Does adding chlorine tablets raise pH?

Adding chlorine tablets to pool water is a common way to sanitize pools and keep them free of harmful bacteria and algae. However, chlorine tablets can affect the pH of pool water. So does adding chlorine tablets raise the pH of pool water?

Quick answer: Yes, adding chlorine tablets tends to raise the pH of pool water over time. This is because chlorine tablets dissolve into hypochlorous acid and hypochlorite ions, which are alkaline chemicals that increase pH. However, the pH rise is usually gradual and often takes weeks or months to occur.

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How Do Chlorine Tablets Work?

Chlorine tablets, also known as trichlor tablets, are compressed pucks of trichloroisocyanuric acid (trichlor) and other stabilizers. Here’s an overview of how they work:

Trichlor tablets dissolve slowly when submerged in pool water.
The trichlor reacts with water to form hypochlorous acid (HOCl) and hypochlorite ions (OCl-).
Hypochlorous acid and hypochlorite provide free available chlorine (FAC), the active sanitizing agent.

FAC kills bacteria, oxidizes organic matter, and prevents algae growth.
The cyanuric acid stabilizers in the tablets reduce chlorine loss from UV rays.

So in summary, trichlor tablets rely on dissolving slowly and delivering hypochlorous acid and hypochlorite to disinfect the pool over an extended time period.

How Do Chlorine Tablets Raise pH?

When chlorine tablets dissolve, they form hypochlorous acid (HOCl) and the hypochlorite ion (OCl-). Both of these chlorine-containing chemicals are alkaline with high pH values around 12-13.

As these chemicals accumulate in the pool from the dissolving tablets, they raise the overall pH, making the water more basic. A typical chlorine tablet contains about 90% trichlor, which breaks down into about 70% hypochlorous acid and 30% hypochlorite when dissolved in water.

The hypochlorite ion is particularly long-lasting and builds up faster than hypochlorous acid. So the accumulation of hypochlorites is the main driver of pH rise when using chlorine tablets.

Gradual pH Rise

Importantly, the pH rise is gradual. Trichlor tablets are designed to dissolve slowly over days or weeks to provide sustained chlorine release. So the alkaline hypochlorous acid and hypochlorite also enter the pool water slowly.

This slow buildup typically prevents drastic spikes or fluctuations in pH. However, over weeks and months, the pH can creep higher and higher if left unchecked. The rate of pH rise depends on factors like:

Chlorine tablet dose and frequency of addition

Pool size and volume
Weather and sun exposure that breaks down chlorine
Other chemical additions that affect pH

Regular pH testing and balancing is important to monitor this slow climb in alkalinity.

How High Can pH Rise?

If enough chlorine tablets are continually added without managing pH, the levels can rise well above ideal ranges. Here are some example pH increase scenarios:

Adding the weekly recommended dose may raise pH by around 0.1-0.3 over a month.

Doubling the chlorine dose could raise pH by 0.2-0.6 in a month.
In summer, pH could climb by 0.5-1.0 per month due to faster chlorine degradation requiring more tablets.
Over six months, pH could potentially climb 1-3 points if not managed.

However, pools with automatic chemical dosing or companies that manage water chemistry tend to prevent extreme pH spikes. The rise is more gradual and spread out over a longer time frame.

Ideal Pool pH Range

For proper sanitization and swimmer comfort, pool water should be maintained in an ideal pH range. The recommended target is:

7.4 – 7.6 pH for chlorine pools.

7.2 – 7.8 pH for saltwater pools.

Below 7.2, the water can become irritating to skin and eyes. Above 7.8, chlorine disinfection starts to decline and scale formation increases.

So chlorine tablet pH increases should be minimized and managed through regular testing and balancing. Acid such as muriatic acid or sodium bisulfate is added to lower pH if it creeps too high.

Other Impacts of High pH

Rising pH from chlorine addition can also:

Reduce chlorine effectiveness and require using more tablets to maintain disinfection.
Contribute to cloudy water by reducing water’s ability to hold calcium in solution.

Increase the rate of chlorine consumption due to faster degradation at higher pH.
Lead to potential scaling and deposit buildup on pool surfaces and equipment from increased calcium carbonate saturation.

All these issues can raise pool maintenance costs if pH is not controlled.

Tips for Managing pH with Chlorine Tablets

Here are some tips for successfully managing pH rise when using trichlor tablets:

Test pH weekly to monitor changes.
Anticipate and account for pH rise from chlorine addition when balancing water.

Reduce chlorine dosing levels if rise exceeds 0.2 pH units weekly.
Shock pool water periodically to burn off built up hypochlorites.
Add acid (e.g. muriatic or sodium bisulfate) to lower pH as needed.

Use an automatic pH controller to maintain ideal levels.
Switch to unstabilized chlorine (liquid or granular) to avoid pH rise.

Proper chlorine tablet management prevents scaling, maintains sanitizer efficiency, and improves swimmer comfort.

Alternative Sanitizers Without pH Rise

Switching to unstabilized chlorine products or non-chlorine sanitizers can also help avoid pH increase:

Liquid chlorine – Unstabilized, does not raise pH.
Cal hypo (granular chlorine) – Unstabilized, contributes some alkalinity.

