How to Calculate the Langelier Index

Written by james porter
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How to Calculate the Langelier Index
The LSI is useful in predicting whether pool water will dissolve or deposit calcium from the pool walls. (Swimming pool and pool house image by Jim Mills from Fotolia.com)

The Langelier Index, or Langelier Saturation Index (LSI), is a widely used method of measuring the saturation of calcium in water. Calcium concentration comprises the most common and significant factor of water hardness. Hard water has a relatively high amount of dissolved solids, mainly calcium, while soft water has a smaller amount than usual. People use the Langelier Index to predict whether water will corrode or calcify surfaces and how to rebalance the hardness or softness of the water. Calculating the LSI is fairly straightforward once the required measurements have been taken.

Skill level:
Moderate

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Things you need

  • Total Dissolved Solids (TDS) measurement in mg/L
  • Calcium measurement in mg/L
  • Alkalinity measurement in mg/L
  • pH measurement
  • Temperature of water in degrees Celsius
  • Scientific calculator

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Instructions

  1. 1

    Take the logarithm (base ten) of the TDS measurement. Subtract one from that, then divide by 10. Write down the result. This is the adjusted impact of the TDS on the pH.

  2. 2

    Add 273.15 to the temperature (to express it in Kelvin, rather than in degrees Celsius), then take the logarithm (base ten) of that. Multiply this number by -13.12 and add 34.55. Write down the result. This is the adjusted impact of the temperature on the pH.

  3. 3

    Take the logarithm (base ten) of the calcium measurement, and then subtract 0.4. Write down the result. This is the adjusted impact of calcium concentration on the pH.

  4. 4

    Take the logarithm (base ten) of the alkalinity measurement and write down the result. This is the impact of alkalinity on the pH.

  5. 5

    Add the two values from Steps 1 and 2. Add 9.3. Subtract the two values from Steps 3 and 4. This is the pH at which the water will be perfectly saturated with calcium and will neither dissolve calcium from surfaces nor deposit it onto them. In other words, this is the pH at which the LSI will be zero.

  6. 6

    Subtract the pH from Step 5 from the measured pH of the water. This is the LSI. If it is below zero, the water is over saturated and will tend to precipitate out calcium. If it is above zero, the water is under saturated and will tend to dissolve calcium. If it is within the range of --0.5 to 0.5, it is within what is generally considered to be acceptable parameters. If it is outside this range, you should take measures to adjust the water hardness.

Tips and warnings

  • If you don't know the TDS, you can estimate it based on the water's conductivity. See the Total Dissolved Solids page in the resources section.

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