Faking Lateral Flow Tests: the problem with pH

Fruit juices can be used to generate a fake positive on COVID-19 LFTs

On Thursday last week, I got a message from Prof Mark Lorch — my sometime collaborator on supercharacter-based ramblings.

“Have you seen the reports of kids fooling the Covid lateral flow tests and getting false +ve results by adding orange juice to the devices?” he wondered.

At this point, I had not – but I quickly got up to speed. Mark had previously made an excellent video explaining how lateral flow test (LFT) devices work, so it was just a case of working out, firstly, whether the false positives were reproducible, and secondly, speculating what, exactly, was causing them.

Thus ensued some interesting discussion which ultimately led to a couple of articles from Mark. One at The Conversation and another, slightly more recently, at BBC Future.

I won’t delve into LFT-related science, because Mark has covered it (really, check the video and those articles), but I am going to talk a little bit about pH – the scale chemists use to measure how acidic or alkaline solutions are – because as soon as news of this started to gain traction people, predictably, started trying it out themselves. And that was when things got really interesting.


The buffer included with LFTs is effective at neutralising the pH of solutions, for example, cola

Now, firstly, and importantly: the test kits come with a small vial of buffer solution. Buffers are substances which resist pH changes. As I’ve written before, our bodies naturally contain buffer systems, because keeping the pH of our blood and other body fluids constant is important. In fact, if blood pH varies even a little, you’re in all sorts of serious trouble (which is how we can be certain that so-called “alkaline” diets are a load of hooey). Anyway, the important message is: don’t mix any liquid you’re testing with the contents of that phial, because that will neutralise it.

If you want to try this for yourself, just drop the liquid you want to test directly into the window marked S on the test.

That out of the way, let’s get back to pH. It’s a scale, usually presented as going from 0–14, often associated with particular colours. The 0 end is usually red, the 7 in the middle is usually green, while the 14 end is usually dark blue.

These colours are so pervasive, in fact, that I’ve met more than one person with the idea that acids are red, and alkalis are blue. This isn’t the case, of course. The red/green/blue idea largely comes from universal indicator (UI), which is a mixture of dyes that change colour at different pH values. There’s also a common indicator called litmus (people sometimes mix up UI and litmus, but they’re not the same) which is also red in acid and blue in alkali.

Some species of hydrangea produce pink flowers in alkaline soil, blue in acid soil.

There are actually lots of pH indicators, with a wide variety of colour changes. Phenolphthalein, for example, is bright pink in alkali, and colourless in acids. Bromocresol purple (they have such easy-to-spell names) is yellow in acids, and violet-purple in alkalis.

Many plants contain natural indicators. Just to really mix things up, some species of hydrangea produce flowers that are blue-purple when they’re grown in acidic soil, and pink-red in alkaline conditions.

Bottom line? Despite the ubiquitous diagrams, pH has nothing to do with colour. What it is to do with is concentration. Specifically, the concentration of hydrogen ions (H+) in the solution. The more H+ ions there are, the more acidic the solution is, and the lower the pH. The fewer there are, the less acidic (and the more alkaline, and higher pH) it is.

In fact, pH is a log scale. When the concentration changes by a factor of 10, the pH changes by one point on the scale.

This means that if you take an acid with pH of 2, and you dilute it 1 part to 10, its pH changes to 3 (i.e. gets one point more alkaline, closer to neutral). Likewise, if you dilute an alkali with a pH of 10 by 1:10, its pH will shift to 9 (again, closer to neutral).

And what this means is that the pH of substances is not a fixed property.

Louder for anyone not paying attention at the back: the pH of substances is not a fixed property!

Yes, we’ve all seen diagrams that show, for example, the pH of lemon juice as 2. This is broadly true for most lemons, give or take, but if you dilute the lemon juice, the pH rises.

Apple juice dropped directly into the test window gives an immediate “positive” result.

