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Accurate Pool Chemical Dosages

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Some of you may have realized that some of the pool industry dosage charts for alkalinity adjustments differ with other charts. Here is why:

Previous to 1994, the dosage charts used in the pool industry for lowering alkalinity using muriatic acid and dry acid (sodium bisulfate) were off about 20% – leading one to use insufficient acid for the job at hand. Dosage charts for raising alkalinity using sodium bicarbonate were also off, by about 7%.

In a research report (published by Service Industry News in their March 11, 1994 issue), onBalance members (back before onBalance existed as such) announced to the industry both the error, the correct dosages, and why the errors existed! Most publishers of dosage charts changed their material in accordance with our corrections… but a couple to this day has not done so.

“Muriatic” acid is a slang term for hydrochloric acid, or a solution of water and HCl. The specific strength of acid we use in the pool industry is 31.45% HCl. However, the strength commonly used in laboratory applications, sometimes referred to as “concentrated” HCl, is 37%. About a 20% difference. Hmmm…

Previous to our research publication, the commonly quoted dosage for lowering alkalinity by 10 ppm in a 20,000 gallon pool was 1.3 quarts. Our chemical calculations showed that the correct dosage was 1.6 quarts – a 20% difference. Are we sensing a pattern here?

Apparently, sometime in the past someone asked a chemist to figure up a dosage chart for “muriatic” acid in pool water. He pulled his technical sheet off the shelf, and figured the chart on lab acid – not pool acid. Then perhaps, this chemist used his incorrect acid chart to do a conversion for dry acid dosages, which is also more than 20% off.

The sodium bicarbonate, or baking soda charts, as mentioned, used to be about 7% off – this time telling you to add too much for the effect needed. Of course, overdosing sodium bicarbonate is not particularly detrimental, only resulting in a slight overshooting of the alkalinity and 7% isn’t that much… however the correct dosage charts will say 2.8 pounds for a 10 ppm alkalinity increase in 20,000 gallons, where the old charts will say 3.0 pounds and some say to add 3.3 pounds, an 18% error.

So if your dosage chart of choice calls for 1.3 quarts of acid for 10 ppm in 20,000 gallons (instead of the correct 1.6 quarts), and/or calls for 3.0 pounds of sodium bicarbonate for 10 ppm in 20,000 gallons (instead of the correct 2.8 pounds) – change charts!

Below are some formulas used to calculate the dosage for various swimming pool chemicals.

 

SIMPLIFIED  FORMULAS for CHEMICAL ADDITIONS to POOLS

 

Lowering Total Alkalinity with Muriatic Acid:
(Volume of water in pool ÷ 125,000) X ppm desired decrease = quarts to add

Lowering Total Alkalinity with Sodium Bisulfate:
(Volume ÷ 47,000) X ppm desired decrease = lbs. to add

Raising Total Alkalinity with Sodium Bicarbonate:
(Volume ÷ 71,400) X ppm desired increase = lbs. to add

Raising Total Alkalinity with Sodium Carbonate:
(Volume ÷ 113,200) X ppm desired increase = lbs. to add

Raising Total Alkalinity with Sodium Sesquicarbonate:
(Volume ÷ 80,000) X ppm desired increase = lbs. to add

Sodium Hypochlorite: (Volume ÷ 30,000) X ppm desired increase = quarts to add
(Based on 10%/weight – this can vary slightly based on the strength and age of the product - approximation)

Calcium Hypochlorite: (Volume ÷ 78,000) X ppm desired increase = lbs. to add
(Based on 65% av Cl)

Calcium Hypochlorite: (Volume ÷ 60,000) X ppm desired increase = lbs. to add
(Based on 50% av Cl)

Lithium Hypochlorite: (Volume ÷ 42,000) X ppm desired increase = lbs. to add

Trichlor: (Volume ÷ 108,000) X ppm desired increase = lbs. to add

Dichlor (56% av Cl): (Volume ÷ 67,200) X ppm desired increase = lbs. to add
Dichlor (62% av Cl): (Volume ÷ 74,400) X ppm desired increase = lbs. to add

Sodium Sulfite: (Volume ÷ 67,250) X ppm unwanted chlorine = lbs. to add
(Amount in pounds to reduce excess chlorine)

Sodium Thiosulfate: (Volume ÷ 117,600 X ppm unwanted chlorine = lbs. to add
(Amount in pounds to reduce excess chlorine – based on 100% sodium thiosulfate pentahydrate)

Calcium Chloride: (Volume ÷ 83,300) X ppm desired increase = lbs. to add
(Amount in pounds to increase calcium hardness with 77% calcium chloride strength – use 101,700 as divisor for 94% strength)

Cyanuric Acid: (Volume ÷ 120,000) X ppm desired increase = lbs. to add
(Amount in pounds to increase cyanuric acid – based on 100% cyanuric acid strength)

Borax: (Volume ÷ 17,800) X ppm desired increase of boron = lbs. to add
(Based on using sodium tetraborate pentahydrate to increase ppm of boron

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Comment by Terry Arko on April 18, 2012 at 8:05am

Once again a very imformative, useful and helpful tool for pool pros!...Great to have the chemical conversions! Thanks Kim!

Comment by Kim Skinner on April 16, 2012 at 7:06am

Borax is probably more often sold as a "decahydrate" which contains a higher water content than the pentahydrate.  Because of that, that product it is more soluble than the pentahydrate. The forumula number for that kind of borax is "13,600."  

So "Volume divided by 13,600 times the ppm desired amount of boron equals the pounds" of sodium tetraboratepentahydrate (borax) to add.

 

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