Water is in almost everything. It sits in crystal lattices, clings to powder surfaces and dissolves readily into polar solvents left open on the bench. For many chemicals it is the largest single impurity, and for moisture-sensitive chemistry it is the one that decides whether a reaction works at all.
Karl Fischer titration is the standard way to measure it. Understanding how it works helps you read the water line on a certificate and decide when to measure it yourself.
The chemistry in brief
Karl Fischer titration is based on the reaction of iodine with sulfur dioxide in the presence of water, a base and an alcohol, usually methanol. One mole of iodine is consumed per mole of water. The endpoint is detected electrochemically, when a small excess of iodine appears in the cell.
The reaction is specific to water, which is its great advantage over loss on drying. Loss on drying measures everything that leaves the sample on heating, including residual solvents and volatile decomposition products.
Volumetric or coulometric?
Volumetric
Iodine is added from a burette as a titrant of known strength. This technique suits samples with moderate to high water content, from roughly 0.1 percent up to 100 percent, and samples of a gram or more. It is the usual choice for salts, hydrates and many APIs.
Coulometric
Iodine is generated electrochemically inside the cell, and the charge passed gives the amount of water directly. This is ideal for very low water levels, in the ppm range, such as anhydrous solvents, oils and gases. Sample sizes are small and the method is very sensitive, so ambient moisture must be tightly controlled.
Common interferences
Karl Fischer is specific to water, but some samples react with the reagents and give false results:
- Aldehydes and ketones react with methanol to form acetals and ketals, releasing water. Use methanol-free reagents designed for ketones.
- Strong acids and bases shift the pH outside the optimal range, roughly 5 to 7, slowing or distorting the reaction. Buffer the working medium.
- Oxidising or reducing agents, such as ascorbic acid, thiols or some metal salts, react with iodine directly.
- Insoluble solids release water slowly. A Karl Fischer oven heats the sample and carries the water into the cell with dry gas, avoiding both solubility problems and many side reactions.
Why the water figure matters
Stoichiometry
If a reagent contains 2 percent water, every gram contains only 0.98 g of the substance. For a coupling reaction or a titrant preparation, that difference matters.
Moisture-sensitive reactions
Grignard reagents, organolithiums, acid chlorides, many coupling agents and silylating agents react with water before they react with your substrate. Here even ppm levels in the solvent can reduce yield.
Hydrate confirmation
Water content tells you which hydrate you have. Copper(II) sulfate pentahydrate contains about 36 percent water by mass. A result far from that suggests partial dehydration or the wrong form.
Assay on the right basis
APIs and reference standards are often assayed on the anhydrous basis. You need the water content to convert that to an as-is value for weighing.
For solvents used in synthesis, it helps to know typical levels. A good anhydrous grade is often specified at a few tens of ppm of water, while a general reagent grade of the same solvent may contain several hundred ppm or more. Once a bottle is opened and used repeatedly without inert gas protection, its water content can climb steadily toward the higher figure.
Lyophilised biochemicals are a special case. Residual moisture after freeze-drying affects both stability and the true protein or peptide content per vial, so it is often reported on the certificate.
Reagents that commonly gain water
- Polar aprotic solvents such as DMSO, DMF and NMP
- Lower alcohols, acetonitrile and THF once opened
- Sodium hydroxide, potassium carbonate and calcium chloride
- Amine hydrochlorides and many other organic salts
- Lyophilised biochemicals
Keeping water out
- Buy anhydrous solvents in septum-sealed bottles if you use them in small portions, and withdraw by syringe under dry nitrogen.
- Store hygroscopic solids in tightly closed containers inside a desiccator.
- Use molecular sieves for drying solvents. 3A sieves suit methanol, ethanol and acetonitrile, while 4A sieves suit many other aprotic solvents. Activate sieves before use.
- Choose smaller pack sizes for reagents you use slowly, so each bottle is consumed before it absorbs much water.
- Record the opening date on the label and recheck water content on critical reagents after opening.
Getting reliable Karl Fischer results
A few habits improve accuracy considerably, whether the test is done by the supplier or in your own laboratory.
- Standardise the titrant regularly for volumetric work, using sodium tartrate dihydrate or a certified water standard. Titrant strength drifts as the reagent ages.
- Check the drift before each sample. A high background drift means moisture is entering the cell.
- Match sample size to the method. Too little water gives poor precision, too much exhausts the reagent or prolongs titration.
- Transfer quickly. Hygroscopic samples gain water in seconds in a humid lab. Use a dry syringe for liquids and a weighing boat or oven vial for solids.
- Replace the working medium when it is saturated or after a set number of samples, since capacity is limited.
Running a certified water standard alongside samples, at a level similar to the expected result, is a simple way to confirm the system is performing before you trust the numbers.
When to measure it yourself
The certificate reports water at the time of testing. If the bottle has been opened, stored in humid conditions or is critical to a moisture-sensitive process, a quick in-house Karl Fischer test is worth the time.
Zemixal lists anhydrous solvents and Karl Fischer grade reagents in the Solvents (HPLC and GC) and Lab Reagents divisions. Product certificates state water content where it is part of the specification. If you need a specific maximum water level, mention it in your enquiry through the RFQ form.