Bromine -Typically does not impact pH but can at very high levels.
Baquacil – Does not alter pH but lowering is often needed.
Salt systems – Produce chlorine directly in water and have minimal pH impact.

Each option has pros and cons to weigh like cost, convenience, maintenance, etc. But avoiding pH rise is possible with alternatives to trichlor.

Does Stabilized Chlorine Like Dichlor Also Raise pH?

Trichlor is not the only stabilized chlorine product used in pools. Dichlor (sodium dichloroisocyanurate) is another popular option containing about 60% available chlorine.

Like trichlor, dichlor also contains cyanuric acid stabilizers that slow chlorine degradation from UV light. This allows dichlor to provide longer-lasting sanitizing power.

However, dichlor has less impact on pH compared to trichlor tablets when used properly:

Dichlor breaks down into more hypochlorous acid and less hypochlorite than trichlor.
Hypochlorous acid has a neutral pH and does not affect pH like hypochlorite.

Dichlor has lower cyanuric acid levels (40-50%) vs trichlor (85-90%).
The lower stabilizer concentration reduces long-term buildup of hypochlorites.

So dichlor can still potentially raise pH, but generally has only about 25% the impact on pH compared to the same amount of available chlorine from trichlor tablets.

How to Minimize Dichlor pH Rise

Following proper dichlor dosing and maintenance practices can minimize pH increase:

Maintain cyanuric acid levels below 80 ppm.
Avoid overdosing dichlor beyond pool requirements.

Shock periodically to oxidize hypochlorite buildup.
Test and balance pH regularly.
Add acid (e.g muriatic) to reduce high pH as needed.

So dichlor has advantages over trichlor when pH management is a major concern. But proper dosing and oversight are still required to control pH.

Can You Add Too Much Trichlor Causing a Sudden pH Spike?

While the pH rise from chlorine tablets is usually gradual, it is possible to cause a sudden spike by severely overdosing trichlor.

Adding dozens of tablets at once far beyond the pool’s chlorine demand can rapidly boost pH. The excess trichlor dissolving can flood the pool with hypochlorous acid and hypochlorite.

This is most likely with very small pools, but improper dosing in bigger pools can also overwhelm the buffers and cause a pH jump.

Risks of Drastic pH Spikes

Spiking the pH suddenly with chlorine overdosing carries some risks:

Eye and skin irritation to swimmers.

Rapid loss of chlorine effectiveness.
Potential damage to equipment from highly alkaline water.
Cloudy water and scaling issues.

Thankfully such spikes are usually temporary. The pH will naturally drop over time. Diluting the water with fresh lower pH water can also help reduce severe spikes faster.

Preventing Sudden Spikes

The best way to prevent drastic pH spikes is by:

Adding chlorine according to label directions based on pool volume.

Avoiding dumping in lots of tablets at one time.
Testing pH before and after adding large amounts of chlorine.
Inspecting pool pH daily when first increasing chlorine doses.

With proper dosing technique and testing, trichlor addition can raise pH gradually without risk of sudden spikes.

Pool pH Changes When Switching to Chlorine Tablets

When transitioning a pool from another sanitizer to chlorine tablets, the change can alter pH in either direction:

From liquid or unstabilized chlorine – pH will rise from the switch to trichlor.

From dichlor – Possible slight pH increase from trichlor’s greater stabilizer concentration.
From bromine – pH may drop slightly since bromine tends to increase pH more than chlorine.
From Baquacil – pH will likely drop due to Baquacil’s pH rising effect.

From salt generator – Trichlor will usually raise pH relative to salt chlorine generation.

When transitioning sanitizers, give the pool water a few weeks to stabilize and adjust dosing and balancing accordingly. Getting pool water into the ideal pH range with trichlor may require some acid addition.

Do Chlorine Tablets Lower pH?

Chlorine tablets do not directly lower pH when dissolved in water. However, they can have an indirect lowering effect in some cases:

Heavy chlorine use can degrade cyanuric acid over time, reducing long-term buildup of hypochlorite at high pH.
Shocking to oxidize organic matter also burns off hypochlorite, which may slightly lower pH.
High chlorine concentrations kill algae. Dead algae release acids that can slightly reduce pH for a period.

But in general, the dominant effect of trichlor tablets is a slow rise in pH over time as stabilizer concentration and hypochlorite levels increase. The pH lowering mechanisms are secondary effects and minimal compared to the overall pH rise.

Conclusion

Adding chlorine tablets such as trichlor to pools does tend to raise the pH due to accumulation of hypochlorous acid and hypochlorite from the dissolving tablets. However, the rise is gradual over weeks and months rather than sudden spikes.

With proper routine testing and pH management using acid to lower alkalinity, trichlor tablets can effectively sanitize pools while keeping pH in the ideal range. Switching to dichlor or unstabilized chlorine products can also minimize long-term pH impacts.

Regular monitoring and adjustment is key to balancing pH for healthy pool water when using chlorine tablets as a sanitizer.