I am by no means an expert in commercial, bottled lemon juice, but I reckon a lot of them have water added – which may well explain why @chrismiller_uk was able to get a positive result, while @BrexitClock, using a French bottle of lemon juice, couldn’t.

Mark and I concluded that the pH you need to aim for is probably around 3–4. Go too low, and you don’t get a positive (and you might wipe out the control line, too). Likewise, too high also won’t work.

Myself, I tried apple juice. I couldn’t find the indicator colour key for my indicator paper (I really must clear out the drawers one of these days) but it’s broadly the same as Mark’s cola photo, up above. In other words, the apple juice is about pH 3. And it gives a beautiful positive result, immediately.

One more interesting observation: Mark recorded some time-lapse video comparing orange juice to (sugar-free) cola. It shows the cola test line developing a lot more slowly. We’re not entirely sure why, but it may be pH again: orange juice almost certainly has a lower pH than cola.

For any parents reading this thinking we’re being terribly irresponsible, fear not: as Prof Lorch has made clear in his articles, you can identify a fake by waiting a few minutes and then dropping some of the buffer solution provided in the test window. As I said above, this will neutralise the pH, and the positive test line will disappear. Extra buffer won’t change a genuinely-positive test, because the antibodies bind very tightly (more technical info here). To quote Mark: “you’d need a swimming pool’s worth of buffer to have any chance of washing [the antibodies] off.”

Alternatively, you can just watch your teenager as they do their tests, and make sure they’re not getting up to anything nefarious…

Have you tried to trick an LFT? If you have, share your results! Look us up on Twitter: @chronicleflask and @Mark_Lorch or add a comment below. We’d love to see your photos!

Do you want something non-sciency to distract you from, well, everything? Why not take a look at my fiction blog: the fiction phial? You can also find me doing various flavours of editor-type-stuff at the horror podcast, PseudoPod.org – so head over there, too!

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Alkaline water: if you like it, why not make your own?

Me* reading the comments section on the Amazing Alkaline Lemons post (*not actually me)

Alkaline water seems to be a trend at the moment. Not quite so much in the UK, yet, but more so in the US where it appears you can buy nicely-packaged bottles with the numbers like 8 and 9.5 printed in large, blue letters on their sides.

It’s rather inexplicable, because drinking slightly alkaline water does literally NOTHING for your health. You have a stomach full of approximately 1 M hydrochloric acid (and some other stuff) which has an acidic pH of somewhere between 1.5 and 3.5. This is entirely natural and normal – it’s there to kill any bugs that might be present in your food.

Chugging expensive water with an alkaline pH of around 9 will neutralise a bit of that stomach acid (bringing the pH closer to a neutral value of 7), and that’s all it will do. A stronger effect could be achieved with an antacid tablet (why isn’t it antiacid? I’ve never understood that) costing around 5p. Either way, the effect is temporary: your stomach wall contains special cells which secrete hydrochloric acid. All you’re doing by drinking or eating alkaline substances is keeping them busy.

(By the way, I’m not recommending popping antacids like sweeties – it could make you ill with something called milk-alkali syndrome, which can lead to kidney failure.)

Recently, a video did the rounds of a woman testing various bottled waters, declaring the ones with slightly acidic pHs to be “trash” and expressing surprise that several brands, including Evian, were pH neutral. The horror. (For anyone unsure, we EXPECT water to have a neutral pH.)

Such tests are ridiculous for lots of reasons, not least because she had tiny amounts of water in little iddy-biddy cups. Who knows how long they’d been sitting around, but if it was any length of time they could well have absorbed some atmospheric carbon dioxide. Carbon dioxide is very soluble, and it forms carbonic acid when it dissolves in water which, yes, would lower the pH.

Anyway, there’s absolutely nothing harmful about drinking water containing traces of acid. It doesn’t mean the water is bad. In fact, if you use an ion exchange filter (as found in, say, Brita filter jugs) it actually replaces calcium ions in the water with hydrogen ions. For any non-chemists reading this: calcium ions are the little sods that cause your kettle to become covered in white scale (I’m simplifying a bit). Hydrogen ions make things acidic. In short, less calcium ions means less descaling, but the slight increase in hydrogen ions means a lower pH.

