What Reagents Can You Use Past Their Chemical Expiry Date?

Many chemicals in the lab, including buffer salts, metal salts, sugars, and SDS, are okay to use past their expiry date. Others, such as ammonium persulfate and antibiotic solutions, should not be used past their chemical expiry date. And reducing agents and hydrogen peroxide can be qualitatively checked to see if they still work. So consider the chemistry and biochemistry of your old reagents before throwing them out.

Have you ever picked a chemical that’s years out of date up off the shelf and wondered: “can I still use this?”

Chemicals are expensive. Most labs contain thousands, and few people know which are in date and which aren’t. Replacing them whenever they go out of date is a massive waste of grant money. 

Plus, we use some chemicals so rarely, and it’s also rare that people sort out their benches before leaving. The result is a pile-up of old chemicals and solutions—some potentially useful and worth keeping. 

So what reagents are usable past their chemical expiry date? How can you check if they are still okay? And which ones should you throw out?

Read on to find out.

1. Buffers

In the lab, you’ll encounter buffer solutions (e.g., 1M tris stocks) and buffer salts (e.g., sodium citrate). 

Buffer salts should be fine to use past their expiry date since they are usually stable unreactive solids. So long as they have been stored in their original container, you’re good to go.

Buffer solutions are different, and you should probably throw them out. Why so? Because they are usually pH adjusted to somewhere around physiological pH. 

Consider also that buffer molecules, their breakdown products, and molecules dissolved from the air provide carbon and nitrogen sources, and you have ideal growing conditions for unwanted bacteria and fungus. 

They can also change the pH of the solution away from the one written on the bottle.

Swirl your buffer solutions occasionally and look for floaty bits to check if anything is growing in them.

In a pinch, you could filter sterilize your buffer solutions to remove anything that has grown before reusing. Just check its pH is still correct.

2. General Salts

Most of the chemicals on your shelves will be salts of some sort, like magnesium chloride, potassium phosphate, and calcium carbonate.

Like buffer crystals, many of the salts in your lab are unreactive solids and are okay to use beyond their date.

Be Cautious Using Old Hygroscopic Salts

Just beware that some salts are hygroscopic and will attract atmospheric moisture to themselves. Sometimes so much so they get physically wet. 

Examples of hygroscopic salts include zinc chloride and calcium chloride.

Plus, simple salts are relatively cheap, meaning we buy them in huge tubs that sit around for years.

You’ll find out which old salts are hygroscopic when you open them and are soaking wet.

Throw these out because you can’t weigh them out accurately, meaning there will be an error in the molarity of your solutions.

In a pinch, you could try drying them out by freeze-drying or warming them up gently in low-humidity conditions.

3. Reducing Agents

Let’s split these into two.

BME and DTT

Beta-mercaptoethanol (BME) and dithiothreitol (DTT) are liquid at room temperature.

Avoid reusing solutions containing them, and add them immediately before your experiments.

Depending on the pH and temperature of the solution, their half-life is between a few hours and several days. 

Pure BME and DTT are stable for 1-3 years if stored properly. You’re likely to finish the bottle before it goes out of date.

Don’t use them beyond their shelf life because, if they have lost their potency, it will give you a real headache when you troubleshoot what’s gone wrong.

TCEP

Tris(2-carboxyethyl)phosphine (TCEP) is a solid at room temperature and more stable in aqueous solutions than BME and DTT.

The powder is stable and should be okay to use beyond its use-by date. 

Solutions of it are more stable than BME and DTT solutions but don’t last forever. Discard them after a month or freeze them down at -20°C.

How to Qualitatively Check if Reducing Agents Still Work

Find a sample of a protein in your lab containing two or more peptide chains held together by disulfide bonds and run a gel of it. 

Antibodies work great but check to see if you have something cheaper to hand.

• Run one gel with a sample containing 1-10 mM reducing agent. 
• Run another gel with no reducing agent in the sample.

If the reducing agent is okay, it will break the disulfide bonds, resulting in multiple bands on your gel corresponding to the individual protein chains. 

The unreduced sample will produce a single heavier band corresponding to the disulfide-linked chains.

And finally, because they lose potency quickly, reducing agents are one of the first things to check when things unexpectedly go wrong!

4. Phenylmethylsulfonyl Fluoride

Phenylmethylsulfonyl fluoride (PMSF) is a common serine protease inhibitor.

The powder is okay to use past its use-by date unless you notice your sample starts getting proteolyzed. If this happens, buy some new PMSF. 

Aqueous solutions of PMSF have a half-life of about 30 minutes, so don’t reuse them.

And stock solutions of PMDF are usually prepared at 100 mM in isopropanol. These should be stable for several months at 4°C or several years at -20°C.

5. Heavy Metal Salts and Solutions

Heavy metal salts include chemicals like nickel sulfate, cobalt chloride, silver nitrate, and gold chloride. 

These stable compounds last almost indefinitely and can be used well beyond their shelf life.

Spillage and contamination are more likely to ruin them before they become chemically unsuitable for your experiments. 

