How to Raise Alkalinity in a Hot Tub (Step-by-Step Guide)


Ensuring that alkalinity levels are within the correct range is vitally important for the safe running of your hot tub and its components. So how do you raise the alkalinity when it’s low?

There are two ways to raise alkalinity in a hot tub: baking soda (sodium bicarbonate) and soda ash (sodium carbonate). Baking soda raises alkalinity levels with minimal effect on the pH. On the other hand, soda ash raises both the alkalinity and pH, so it should only be used if both are low.

In this article, you’ll learn:

  • The two best ways to raise alkalinity
  • The damaging effects of low alkalinity
  • The cause of low alkalinity
  • How to prevent it from reoccurring

Two ways to raise alkalinity in a hot tub

Before adjusting the total alkalinity (TA), make sure you know the capacity of your hot tub so you can calculate the required dosage.

The ideal total alkalinity is between 80-150 parts per million (ppm). Ensure that the TA is in the optimal range before moving on to the pH. In some cases, achieving the right TA level may get your pH in the correct range.

How to raise alkalinity using sodium bicarbonate

Sodium bicarbonate, which is commonly known as baking soda (or bicarb in the UK), is one of the most efficient ways of raising alkalinity in spa water.

Baking soda rapidly raises alkalinity without increasing the pH too much, so use it if the alkalinity is low and the pH is within range.

Baking soda is not harmful, but it can irritate the skin and eyes, so you may want to wear protective gear.

What you’ll need:

How to raise alkalinity using sodium bicarbonate:

  1. Use a liquid test kit (most accurate) or test strips to take a reading of the current alkalinity level.
  2. Calculate the dosage your hot tub requires by following the instructions on the bottle.
  3. Put on rubber gloves and goggles.
  4. For best results, fill a clean bucket with water from the spa. Add the baking soda to the bucket and stir until fully dissolved. Alternatively, you may sprinkle the baking soda directly into the spa water.
  5. Turn on the pump and jets. Pour (or sprinkle) the solution into the spa and allow it to circulate for six hours with the cover removed.
  6. Retest and adjust if necessary.

Sodium bicarbonate has a pH of 8.4. This means that it’s only slightly higher than the pH of your spa water and is the reason why it has little effect on raising pH levels.

The exact amount of baking soda your spa needs will differ each and every time you have to raise alkalinity levels. This is because the amount required depends on the current levels of pH, TA, free chlorine, CYA, calcium hardness, TDS, and water temperature.

For this reason, it’s better to increase the alkalinity gradually rather than adding large amounts of baking soda all at once.

As a general rule, you’ll need to use 1/4-ounce (7g) of baking soda per 100 gallons (380 liters) of spa water. A 1/4-ounce of baking soda in 100 gallons will raise alkalinity by approximately 10 ppm. The increase in pH is so insignificant that it cannot be measured using standard test kits.

ProductDosage (imperial) Dosage (metric)
Sodium bicarbonate1/4 ounce per 100 gallons of water7g per 380 liters of water

I’ve listed Alkalinity Increaser in the tools list as it has clear user guidelines for those that aren’t sure. However, it’s not necessary to shell out so much as it’s no different from pure baking sodaOpens in a new tab..

Make sure to keep the cover off whenever you add chemicals to allow the water to breathe. This releases obnoxious chemical odors into the air and prevents damage to your vinyl cover.

I recommend using a liquid test kit as it provides you with more precise readings for making small adjustments that aren’t possible with simple test strips.

How to raise alkalinity using soda ash

Soda ash (sodium carbonate), also known as washing soda, is an alternative way to raise alkalinity. While soda ash does raise alkalinity, it is far more effective at raising pH. Only use soda ash if both the total alkalinity and pH levels are low.

It’s best to wear protective gear when handling soda ash as direct contact with skin and eyes may cause harm. Sodium carbonate dust is harmful if inhaled, causing irritation to the respiratory tract and a burning sensation in the nose and throat.

What you’ll need:

How to raise alkalinity using soda ash:

  1. Take a sample from your spa and test the current total alkalinity level.
  2. Calculate the dosage your hot tub requires by following the instructions on the bottle.
  3. Put on rubber gloves, goggles, and a mask.
  4. Use a clean bucket and fill it with water from the spa. Add the soda ash to the bucket and stir until fully dissolved.
  5. Turn on the pump and jets. Pour in the solution and allow it to circulate for six hours.
  6. Retest and make adjustments if necessary.

