Method Used:
- Gravimetric Method: A water sample is evaporated, and the residue is weighed.
Gravimetric Method (Standard Laboratory Procedure):
Equipment used:
Evaporating dish – It holds the water sample during heating and collects the residue after evaporation for weighing.
Water- bath/ Steam-bath – In a water bath, warm water utilizes to distribute temperature uniformly to samples for incubation
Membrane filter- It is used to filter the sample through 0.45 micron filter paper to eliminate the contamination.
Hot Air Oven – The oven evaporates water at 103°C–105°C or 179°C-180°C, leaving only dissolved solids behind for accurate weighing and TDS calculation.
Desiccator – The desiccator cools the evaporating dish after drying, preventing moisture absorption from air, ensuring accurate and stable weight of the dried residue.
Analytical balance – It is used to accurately measures the weight of the residue before and after evaporation, helps to determine the exact amount of total dissolved solids in the water sample.
Measuring cylinder – Used to accurately measures the water sample volume, ensuring correct TDS calculation.
Procedure:
- For one hour,heat the clean the evaporating dish to 180°C.
- After cooling in the desiccator, weigh it and keep it there until you’re ready to use it.
- Use membrane or filter paper to filter a portion of the sample.
- Take a known volume of the water sample in a measuring cylinder—for example, 100 ml water into an evaporating dish which is previously weighed and set over a steam bath.
- Once the water in the residue has completely evaporated kept it into preheated hot air oven.
- Set the oven to either 103°C or 179°C to dry the dish for about 1-2 hour to a uniform mass.
Calculation:
Total Dissolve Solid ( TDS ) , mg/I = 1000 M/ V
where
M= mass in mg of filterable residue, and
V- volume in ml of the sample
Purpose of testing:
- Assess Water Quality:
TDS testing helps determine the overall purity of water. It indicates the presence of inorganic salts (like calcium, magnesium, sodium, and potassium) and small amounts of organic matter. - Health Safety:
Excessively high or low TDS levels can affect human health. For example, high levels of sodium or nitrate can be harmful, especially for people with health conditions. - Taste :
TDS affects the taste of water. Very high or low levels may give water a bitter, salty, or flat taste, which is unpleasant for drinking. - Indicator of Contamination:
Sudden changes in TDS levels can signal pollution or contamination from industrial waste, sewage, or agricultural runoff. - Check Effectiveness of Purification Systems:
To monitor the performance of water filters, reverse osmosis (RO) systems, and other purification units to ensure they are working properly. - Compliance with Standards:
TDS testing ensures water meets regulatory standards, such as IS 10500 in India or WHO guidelines, which recommend a maximum limit of 500 mg/L for safe drinking water. - But we, Pro Research & Testing Laboratory, recommend that the TDS limit of your drinking water should be between 75-140 mg/l because low TDS contains low level of minerals which ultimately doesn’t impact our health.
Basic handling mistakes we should keep in mind during the testing:
The accuracy of the test results may be impacted by additional measures that should be taken in along with wearing gloves or masks.Make sure there is no splashing or spilling throughout evaporation to prevent sample loss.During TDS testing of drinking water, use clean and dry equipment to avoid contamination.Once the dish has cooled, weigh it to prevent residue from remaining for too long because some residues are hygroscopic and can collect water from partially dried desiccant. The water sample should be measured accurately, and distilled or deionized water should be used for rinsing glassware.In the gravimetric method, the sample must be evaporated carefully in an oven at the correct temperature and duration,typically between 103°C-105°C or 179°C-180°C for 1-2 hour , and then cooled in a desiccator to prevent moisture absorption before weighing.Consistent temperature and immediate recording of results are also crucial for accurate analysis.
FAQs
- What is Total Dissolved Solid / TDS in drinking water?
TDS determines the overall purity of water. It indicates the presence of inorganic salts (like calcium, magnesium, sodium, and potassium) and small amounts of organic matter.
- Why is TDS testing important for drinking water quality?
Excessively high or low TDS levels can affect human health. For example, high levels of sodium or nitrate can be harmful, especially for people with health conditions. It also differs the taste of the water.
- What is the acceptable TDS limit for drinking water as per IS 10500?
TDS limit of drinking water should be maximum of 500 mg/L
- What are the main steps involve in the TDS testing procedure?
Use clean and dry equipments to avoid contamination.Filtration of the sample should be done.Maintain the temperature properly.Consistent temperature and immediate recording of results are also crucial for accurate analysis.
- Why filtering of sample is important for TDS testing before start the process?
Filtration of sample ensure the elimination of contamination.
- What is the drying time for TDS Testing?
The drying time should be 1-2 hour to a uniform mass.
- Why gravimetric method is more useful method for TDS Testing?
The gravimetric method gives more accurate and precise result as digital meters only detect ionic charged particles not neutral dissolved substances which can differ from the actual result.
- What is the maximum limit we can detect by this method?
The maximum limit can be detected is 400000 mg/l by taking small volume of sample i.e 5-10ml.
- How many times testing should repeat to get accurate result?
Minimum 5-6 times testing should be repeated & dry until the constant weight.Average result should be calculated in 10% variation among the results.
Conclusion-
Water is the basic need in our life but contaminated one can leads to health hazard. Digital TDS meter is easily available in the market but for the accurate result you should have to test by gravimetric method. The upper or lower range of the TDS does not suitable for human consumption as it reflects the preliminary sign of water pollution. By following these method, you can easily test the TDS or Total Dissolved Solid at any laboratory or manufacturing industry with availability of the equipments.With that you have to keep in mind about basic handling mistakes to be avoided and the proper calculation should be done. Also you can reach to Pro Research & Testing Laboratory for the testing purposes.