Determination of Total Dissolved Solids
TDS in water is due to the dissolved salts and minerals in water which are usually present in the form of ions; ex- sodium, potassium, carbonates, sulphates etc. Sometimes these dissolved solids can be toxic and also causes formation of scales in pipes and hence determination of the same is essential.
TDS can be determined by two methods:
- Gravimetric analysis: This method is a laboratory method and is time taking but results are accurate. Here, water sample is prepared by filtering water by 1.5 micron filter so as to separate suspended soilds from the water.
Step1- Take an empty beaker and note down it’s weight, say- 20 g.
Step2- Put water sample in the beaker and take weight again, say-220 g which, means weight of water=220 g – 20 g=200 g.
Step3- Heat the beaker to evaporate water and once all the water is evaporated take the weight of beaker along with the residue, say-21 g which means weight of residue is= 21g – 20 g=1 g.
weight of residue= 1 g= 1000 mg
weight of water=200 g
Volume of water=200/1(density of water= 1 g/cc)
=200 cc=200 mL=0.2 litres
TDS=1000 mg/0.2 Litres= 5000 mg/L=5000 ppm.
*Note- when water is evaporated, dissolved solids in the form of ions combines to form solid residue.
2. TDS meter: Dissolved solids are usually present in water in the form of ions and ions conduct electricity. This principle is utilized in finding the TDS of water. TDS meter tip is dipped inside water which measures the amount of electricity getting conducted and this electricity value is calibrated to TDS value in ppm or mg/L. This method is very quick to use and is widely popular. Results of this test are approximate because all the dissolved solids present in water are not present as ions.
Determination of Total Suspended solids
Total Suspended solids are determined by Gravimetric Analysis in laboratory. Since water contains both dissolved as well as suspended solids, hence for the determination it is essential to separate the two solids.
First, water sample is filtered using 1.5 micron filter and the residual contents over filter contains suspended solids. This filter is kept inside oven at 104°C and taken out after an hour. The dried weight of filter along with dried residue is taken. After subtracting the weight of filter in this weight , the suspended solids weight is found out and by dividing the weight of suspended solids by volume of water, the Total Suspended Solids value in ppm or mg/L can arrived at.