A Brief Discussion on the Key Points of the Chloride Ion Content Determination Method in Cement

The source of chloride ions in cement mainly comes from raw materials, fuels, mixed materials, and additives. Chloride ions in cement are an important factor in the corrosion of steel bars in concrete. Since rebar corrosion is one of the main forms of concrete damage, various countries have made corresponding regulations on the chloride ion content in cement. Therefore, in the new cement standard in our country, the requirement has been added that "the allowed addition of grinding aids in cement production must be ≤0.5% and the chloride ion content in cement must be ≤0.06%", fully reflecting the cement industry's commitment and responsibility for concrete quality assurance. There are many methods for determining chloride ions, and the GB/T176-2008 standard for "Chemical Analysis Methods of Cement" provides two methods for determining chloride ions, namely the ammonium thiocyanate titration method (reference method) and the distillation separation - mercuric nitrate complexometric titration method (substitute method). Below, the steps of the two methods will be introduced in detail, and corresponding operational points will be provided for steps that are prone to problems, in order to improve the accuracy of the tests and reduce human errors in the experiments.
1 Ammonium Thiocyanate Titration Method
1.1 Principle
The sample is decomposed with nitric acid while eliminating the interference of sulfides. A known amount of silver nitrate standard solution is added to precipitate chloride ions in the form of silver chloride. After boiling and filtering, the filtrate and wash liquid are cooled to below 25°C. Using iron (III) salt as an indicator, the excess silver nitrate is titrated with the ammonium thiocyanate standard solution. The reactions are as follows:
Chloride ions react with the added silver nitrate standard solution: Cl- + Ag+ = AgCl↓
Ammonium thiocyanate reacts with excess silver nitrate: CNS- + Ag+ = AgCNS↓
1.2 Analysis Steps and Operational Points
(1) Weigh about 5g of the sample and add 50ml of water.
Stir quickly to ensure it is completely dispersed and mixed; otherwise, the sample may settle at the bottom of the beaker.
(2) Under stirring, add 50ml of nitric acid (1+2) and heat to boiling.
After adding nitric acid, stir continuously and boil to allow the generated hydrogen sulfide to escape, preventing interference with the measurement, while also ensuring the sample dissolves more uniformly.
(3) Accurately transfer 5ml of silver nitrate standard solution into the solution and boil for 1-2 minutes.
The accuracy of the silver nitrate standard solution directly determines the accuracy of the test results, so the silver nitrate standard solution must be prepared strictly according to the standard requirements and added accurately using a pipette or burette.
(4) Add a small amount of filter paper pulp.
Do not add too much filter paper pulp to avoid affecting the filtration speed.
(5) Use pre-washed slow filter paper or glass sand core funnel for vacuum filtration, collecting the filtrate in a 250ml conical flask.
Before filtering, both the slow filter paper and glass sand core funnel must be washed with nitric acid (1+100) to avoid introducing errors into the experiment.
(6) When the total volume of the filtrate and wash liquid reaches about 200ml, cool the solution in weak light or darkness to below 25°C.
Titration should be performed at room temperature; at higher temperatures, the red complex is prone to fading.
(7) Add 5ml of ammonium iron(III) sulfate indicator solution and titrate with the ammonium thiocyanate standard titration solution until the produced reddish-brown color does not disappear upon shaking.
During titration, shake the solution sufficiently to avoid the precipitation of silver ions, which could cause the endpoint to appear too early.
(8) If the volume of the ammonium thiocyanate standard titration solution consumed is less than 0.5 ml, re-test using half the sample mass.
2 Distillation Separation - Mercuric Nitrate Complexometric Titration Method
2.1 Principle
Using a specified distillation apparatus at a temperature of 250-260°C, decompose the sample with hydrogen peroxide and phosphoric acid, using purified air as a carrier to distill and separate chloride ions. Use dilute nitric acid as the absorbing liquid, and after distilling for 10-15 minutes, rinse the condenser and its lower end with ethanol, with the amount of ethanol added being more than 75% (by volume fraction). At around pH 3.5, using diphenylcarbazone as an indicator, titrate with the mercuric nitrate standard titration solution. The reaction equations are as follows:
Distillation reaction: 3Cl- + H3PO4 = HCl↑ + PO43-
Titration reaction: Hg2+ + 2Cl- = HgCl2↓
At the endpoint: Hg2+ + diphenylcarbazone = Hg-diphenylcarbazone (cherry red)
2.2 Analysis Steps and Operational Points
(1) Add 3ml of water and 5 drops of nitric acid.
