People often ask this question -- "Why doesn't a chelator work very well to reduce soap scum when I use soap in a sink full of dishwater or doing laundry or bathing in the tub?" Here is the answer --
A chelator like citrate (made from citric acid) or EDTA in your soap is effective at treating small amounts of hard water to reduce soap scum. An example would be the water that is in your washcloth when you shower or when you "spot wash" a dish with a soapy sponge.
When you wash clothes in a washing machine, wash dishes in a sink full of water, or take a tub bath, a lot more water involved, and thus there are more hard water minerals to treat.
It is physically impossible to pack enough chelator into a soap so it can effectively treat a lot of hard water. You must instead treat the water with a whole house water softener or with a water-softener product such as borax, washing soda, or zeolites.
Here are the details about why a chelator added to soap does not work to soften a lot of hard water --
How much chelator is typically added to soap?
Let's say I make laundry soap with citric acid at 3% of the total soap weight plus enough extra NaOH to react with the citric acid. That means I am adding 3 grams of citric acid (3) to every 100 grams of soap.
I use about 15 grams of soap per load of laundry. Since citric acid is 3% of the soap, this means I am adding 0.45 grams of citric acid to each load.
How much hard water mineral can this chelator remove?
Using simple chemical calculations (well, simple for a chemistry geek!), I estimate the citrate created by this amount of citric acid will chelate roughly 0.23 grams (230 milligrams) of water hardness, assuming the hardness is all calcium carbonate.
The US Geological Survey says --
"...0 to 60 mg/L (milligrams per liter) as calcium carbonate is classified as soft [water]; 61 to 120 mg/L as moderately hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard..." (1)
The hardness of my water is about 100 mg/L (somewhere in the moderately hard range.) The citrate in my 15 grams of laundry soap can treat the hard water in 2.3 liters (about 2.5 quarts) of water.
How much water is used to wash a load of laundry?
"...Most high-efficiency washers use only 15 to 30 gallons (56.8 to 113.6 L) of water to wash the same amount of clothes as older washers [using] 29 to 45 gallons per load (109.7 to 170 L). The most efficient washers use less than 5 gallons (18.9 L) per cubic foot of capacity...." (2)
I have a high efficiency washer. I'm going to assume the actual water per wash load is 56 liters (15 gallons).
How much citric acid is needed to soften this amount of hard water?
I calculated the 3% citric acid in my 15 grams of laundry soap is able to soften 2.3 liters of water. But I need to soften 56 liters of water!
I would need to use 56/2.3 = 24 times more soap to each load to get that much citric acid into the wash water. That is 15 grams X 24 = 365 grams of soap.
This is an unrealistic amount of soap to use per load of laundry.
Another approach would be to add more citric acid to the soap. I would have to increase the citric acid content in the soap recipe from 0.45 g per load to --
0.45 g X 56/2.3 = 11 grams citric acid per load
That increases the citric acid content of the soap from 3% to a whopping 73%. This is not a good idea.
Rather than add citric acid to the soap, a person might wonder, "Why not add the citric acid directly to the laundry water?" Near the end of this article, I explain why this is not a good idea either.
Okay, I get it about citrate, but what about EDTA or other chelators?
The situation remains essentially the same no matter what chelator you add to your soap. As I said earlier, it is physically impossible to pack enough of any chelator into a soap so it can effectively treat a lot of hard water. You must instead treat the water with a whole house water softener or with a water-softener product such as borax, washing soda, or zeolites.
So why is washing soda or borax a better choice than a chelator?
Washing soda or borax is added separately from the soap, so you can use as much or as little as needed to get the results you want.
Washing soda and borax are less expensive and there are fewer environmental concerns compared with some chelators, such as EDTA, typically used in soap.
How much washing soda to use?
Very roughly speaking, washing soda can remove about its own weight in hard water minerals.
My water has 100 mg/L of hardness as calcium carbonate (CaCO3). If my washer uses 56 liters of water per load, I need to add at least this much washing soda to soften the water --
Washing soda weight = (1 part washing soda/1 part CaCO3) X 100 mg CaCO3/L X 56 L/washer load = 5600 mg / 1000 mg/g = 5.6 grams / washer load
My laundry mix recipe calls for a 1:1 ratio of soap and washing soda by weight. I use about 30 grams of this mix per load. So for every 15 grams of soap I add per load, I am also adding 15 grams of washing soda.
That's about 2.5 times more washing soda than is strictly needed for softening the water. (I explain why in the section after next.)
How much borax to use?
Borax is not as efficient at softening water. Very roughly 4 parts borax by weight are needed to remove 1 part hard water minerals.
Since my moderately hard water has 100 mg/L of hard water minerals, that means I have to add about 400 mg/L borax to every liter of water just to soften the water.
Putting that in everyday terms, I would need to add this much borax per load --
Borax weight = (4 part borax/1 part CaCO3) X 100 mg CaCO3/L x 56 L/washer load = 22400 mg / 1000 mg/g = 22.4 grams borax / washer load
As with washing soda, it is best to use more borax than this (see next section). I might choose to use twice as much borax, which means I would need about 45 grams of borax and 15 grams of soap per laundry load. That is a ratio of 1 part soap to 3 parts borax by weight.
Why not add just enough washing soda or borax to treat the hard water and no more?
Washing soda or borax has another job to do in the wash water besides treating hard water minerals. It also must keep the wash water alkaline.
If I add only enough washing soda or borax to just treat the hard water minerals, there will not be enough washing soda or borax to also maintain a high pH. If the pH in the wash water is not high enough, the soap cannot do a good job of cleaning.
Why not just add citric acid in the wash water to treat hard water?
Citric acid fails at both jobs -- removing hard water minerals and keeping the wash water alkaline.
Any acid, including citric acid, will lower the pH of the wash water below a pH of 7. Soap must remain alkaline to be a functional cleaner. In acidic conditions, soap entirely breaks down chemically and cannot clean.
Also, citric acid is not a chelator. It must chemically react with a sufficient amount of an alkali, such as sodium hydroxide, to form citrate. It is the chelator citrate that is able to react with hard water minerals and soften water.
See also https://www.soapmakingforum.com/threads/laundry-soap-and-hard-water-questions.75237/
(1) Water Hardness School. Hardness of Water. US Geological Service. https://www.usgs.gov/special-topic/water-science-school/science/hardness-water
(2) Home Water Works. Clothes Washer: Crisp, clean clothes without the waste. Alliance for Water Efficiency. https://www.home-water-works.org/indoor-use/clothes-washer
(3) We sometimes talk as if citric acid is a chelator, as I have in this article, but this is only a shortcut figure of speech, not strictly correct chemistry. You must neutralize the citric acid with a base, such as NaOH, to form citrate. It is citrate that is the chelator, not citric acid. More about the correct use of citric acid in soap...
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