Sodium carbonate has a pH between 11.3 and 11.7, which means that it’s highly alkaline. This is why it has such a dramatic effect on raising pH levels.

Just as with baking soda, the exact amount of soda ash your spa requires will differ each time you have to raise the total alkalinity levels.

The current levels of pH, TA, free chlorine, CYA, calcium hardness, TDS, and water temperature in your spa water all affect the dosage amount that is needed.

Err on the side of caution, increasing the alkalinity gradually rather than adding large amounts of soda ash all at once.

As a general rule, you’ll need to use 1/8-ounce (3.5g) of sodium carbonate per 100 gallons (380 liters) of spa water. 1/8-ounce will raise alkalinity by approximately 8.6 ppm. Keep in mind that this dosage will increase the pH by 0.4.

ProductDosage (imperial) Dosage (metric)
Sodium carbonate1/8 ounce per 100 gallons3.5g per 380 liters of water

Again, ensure you keep the cover off after adding soda ash to release chemical odors into the air and prevent damage to your vinyl cover. Using a liquid test kit provides more accurate readings than test strips.

Note: Soda ash can turn hot tub water cloudy. If this happens, check out my guide on how to clear cloudy water.

Why is the alkalinity low in my hot tub?

If you do your best to keep on top of your spa’s water chemistry, you might wonder why it’s suddenly become so low. Here are the reasons why you’re experiencing low alkalinity.

The causes of low alkalinity in a hot tub are contaminants introduced by bathers, overuse of alkalinity lowering products (sodium bisulfate or muriatic acid), overuse of chlorine tablets, and excessive rainwater. Alkalinity levels need to be checked on a weekly basis to ensure they’re within the correct range.

Every time you use your hot tub, some 100 million new bacteria are introduced into the water from your skin and from personal care products. This has a significant effect on the water chemistry, which is why it’s important to shower and rinse off before entering the tub.

Another reason could simply be user error. As you treat the water with alkalinity and pH decreaser to rid the water of contaminants, it might well be that you simply added too much, lowering the alkalinity levels.

If you use chlorine tablets (trichlor), it could be the cause of low alkalinity levels. Many hot tub owners wrongly view chlorine tablets as ‘set and forget’, so they fail to test and adjust their spa water.

Because chlorine tablets are highly acidic (2.9 pH), they reduce alkalinity and pH levels more quickly than pH neutral products like chlorine granulesOpens in a new tab.. Remember to balance the water more often if using chlorine tablets.

If you’re looking for a system that is truly ‘set and forget,’ I highly recommend that you check out Silk BalanceOpens in a new tab..

Another reason why alkalinity levels drop is rainfall. As rain falls to the ground, it combines with air pollutants, which make the droplets acidic. A heavy downpour of rain can quickly lower the alkalinity and pH levels.

As long as there isn’t a storm, using a hot tub in the rain is mostly safe. However, you might want to invest in some sort of shelter to avoid diluting the water with acidic rainwater.

Why is low alkalinity in hot tubs bad?

None of us signed up for a chemistry degree when we purchased our hot tub, but that’s often how we’re left feeling. But understanding how low alkalinity negatively affects your hot tub is important for a number of reasons.

Low alkalinity is bad because it causes pH levels to become unstable, causing the water to become too acidic. When the alkalinity level is too low, the pH will fluctuate rapidly as it can’t properly counteract the effects of acids. Low pH is a health hazard and causes damage to the hot tub and its components.

Total alkalinity is the measurement of water’s ability to resist changes in pH. Low alkalinity is more serious than high alkalinity. It usually causes the pH of the water to decrease and become acidic, resulting in discomfort to bathers.

Low pH results in itchy and dry skin, irritation to the eyes and nasal passages, and may even cause respiratory problems in some people.

Because low-alkaline water cannot balance the number of phosphates, the water can turn green as it acts as food for algae.

Low alkalinity has a significant effect on your hot tub too. Corrosive water can cause costly damage to the pipes, pump seals, heater, heating elements, and other metal components in the water handling system.

The high water temperatures and rapid flow rate in hot tubs mean that the water is even more corrosive than usual. Acidic water can cause pitting, etching, cracking, and delamination of the spa shell.

Calcium scaling caused by low alkalinity levels can result in a build-up that clogs plumbing and also causes staining, making it difficult to keep surfaces clean.

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