potential margin of error. Always use a certified water standard provided by a chemical supplier to verify the concentration of your Karl Fischer titrant. Karl Fischer titrants typically have a certified value of ±10% of the nominal value and should be standardized every time a new bottle is open and every 2-4 weeks thereafter, depending on accuracy requirements. Once a bottle of titrant is opened, and as its used, the air in the headspace of the bottle will react with the titrant. This causes the titrant strength to vary. Repeat the standardization procedure at least 3 times and take the average result to input into your KF sample method. You can enter the actual strength of the titrant in the titrant database found in the general options on your titrator.
Read our Karl Fischer titrant standardization blog.
#4. Avoid side reactions.
Knowing the composition of your sample can help you avoid any potential side reactions. Side reactions can cause falsely high, or falsely low, results. Common side reactions occur in samples with aldehydes and ketones, oxidizing agents, and samples with pH outside of the ideal range (4-7 pH). You can take pre-emptive steps during sample preparation to avoid issues. Examples include using reagents especially designed for minimizing side reactions with ketones and aldehydes or using Karl Fischer grade buffering agents to ensure you achieve and maintain a proper pH.
#5. Calculate your sample size.
When calculating your sample size, you must take your titrant strength, your expected water concentration, and burette volume into account. If you know all of those variables, calculating your sample size is quick and easy. Simply use the calculation below to determine your ideal sample size. Just keep in mind, that your sample size is in grams, even if it is a liquid.
#6. Prepare your sample (don't skip this step).
Proper sample preparation can help negate issues with regular and complex samples. Samples that have extremely high water content, are insoluble in the solvent, release water slowly, or have inhomogeneous water distribution, or interferences, special sample preparation may be needed. An external extraction can be used to pull moisture out of a sample that would usually have issues dissolving in a solvent. An external dissolution can be used to pre-dissolve your sample in a solvent. A homogenizer or shaker device can be used to mix your sample during this sample preparation. These methods will create a liquid sample that can be easily added to the cell via a syringe.
#7. Care sample introduction is key.
How you introduce your sample into the titration cell can greatly influence your results as excess moisture could be introduced. To avoid extraneous moisture, follow these steps when introducing a sample via syringe.
- For liquid samples using the recommended syringe with a non-coring needle, draw a small amount of sample into the syringe to coat the inside of the syringe before taking your sample for injection.
- Expel the small amount of sample into a waste beaker.
- Mass the syringe and zero your scientific balance.
- Carefully draw the sample up into the syringe.
- Mass the syringe and record the value.
- Pierce the septum vertically and lower the needle until the tip is just below the level of solvent in the titration cell. (This ensures that the sample does not splash onto the sides of the titration cell).
- Administer the sample.
- Remove the syringe and re-mass the syringe.
- Take the difference between the first mass (g) and the second mass (g). This is your sample size in (g).
- Enter this sample size into the titrator.
- Begin analysis.
Always remember that any septum is only good for 50 uses, so tracking this and replacing the septum is a necessary maintenance procedure. For compatible solid samples that will dissolve completely in the cell, you can remove the septum cap and pour the sample directly into the sample cell. It is recommended to use a tool such as our HI900950 chemical spoon to avoid the sample sticking to your implement.
#8. Moisture contamination can be a big deal.
KF titrators are built to function as a sealed system to minimize atmospheric moisture from contaminating your analysis. Ensure your O-rings, tubing, and desiccant cartridges are in good condition to maintain seals and keep the unit in good working health.
#9. Make sure to replace wearable parts.
Replacing tubing, O-rings, syringes, and desiccant are all important maintenance procedures that can be done by a user or by Hanna staff with the purchase of a service contract or onsite service. We recommend an annual system assessment and preventative maintenance performed by Hanna personnel which is included with our annual service contract HI903933-SC.
|Wearable Part Maintenance|
|Part||When to Replace|
|Syring and tubing||12 months|
|Septa||Every 50 injections|
|Factory Calibration||Every 2 years|
Get Peace of Mind with Hanna Titrator Service Plan
#10. Maintain your electrode and cell.
Keeping your Karl Fischer instrumentation clean will help you to maintain a closed system without extra moisture creeping in. To clean the sensing electrode and cell, soak the bottom electrode and the cell in concentrated nitric acid solution (~70%) for at least 1 hour. After the hour is up, rinse the electrode with tap water and then deionized water for a minimum of 15 minutes. Finally, rinse the sensing electrode and cell with Karl Fischer grade methanol. Alternatively, for the cell only, a drying oven can be used. For long-term storage, the electrode and cell can be stored dry. Ensuring maintenance is done correctly and often will keep your Karl Fischer up and running reliably.
NOTE: Do not submerge the top or cable of the electrode in any liquid.
Download FREE printable Karl Fischer best practices
For more information regarding how Hanna Instruments can help you with your titration needs, contact us, at firstname.lastname@example.org or 1-800-426-6287.
Allison graduated from Bryant University with a Master’s Degree in Global Environmental Studies. She is passionate about nature, and how science is connected to the world around us. At Hanna, she provides an array of content and support to customers through the Hanna Blog, SOPs, and Data Sets.
Allison may be reached at email@example.com.