How To Measure Sulfate In Water

Determination Method of sulfate in water

Flame atomic absorption spectrophotometry

Sulfate reacts with barium chromate suspension in the ammonia medium of water-ethanol.

The reaction formula is :

SO42－＋BaCrO4→BaSO4↓＋CrO42－

We can calculate the content of sulfate by measuring the chromate released from the reaction by atomic absorption spectrometry. The flame used is an air acetylene-rich yellow flame and the measurement wavelength at 359.3nm.

EDTA disodium titration

Determination range:

The best determination range of this method is 10 ~ 150mg / lso42 -.

Under slightly acidic conditions, add excess barium chloride to quantitatively generate barium sulfate precipitation from sulfate ions in water samples. The remaining barium is titrated with disodium ethylenediamine tetraacetate standard solution in a medium with pH = 10 and Chromium Black T as an indicator. The original calcium and magnesium in the water sample will also be titrated together. We should deduct the consumed titrant by titrating another water sample of the same volume without precipitant under the same conditions.

In order to make the titration end point clear, it should ensure that the test solution contains a certain amount of magnesium. Therefore, we can use the mixture of barium and magnesium as precipitants. From the titrant volume consumed by the added barium and magnesium determined through the blank test, minus the titrant volume consumed by the remaining barium and magnesium after the precipitation of sulfate, so we can calculate the amount of barium consumed in the precipitation of sulfate and then the calculate the sulfate content.

Barium chromate colourimetry

Determination range:

The minimum measurement of this method is 0.05mg. If take a 10ml water sample for determination, the minimum detection concentration is 5.0mg/l.

Carbonate and bicarbonate in water samples can also react with barium to produce precipitation under alkaline conditions, it will cause interference, but we can eliminate it by calcium ammonia solution.

In an acidic solution, barium chromate and sulfate produce barium sulfate precipitation and chromate ion. After neutralizing the solution to be alkaline, then we can filter and remove the excess barium chromate and the generated barium sulfate precipitation. The filtrate contains chromate ion replaced by sulfate, which is yellow and quantitative by colourimetry.

Adding calcium ammonia reagent can make the solution alkaline and remove the interference of carbonate and bicarbonate. We can reduce the solubility of barium chromate in an aqueous solution by adding ethanol.

Barium Sulfate Turbidimetry

Determination range:

The minimum detection amount of this method is 0.05mg. If take a 50ml water sample for determination, the minimum detection concentration is 1mg / L.

Sulfate ions in water react with barium to produce fine barium sulfate crystals, making the aqueous solution turbid. The turbidity is directly proportional to the sulfate content in water samples within a specific concentration range.

Ion chromatography

The anion was analyzed qualitatively and quantitatively using the ion exchange principle. Through the ion exchange resin, the anion to be tested is separated from each other based on the different relative affinity of the anion to the low-capacity strong alkaline anion resin (separation column). The separated anions are converted into acid type with high conductivity when flowing through the strong acid cation resin (inhibition column) chamber. Carbonate bicarbonate is converted into carbonic acid with weak conductivity (removing background conductivity). We can measure the anion converted to the corresponding acid type by conductivity detector and compare it with the standard. The peak was determined qualitatively according to the retention time, high peak or peak area quantification.

Paper, board and pulp – Determination of water-soluble sulfate (conductometric titration method)

We can determine the water-soluble sulfate in pulp, paper and paperboard by conductometric titration. The minimum limit of sulfate ion of the material analyzed by this method is 20 mg/kg.

Principle

At least 4g flake sample is extracted with 100ml hot water for 1h, then filter the extraction solution, and the sulfate ion in it is precipitated with excess barium ion. The excess barium ion is determined by lithium sulfate according to conductometric titration.

Trial production

The analysis should use the reagent of analytical purity (A.R.) and specified water.

Distilled water or deionized water: the conductivity is more minor than 1.0ms/m.

Ethanol (C2H5OH): 95% (V / V).

Barium chloride solution: C (BaCl2 • 2H2O) ≈ 5mmol / L.

Dissolve 1.25g of barium chloride in two crystal drinks of water and dilute to 1L.

Hydrochloric acid: C (HCl) ≈ 1mmol / L solution.

Lithium sulfate standard solution: C (Li2SO4 • H2O) = 5mmol / L.

Accurately dissolve 0.640g of dry monocrystalline lithium sulfate with water, put it in a 1L volumetric flask, and dilute it with water to the scale.

instrument

Conductivity meter with a sensitivity of 0.001ms/m.

Micro burette, 5ml, scale 0.02ml. If possible, we can use an automatic titration device.

A constant temperature water bath can control and adjust the temperature of 25 ° C ± 0.5 ° C or other temperatures close to room temperature and keep the temperature constant during titration. In the whole titration process, the temperature of the test solution remains constant, which is essential for the accuracy of the experimental results.

Agitator and automatic titration device that can control and adjust the temperature.

Sample taking and preparation

Sample the pulp sample according to the standard of GB 740 and the average sample of paper and paperboard according to the standard of GB 450. In sampling, wear clean gloves to take the sample and prepare the paper. Take it carefully during operation to prevent contamination of the sample. Keep the sample away from acid mist and prevent dust.

Step:

Two tests per sample. The blank test of the reagent should also be done according to the operation steps of the sample.

Extraction of paper sample

Weigh the dried sample not less than 4g (accurate to 0.01g), and weigh the sample to determine the moisture. We should determine the moisture of paper and paperboard according to GB 462 and the moisture of pulp samples according to GB 741. Cut or tear the sample to about 5mm × 5mm flakes and put it into a 250ml conical bottle with a standard interface. It should be separated and layered before pumping if it is the thick paperboard.

