Hot vs Cold Descaling Solution

[BaristaMcBob]

Since limescale is soluble in cold water, would a cold descaling solution be more effective?

[BaristaBoy E61]

What are you planning to use to descale?

[homeburrero]

Since limescale is soluble in cold water, would a cold descaling solution be more effective?

No. Descaling is not so much about dissolving limescale as calcium carbonate and calcium bicarbonate, it's about using the acids to create a chemical reaction that converts the limescale to a soluble calcium salt. For vinegar (acetic acid) the calcium carbonate reacts with the acetic acid to eventually form calcium acetate, carbon dioxide, and water. For citric acid it forms calcium citrate, carbon dioxide, and water. These reactions progress faster if the solution is warmer.

P.S
In case someone wants to resurrect a little of their high school chemistry, and see the reaction's formula, For acetic acid:
CaCO₃ (s) + 2CH₃COOH (aq) → Ca(CH₃COO)₂ (aq) + H₂O (l) + CO₂ (g)

[WWWired]

Since limescale is soluble in cold water, would a cold descaling solution be more effective?

Both BaristaBoy E61 and homeburrero's posts hit the mark perfectly! Its important to consider what the materials are being descaled and also that the descale is chemical process, not a mechanical dissolving process.

There is a lot to consider with such a question. It would depend on how "effective" is defined possibly. Possible "effective" definition might include ability to:

1. Remove "limescale;"
2. (2) Do as little damage to all machine components as possible;
3. (3) Provide any protective benefits of "descale solutions" (passivation etc.).

First things first - Why is descaling-effectiveness important?
The first best defense against "limescale" is suggested by many to be through use of properly treated water (filtered or for higher end plumbed in machines through use of an inline water treatment system). Limescale is likely an umbrella term that includes compounds of Calcium, magnesium and silica found in most water supplies. Descaling has been suggested to be one of the most, if not most, important maintenance activities for higher quality espresso machines. This might be particularly true for machines that have a lot of thermal and other data collecting probes and sensors. Some espresso machine models have PID control systems that depend on receiving data from various sensors throughout a machine often. Any mineral sale buildup on sensors or probes can affect a PID significantly and interfere with the attempts by the computer algorithm to reach its set point. This is also true in non-PID systems such as Heat-Exchange (HX) machines that that have thermal and pressure monitoring components that depend on efficient transfer of kinetic energy between two fixed-volume-pressure-vessels. Scaling and fouling will increase most systems resistance to heat transfer that is often crucial for machines whose central functions involve heating and cooling water (like espresso machines).

The Descaler Goldilocks Temperature Zone:
"Not too hot and not too cold, but just right" the Goldilocks and the Three Bears Quote goes. Steven Hawking suggested an Earth-like planet anywhere in the Universe would be "like Goldilocks . . . just right" for being habitable. In keeping with this concept, some suggest at about 55℉ (approx. 12℃), the exothermic equilibrium chelation chemical reaction in a 10% solution of Citric Acid or Acetic Acid is desireable relating to descaling certain metals and surfaces. Here, "nominal" might be in relation to the characteristics of the chemical reaction occurring are effective but not damaging to the machine components/metals etc.

However, as homeburrero has mentioned above, there may be a "faster" to be had here by increasing temperature (kinetic energy in the system) . . . and this may not be as "efficient" a reaction as at a lower temperature, but it may in deed be faster . . . someone hopefully will add some further insight in another comment. Many of us, me included, have considered at least "luke warm" to be ok and effective for our 2 Tblsp to 1 Liter (quart) of water Citric Acid descaling solutions. I've never actually done any tests on identical strips of different metals using different descale solutions and different temperatures, pressures, and concentrations of chemical reactants the way YouTuber "Project Farm" might lol . . . but perhaps we can get HB's founder to reach out to "Project Farm" creators on YouTube to do a special cross-platform collaboration between HB and Project Farm?

So if a descale solution is 10% (14 Tbspns Citric Acid to 1L/quart or 10% Acetic Acid), and 55℉ (12℃) is considered to be "cold," then this might be considered to be "effective" if the mineral affected surfaces are subject to corrosion or are part of a plating of one metal over another etc. If the underlying metal or polymer (a Teflon tube for example) is resistant to chemical attack by acids (even weak acids) then increasing the rate of reaction can be considered more "effective" possibly. The three ways to possibly increase a rate of reaction are by varying Temperature, Pressure, and Concentration of the chemical reactants. Consideration when attempting to increase the rate of reaction as to whether the reaction is endothermic or exothermic and what direction the reaction will be favored in (forward or back) when temperature, pressure and/or concentration of reactants is varied in can also assist in determining "effectiveness." (spoiler alert: it appears from many posts that a lower temperature favors a more efficient descale chemical reaction involving an exothermic chelation reaction in the forward direction and thus the 55℉/12℃ temperature for a 10% solution of descale acids - but again, this may not mean "faster").