(a) Absorb the hydrogen chloride generated during heating and distillation.
(b) Further eliminate the interference of the very small amount of hydrogen sulfide that is distilled.
(c) Ensure that the lower end of the tube is in good contact with the liquid surface.
(2) Weigh the sample and place it in a dried quartz distillation tube, ensuring that the sample does not adhere to the tube wall.
If any sample adheres to the tube wall, a portion of the sample will not react, leading to a lower test result.
(3) Add 5 drops of hydrogen peroxide and shake well.
(a) Disperse the sample to prevent clumping.
(b) The hydrogen sulfide generated during distillation is oxidized to sulfuric acid by hydrogen peroxide and is not distilled out.
(4) Add 5ml of phosphoric acid.
(a) Phosphoric acid has a relatively high boiling point and strong ability to dissolve minerals. While decomposing the sample at high temperatures, it can distill out volatile hydrochloric acid from chlorides.
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(b) During the distillation process, other halides and sulfides can also be distilled out in the form of corresponding hydrogen halides and hydrogen sulfide. The dilute nitric acid absorption solution is a weak acid and will not absorb.
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(c) After adding phosphoric acid, the carbonates must be allowed to release carbon dioxide before connecting the gas outlet and inlet.
(d) To speed up the determination and reduce the measurement time, while distilling the previous group, preparations for the second group of samples can be made. After adding phosphoric acid, place it on a test tube rack to wait.
(5) Connection of gas inlet and outlet.
Connect the gas outlet first, then the gas inlet.
(6) Adjustment of gas flow meter.
Adjust the gas flow rate to 100~200 ml/min. At this time, there should be continuous bubbles produced in the conical flask. If not, check its gas tightness. The gas flow rate will also affect the measurement results.
(7) Distillation time.
Distill for 10~15 min. The distillation time can be determined based on the chloride ion content. When the chloride ion content is between 0.2%~1.0%, the distillation time should be 15~20 min, and a more concentrated standard titration solution of mercuric nitrate should be used for titration.
(8) Amount of ethanol.
The amount of ethanol should ensure that the indicator can dissolve fully, making the endpoint sharp.
(9) pH adjustment.
The color change range of bromothymol blue indicator is 3.0~4.6. Adjust to blue with sodium hydroxide, then adjust to yellow and add one drop in excess, at which point the pH is just around 3.5.
(10) When preparing diphenylcarbazone solution, the diphenylcarbazone solution must be completely dissolved in ethanol; otherwise, insufficient indicator concentration will affect its sensitivity.
(11) Both bromothymol blue and diphenylcarbazone indicators are dissolved in ethanol, so do not prepare a large amount at once; otherwise, prolonged time will cause ethanol to evaporate, changing the concentration of the indicator solution, making the endpoint color change less sharp and affecting the titration results.
(12) Preparation of mercuric nitrate standard titration solution.
(a) Mercuric nitrate solution is a heavy metal solution and is toxic. Wear gloves during preparation to avoid skin contact.
(b) To prevent hydrolysis, always add to nitric acid first, then dilute with water, and use a dry beaker.
(c) Solid mercuric nitrate is highly hygroscopic; it should be sealed immediately after weighing.
(d) There is no need to dry mercuric nitrate before weighing.
3 Conclusion
The ammonium thiocyanate volumetric method, as a classic precipitation titration method in analytical chemistry, has a clear principle, simple operation, stable and reliable results, and high accuracy. However, due to the large amount of mixed materials added to some common cements in our country, this method requires a larger sample size, which contains a lot of acid-insoluble residue, severely affecting the filtration speed and prolonging the test time. It is more suitable for the determination of silicate cement.
The distillation separation - mercuric nitrate complex titration method requires a small sample size and has a fast analysis speed, making it more suitable for the determination of common silicate cement with a large amount of mixed materials added. However, this method has many influencing factors. When the chloride ion content is very low, the sensitivity of the method decreases, making it difficult to determine the endpoint. Moreover, the equipment used by different manufacturers in various regions has inherent differences, often resulting in negative deviations. Mercuric nitrate is also a highly toxic substance, so extreme caution is required during use.
Regardless of the method used, the reagents used for calibration and preparation in the experiment are all standard reagents. A blank test should be conducted. The water used should be deionized water that meets laboratory water specifications. Since the measured chloride ion content is very low and is greatly affected by the surrounding environment, it is best to conduct the experiment in a dedicated location.