So, filtered water from such jugs tends to be slightly acidic. Brita don’t advertise this fact heavily, funnily enough, but it’s true. As it happens, I own such a filter, because I live in an area where the water is so hard you can practically use it to write on blackboards. After I bought my third kettle, second coffee machine and bazillionth bottle of descaler, I decided it would be cheaper to use filtered water.

I also have universal indicator strips, because the internet is awesome (when I was a kid you couldn’t, easily, get this stuff without buying a full chemistry set or, ahem, knowing someone who knew someone – now three clicks and it’s yours in under 48 hours).

The pH of water that’s been through a (modern) ion-exchange filter tends to be slightly acidic.

The water in the glass was filtered using my Brita water filter and tested immediately. You can see it has a pH of about 5. The water straight from the tap, for reference, has a pH of about 7 (see the image below, left-hand glass).

The woman in the YouTube video would be throwing her Brita in the trash right now and jumping up and down on it.

So, alkaline water is pretty pointless from a health point of view (and don’t even start on the whole alkaline diet thing) but, what if you LIKE it?

Stranger things have happened. People acquire tastes for things. I’m happy to accept that some people might actually like the taste of water with a slightly alkaline pH. And if that’s you, do you need to spend many pounds/dollars/insert-currency-of-choice-here on expensive bottled water with an alkaline pH?

Even more outlandishly, is it worth spending £1799.00 on an “AlkaViva Vesta H2 Water Ionizer” to produce water with a pH of 9.5? (This gizmo also claims to somehow put “molecular hydrogen” into your water, and I suppose it might, but only very temporarily: unlike carbon dioxide, hydrogen is very insoluble. Also, I’m a bit worried that machine might explode.)

Fear not, I am here to save your pennies! You do not need to buy special bottled water, and you DEFINITELY don’t need a machine costing £1.8k (I mean, really?) No, all you need is a tub of….

… baking soda!

Yep, good old sodium bicarbonate, also known as sodium hydrogencarbonate, bicarb, or NaHCO3. You can buy a 200 g tub for a pound or so, and that will make you litres and litres and litres of alkaline water. Best of all, it’s MADE for baking, so you know it’s food grade and therefore safe to eat (within reason, don’t eat the entire tub in one go).

All you need to do is add about a quarter of a teaspoon of aforementioned baking soda to a large glass of water and stir. It dissolves fairly easily. And that’s it – alkaline water for pennies!

Me* unconvinced by the flavour of alkaline water (*actually me).

Fair warning, if you drink a lot of this it might give you a bit of gas: once the bicarb hits your stomach acid it will react to form carbon dioxide – but it’s unlikely to be worse than drinking a fizzy drink. It also contains sodium, so if you’ve been told to watch your sodium intake, don’t do this.

If I had fewer scruples I’d set up shop selling “dehydrated alkaline water, just add water”.

Sigh. I’ll never be rich.

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Words of woo: what does ‘alkalise’ mean?


‘alkaline’ diets usually revolve around eating lots of fruit and vegetables – no bad thing, but it won’t change your body’s pH

If you hang around in the unscientific chunks of the internet for any length of time, as I find myself doing from time to time, you start to come across certain words that get used over and over. They are usually words that sound very sciency, and they’re being used to make things sound legitimate when, if we’re honest, they’re really not.

One such word is ‘alkalise’ (or ‘alkalize’). I’ve met it often ever since I wrote my post ‘Amazing alkaline lemons?‘. So, what does this word mean?

Good question. Google it, and at least the first three pages of links are about diets and how to ‘alkalise your body’ featuring such pithy lines as:

“It’s not really a diet… it’s a way of eating” (is there a difference?)
“Alkalise or live a life of misery” (gosh)
“Alkalise or die” (blimey)
“Alkaline water” (apparently this is a thing)
“Why it’s important to alkalise your water” (using our overpriced products)

In fact, I had to click through several pages of Google links before I even got to something that was simply a definition. (I’m aware that Google personalises its search results, so if you try this yourself you might have a different experience.) Certainly, there are no legitimate chemistry, biochemistry – or anything else like that – articles in sight.