Another important consideration might involve what is the descale solution composed of. A quick survey of various descaling solutions by different producers reveals a wide variety of different acids being used in different concentrations. This is why many folks often suggest "Follow the manufacturers recommendations as to what descaler to use." This can be an important consideration since not all machines have the same metals and component materials. Breville machines, for example, may contain stainless steel components, whereas some other models, a classic highest quality Nuova Simonelli espresso machine from the 1990s perhaps, may include only brass and copper alloys.

Citric Acid is considered to be anywhere from three to ten times the strength (effectiveness) of basic Vinegar (acetic acid, also called ethanoic acid). The reason for this is that Acetic acid has a p𝐾ₐ value of 4.76, while Citric acid has a p𝐾ₐ₁ value of 3.13 and a p𝐾ₐ₂ value of 4.76. This might imply that Citric Acid is about ten times as acidic as Vinegar (Acetic or ethanoic) acid . . . however, this is conditional of course on the concentration in solution, so 1 or 2 tablespoons in a liter (quart of water) will be about 3x as strong as regular off-the-shelf 4% Acetic Acid (Vinegar).

The fascinating stories of a disappearing boiler & weeks of citric acid soaking:
There are many great accounts on Home-Barista to explore of how to use Citric Acid and other descalers. They all have great merit to read and learn from. One such story suggested hearing about someone who put their expensive espresso machine boiler into a bucket with Citric Acid and came back after a while only to discover someone had stolen their boiler. Upon closer inspection they noticed the bottom of the pail had a bunch of dark material in it and it turned out that after draining the water, the dark residue had the same mass as their boiler . . . of course this is a complete myth but a fascinating and very fun story. It would take an incredible amount of Citric Acid to do any real damage to a metal. Another great account involves folks who might leave their item to be descaled in a citric acid bath for several days or a week. This is not harmful as what some explanations suggest has occurred is the one or two tablespoons of Citric Acid in the solution will quickly become chemically exhausted and bound up through its chemical reaction with the mineral scale. Once all the chemical reactions have been exhausted, the Citric Acid ceases to be reactive. There is a change in the pH of the resultant solution which may have a minor effect it has been suggested. So like a jar of jelly beans, once you eat the last jelly beans that's it . . . but it's good while it lasted and if you want more you have to get an other jar of jelly beans or refresh your descale solution by replacing the spent solution with a fresh batch.

The higher efficiency reaction of Citric Acid (compared to other weak acids like vinegar) might have something to do with Citric Acid's greater acidity equating to a lower pH in the resulting solution resulting in faster Base protonation. This lower resultant pH in the reactant solution may explain the faster reaction. In a chemical equation the bottom line could be that the Calcium Carbonate + Citric Acid³ ⇌ equation might be more right side shifted than is the Acetic Acid, perhaps due to the Complexation super-powers of Citric Acid, which can result in increased Calcium chemical-reaction-dissolution (not to be confused with mechanical dissolution).

So back to the Original Posters Question:
Is a colder solution more effective? If the chemical equation is a right shifted equilibrium reaction and the reaction exothermic, then lowering the temperature might favor a more "efficient" forward (right shifting) direction of the reaction. There are a lot of very highly trained chemistry folks here and experts on Home-Barista that have a huge amount of experience with this matter that can clarify and correct this conclusion as part of this very interesting question raised by the original poster and the following discussion (including this reply) This was actually counter-intuitive to me as generally I think we all assume that adding some kinetic energy (increasing the temperature or pressure) will increase a chemical reaction. This may in fact be the case as well as it is entirely possible that even though the reaction is not "AS" efficient as it may be at lower temperatures, it could still end up being way faster due to the higher temperatures (increased kinetic energy/activity). It appears that it might be possible that there's a Goldilocks situation present here where actually a lower temperature (of some range) might favor a forward chemical reaction of the exothermic chelation chemical reaction between Citric Acid and Calcium Carbonate (CaCO₃) . . . I'm not sure, but that's why we have forums! haha! awesome! Hope to see more comments and a little more insight to light our paths . . .

[BaristaMcBob (original poster)]

Wow - amazing answers. Thank you!!
I'm an engineer by training, so I appreciate the details, although chemistry is not my forte.

I'm descaling an ECM HX-type home machine. And in this case, I don't have the option to open it or access the boiler directly.

How does one determine the strength (e.g., "5%") of a citric acid solution. Practically speaking, I have a one-pound bag of food grade citric acid in my kitchen. So is it a simple matter of weight? That is, 50g citric acid powder in 1 liter of water = 5% strength?

[homeburrero]

So is it a simple matter of weight? That is, 50g citric acid powder in 1 liter of water = 5% strength?

Yes. When speaking of citric acid, the percentage is conventionally by mass -- grams/grams, and using 1000 grams/liter as the density of the water solution.