Ditto for solutions. Heavy metals are antiseptic. Nothing should grow in gold, silver, cobalt, and nickel solutions. So you can keep them for years on your shelf!

6. Antibiotics

Generally, use the powders regardless of how old they are, but don’t use old solutions. 

A few points to note. 

Generally speaking, the crystals are unreactive solids. That’s why it doesn’t matter if they are old. 

However, once dissolved in solution, some will doubtless be more stable than others. 

Now, there are a lot of antibiotics that we use in research, and I don’t know the stability and chemistry of most of them.

Plus, they will have different stabilities depending on what they are dissolved in and how they have been stored.

Finally, antibiotics are usually a critical component of our experiments.

That’s why it’s best to err on the side of caution with solutions.

7. Ammonium Persulfate

Desiccated ammonium persulfate is stable and will last for years. So you can use the really old stuff that’s in your lab. 

However, ammonium persulfate hydrolyzes rapidly in solution, producing ammonium hydrogen sulfate and hydrogen peroxide.

Never use old ammonium persulfate solutions!

8. Hydrogen Peroxide

Hydrogen peroxide is okay to use past its chemical expiry date if stored in the dark. 

The liquid, supplied as a 30 % w/v solution, is stable but decomposes when exposed to light. So factor this into your decision. 

For this reason, and because reagent bottles are transparent, old working solutions that contain dilute hydrogen peroxide should be discarded.

How to Qualitatively Check Whether Hydrogen Peroxide Is Still Usable

Hydrogen peroxide oxidizes potassium iodide, forming iodine, potassium hydroxide, and hydrogen gas. 

Aqueous solutions of potassium iodide solutions are clear, but iodine solutions are brown. 

Thus, if you prepare 100 mM potassium iodide, add a few drops of hydrogen peroxide into it, and it slowly turns brown—the hydrogen peroxide still works. 

9. Sodium Dodecyl Sulfate

Sodium dodecyl sulfate (SDS) powders and solutions should be fine to use beyond their use-by date. 

Sometimes, sodium dodecyl sulfate may precipitate out of the solution. Especially when it gets cold in the lab. If this happens, heat the solution gently on a hot plate while stirring, and cover the bottle in foil to keep the heat in.

10. Sugars

Old powders should be fine to use but check solutions for growth. 

Like most chemicals on this list (and in general), the crystals are stable, unreactive solids. 

Sugar solutions are similarly stable, but the molecules are high-energy food sources for microbes. Add the fact that sugar solutions will be at or near neutral pH, and things start growing in them really quickly. 

Be sure to check old solutions for contamination before using them. 

Think About the Chemistry

Before discarding a reagent out of superstition, think about its chemistry. For example:

• Is it composed of strong ionic bonds?
• Can it oxidize?
• Is it photosensitive?
• Does it degrade in water?
• Might its composition have changed?

Think About the Biochemistry

Before using old chemicals and solutions, consider biochemistry. 

• What properties make it useful?
• Is it a nitrogen source?
• Is it a carbon source?
• Is it antiseptic?
• Will it retain selectivity?

Can You Use Out-of-Date Chemicals? Answers At a Glance

For easy reference, here’s all that summarized in a convenient table.

Table 1. Summary of whether you can use certain reagents past their chemical expiry date.

Chemical	Fine to use past expiry date?	Notes
Buffer salt powder	Yes	Stable
Buffer solutions	Check	Microbes will grow in them over time
General salts	Yes	Hygroscopic ones will attract atmospheric water
Reducing agents	Check	Run an SDS-PAGE gel on a protein held together by a disulfide bond to check. TCEP is more stable than BME and DTT
PMSF powder	Yes	Stable
PMSF solutions	No	Very short half-life (~30 mins) in aqueous solution
Heavy metal salts	Yes	Antiseptic
Heavy metal solutions	Yes	Antiseptic
Antibiotics powder	Yes	Stable
Antibiotics solutions	No	Diverse array of them available. Best to be safe than sorry
Ammonium persulfate powder	Yes	Stable
Ammonium persulfate solutions	No	Hydrolyzes rapidly in aqueous solutions
Hydrogen peroxide	Check	Stable if kept in the dark. Check by oxidizing potassium iodide to produce a brown iodine solution
SDS	Yes	Stable
Sugar powder	Yes	Stable
Sugar solutions	Check	Microbes will grow in them over time

 

Using Reagents Beyond Their Chemical Expiry Date Summarized

There’s your list of common chemicals, whether or not they are okay to use past their chemical expiry date, and how to check if they are still good! 

Plus, a bit of chemistry to help you understand why some old reagents still work. Hopefully, you can apply this wisdom to chemicals in your lab to help you decide on a case-by-case basis.

Using old reagents can save you time waiting for orders and money otherwise spent on them.

Note that these aren’t hard and fast rules. I’ve based what I’ve written on the cumulation of my lab experience. You can take this advice as generally or literally as you wish. 

For sensitive and critical experiments, try to use the freshest reagents to avoid a heavy heart if it goes wrong. 

Any other helpful tips and lab hacks you want to share? Have I missed a good chemical out? Drop them in the comments section below!