Then use a pipette to suck 100ml of water, connect the air condenser, put it into the water bath, fix the conical flask, heat and extract for 1h, and shake it from time to time.

When the extraction time is reached, take it down and cool it to room temperature, then filter it with a glass filter or Brinell funnel and pretreated ashless filter paper, and collect the filtrate into a clean conical bottle with a plug.

Determination of sulfate

Use a pipette to suck 50.0ml of extracted filtrate, put it into a 250ml beaker, add 100ml of 95% ethanol and 10ml of hydrochloric acid, and add 2.0ml of barium chloride solution accurately.

Put the beaker into a constant temperature water bath (the water bath temperature is 25 ° C ± 0.5 ° C) or at a relatively stable room temperature, insert the electrode of the conductivity meter into the test solution, and stir the test solution with a glass rod or stirring device at an even speed. After the temperature is stable, use a micro burette to add 0.2ml lithium sulfate standard solution each time. After each addition of lithium sulfate, record when the conductivity indicator reaches a constant value, repeatedly add the standard solution and get the corresponding conductivity number until the total volume of lithium sulfate reaches 3.5ml ~ 4.0.

If we use an automatic titrator, it should control the rate of lithium sulfate titrant added at about 0.2ml/min.

Note:

In order to ensure the complete precipitation of sulfate ions, it should use enough excess barium ions at the beginning of titration. If the corresponding consumption of lithium sulfate is less than 1ml, take a small amount of extraction solution (less than 50ml, such as 20ml or 10ml), and add distilled water to make up to the total volume of 50ml, and re-determine.

Calculate the millilitres of lithium sulfate consumed by titration

Draw the titration curve, take the millilitre of lithium sulfate as the X and the conductivity reading of the solution as the Y, and plot the test results. Draw a straight line through each dot and form a “V” shape. Read the volume of lithium sulfate standard solution consumed at the intersection of the two straight lines.

Determination of Sulfate in water – weighing method

It is suitable for determining sulfate ions with a content of less than 10mg / L in water. It is unsuitable for sulfate ions in industrial circulating cooling water or heavily polluted water with barium salt as a scale inhibitor and dispersant.

Instrument

1. Constant temperature water bath

2. High-temperature furnace (0-1000 ℃)

3. Drying oven

4. Porcelain crucible

5. analytical balance

6. Crucible filter (aperture 5-15um)

Reagents

1. Hydrochloric acid solution (1 + 1)

2. Nitric acid

3. Methyl orange indicator (1g / L)

Weigh 0.1g methyl orange, dissolve in 70 ℃ grade III reagent water, cool and dilute to 100ml

4. Barium chloride solution (100g / L)

Weigh 10g barium chloride (BaCl2 · 2H2O), dissolve it in tertiary reagent water and dilute it to 100ml

5. Silver nitrate solution (17g / L)

Dilute 200mg of nitric acid to 0.500ml of concentrated water, and then add 500ml of nitric acid to dissolve.

Step:

1. measure 100 ~ 200ml of water sample filtered by slow quantitative filter paper (containing about 10 ~ 40mg sulfate ion) into the beaker accurately, add a few drops of methyl orange indicator, drop hydrochloric acid (1 + 1) to turn the water sample solution red, then excess 2ml, and add tertiary reagent water to 200ml

2. Boil for 5min, and slowly add 10ml of barium chloride solution at about 80 ℃ under continuous stirring until the upper part of the solution. The clear solution no longer appears to have white turbidity, indicating that the sulfate has been precipitated. Add 2ml more barium chloride solution, put the beaker in a water bath at 80 ~ 90 ℃ and heat it for 2h.

3. Filter with a crucible filter that has been dried to constant weight at 105 ℃ wash the beaker and precipitation with hot three-stage reagent water and put all the precipitation to the filter paper until the filtrate plus a drop of silver nitrate solution does not produce turbidity.

4. Dry the crucible filter at 105 ℃ for 0.5h, and repeat the operation until the constant weight.

5. Obtain sulfate parameters according to P = [(m-m0) M1 / vm2] x106.

Note:

1) Water sample

Phosphate and polyphosphate will produce corresponding barium salt precipitation, interfering with this determination method. When the content of polyphosphate or orthophosphate in the water sample is more than 10mg / L, it will cause higher or lower results, so we should use the potentiometric titration.

If suspended solids, nitrates, sulfites and silica are in water samples, the results can be higher, and Alkali metal sulfate and bisulfate can lower the result.

2) Operation

Add barium chloride precipitant drop by drop under continuous stirring; otherwise, the solution is too concentrated locally, reducing the solubility of precipitation and generating large barite nuclei, resulting in wrapping.

After precipitation, it should be kept in a water bath at 80 ~ 90 ℃ for 2h or placed overnight. This is the ageing process, which makes the crystal grow gradually and easy to filter.

There should be no flame during ashing. Otherwise, it is easy to cause the loss of the object.

“Weighing to constant weight” means that the difference between weighing after drying or burning for two times is less than 0.4mg.

After folding the filter paper for filtration, place it in the funnel, wet it with water, and gently press the filter paper with clean fingers so that there are no residual bubbles between the glass wall and the funnel; Before filtration, fill the glass tube at the lower end of the funnel with water.

The filter paper cannot be folded in half for the second time

The solution should be filtered after cooling to reduce the dissolution loss of precipitation in a hot solution.

Ashing must be complete; otherwise, part of the precipitate is easy to be reduced to barium sulfide by carbon particles during burning.

It should keep air circulation when burning, and the temperature is not over 900. Otherwise, barium sulfate precipitation will decompose into barium oxide.