Be aware that In cases like vinegar, where the acetic acid is normally a liquid, the percentage may be by volume -- ml/ml, which only makes a slight difference. The density of acetic is 1.05 g/ml, so a 5% by volume acetic acid would be around 5.3 % by mass.

P.S.
I'm pretty sure the Ball citric acid that you get in the canning section of the store is pure anhydrous citric acid, but they don't clearly state that. Citric acid is also sometimes sold as the monohydrate, in which case you'd need to correct for the water to be perfectly exact. Each gram of citric acid monohydrate contains about 0.91 grams as anhydrous citric acid. The monohydrate powder is also a little denser, so if you are measuring by tablespoon it comes out close enough irrespective.

[homeburrero]

Is a colder solution more effective? If the chemical equation is a right shifted equilibrium reaction and the reaction exothermic, then lowering the temperature might favor a more "efficient" forward (right shifting) direction of the reaction.

I'm not sure I understand this, although it is true that the reaction rate of a chemical reaction is not always increased by temperature.

What the old hands (e.g., Paul Pratt, Jim Schulman) and chemistry experts (e.g. Dr Pavlis) here on HB have always told us was that heated descale solutions of these weak organic acids dissolve limescale more quickly. Makes sense to my somewhat limited understanding of physical chemistry.

One can easily verify this at home with a bit of crushed calcium carbonate - - a Tums tablet will work great here. Make up your descale solution with a cup of heated in one glass and a cup of room temp in another, then drop a Tums tablet in each. With white vinegar at 68F it fizzes noticably, but at 150F it fizzes violently.




The same should be true for other weak organic acids (citric, sulfamic, lactic) commonly used in decaling solutions. Here's one chart I found online:

[WWWired]

I'm not sure I understand this, although it is true that the reaction rate of a chemical reaction is not always increased by temperature.

What the old hands (e.g., Paul Pratt, Jim Schulman) and chemistry experts (e.g. Dr Pavlis) here on HB have always told us was that heated descale solutions of these weak organic acids dissolve limescale more quickly. Makes sense to my somewhat limited understanding of physical chemistry.

You mentioned some true legends for sure (as are you homeburrero and many of the other posters on HB) The Tums experiment you demonstrated above is fantastic! definitely shows a fundamental difference due to thermal conditions!

Many people possibly equate the dissolving of the Citric Acid crystals in warmer water (solution) with the reactivity of the solution . . . while it is true that the Citric Acid crystals dissolve much more quickly in hot water, this may not mean the higher temperature solution is more efficient or effective than a lower temperature (room or luke-warm) solution. Importantly, trying to dissolve Citric Acid Crystals in cooler liquid just results in the crystals sitting on the bottom as many of us have frustratingly witnessed lol! Paint might actually dry faster and the lawn need mowing sooner if the water is not warm enough to dissolve the crystals into solution . . . One possible preparation method (I've used effectively) is to put the Citric Acid Crystals in the container where the descaling will occur, add increments of 100ml or so of hotter water, and swirling or stirring until the crystals are dissolved in solution. Then add cooler water to lower the temperature and provide more favorable thermal conditions that favor the forward exothermic reaction.

I believe firmly that Paul Prat, Jim Schulman and everyone here (e.g. Dr Pavlis) and other HB posters are fundamentally correct that "heated descale solution" of these weak organic acids dissolve limescale more quickly, possibly because if the Citric Acid Crystals are not dissolving in the cooler solution (as they would do much more readily in hot water), the concentration will be lower . . . by first dissolving the Citric Acid Crystals in warmer water then cooling the solution with cooler water, a possible more favourable condition for the forward exothermic reaction might be achieved. Every year, more than 2 million tons of Citric Acid is produced for use in flavoring, chelating, and to acidify. Just six years after the Declaration of Independence Citric Acid was first crystalized from lemon juice haha! Appropriate since some of George Washington's accounting documents showed he imported his weight in Coffee beans in 1770, just 6 years shy of becoming the first President of the new United States of America! Coffee Powered perhaps? Maybe it wasn't Tea in Boston that powered the revolution after all

Would much appreciate any other insights and or confirmation or correction of the above speculation from me in this post relating to any others who also cool their Citric Acid solutions before descale to increase efficiency, but definitely that homeburrero Tums reference above is very impressive evidence of Acetic Acid thermal efficiencies!

I'm going to keep an eye open for three equal components (perhaps pipe fittings) from an espresso machine with approximately equal amounts of mineral scale buildup on them. I hope then to create a solution of dissolved Citric Acid powder in water and then split it into three portions. One I will put in the fridge to cool slightly, one will be left at room temperature, and the third will be heated slightly. My understanding is that as long as the Citric Acid is fully dissolved in solution, a lower temperature portion of the solution might favor more effective and efficient forward reaction of the exothermic chelation reaction. Not sure, but a fun experiment and something that is fairly regular to our awesome espresso machine hobbies and obsessions!