Hunt specifically for a definition and you get turn basic and less acidic; “the solution alkalized”‘ (The Free Dictionary), to make or become alkaline. (Dictionary.com) and, simply, ‘to make alkaline’ (Collins).


pH 7 is neutral, more than 7 is basic

The first of these is interesting, because it refers to ‘basic’. Now, as I’ve explained in another post, bases and alkalis are not quite the same thing. In chemistry a base is, in simple terms, anything that can neutralise an acid. Alkalis, on the other hand, are a small subset of this group of compounds: specifically the soluble, basic, ionic salts of alkali metals or alkaline earth metals.

Since there are only six alkali metals (only five that are stable) and only six alkaline earth metals (the last of which is radium – probably best you steer clear of radium compounds) there are a rather limited number of alkalis, namely: lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, caesium hydroxide, beryllium hydroxide, magnesium hydroxide, calcium hydroxide, strontium hydroxide, barium hydroxide and radium hydroxide. There you go. That’s it. That’s all of them. (Okay, yes, under the ‘soluble in water’ definition we could also include ammonium hydroxide, formed by dissolving the base, ammonia, in water – that opens up a few more.)

This, you see, is why real chemists tend not to use the term ‘alkalise’ very often. Because, unless the thing you’re starting with does actually form one of these hydroxides (there are some examples, mostly involving construction materials), it’s a little bit lead-into-gold-y, and chemists hate that. The whole not changing one element into another thing (barring nuclear reactions, obviously) is quite fundamental to chemistry. That’s why your chemistry teacher spent hours forcing you to balance equations at school.

No, the relevant chemistry word is ‘basify‘. This is such a little-known word that even my spell checker complains, but it’s just the opposite of the slightly better-known ‘acidify’ – in other words, basify means to raise the pH of something by adding something basic to it. Google ‘basify’ and you get a very different result to that from ‘alkalise’. The first several links are dictionary definitions and grammar references, and after that it quickly gets into proper chemistry (although I did spot one that said ‘how to basify your urine’ – sigh).

What does all this mean? Well, if you see someone using the word “alkalising” it should raise red flags. I’d suggest that unless they’re about to go on to discuss cement (calcium hydroxide is an important ingredient in construction materials) cocoa production or, possibly, certain paint pigments, then you can probably write off the next few things they say as total nonsense. If they’re not discussing one of the above topics, the chances are good that what they actually know about chemistry could safely fit on the back of a postage stamp, with space to spare, so nod, smile and make your escape.

For the record, you absolutely don’t need to alkalise your diet. Or your urine*. Really. You don’t.

And please don’t waste your money on alkaline water.


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There’s no good evidence that drinking lemon juice has a significant impact on urine pH.

* In the event that you actually have problematically acidic urine, perhaps due to some medical condition, there are proven treatments that will neutralise it (i.e. take it to around pH 7, which is the pH urine ought to be, roughly). In particular, sodium citrate powder can be dissolved in water to form a drinkable solution. Of course, if this is due to an infection you should see a doctor: you might need antibiotics – urinary tract infections can turn nasty. Yes, I am aware that the salt of the (citric) acid in lemons is sodium citrate, however there is no good evidence that drinking lemon juice actually raises urine pH by a significant amount. And yes, I’m also aware that dietary intake of citrate is known to inhibit the formation of calcium oxalate and calcium phosphate kidney stones, but that’s a whole other thing. If you have kidney stones there are a number of dietary considerations to make, not least of which might be to cut down on your consumption of certain fruits and vegetables such as strawberries and spinach (and ironically, if you look at some of the – entirely unscientific – lists of acid-forming and alkali-forming foods these are almost always on the alkaline side).