Why PAC Testing Requires More Than One Parameter

PAC is a pre-hydrolyzed aluminum coagulant. Its performance is affected by active aluminum content, degree of hydrolysis, insoluble matter, solution acidity, trace impurities and the characteristics of the water being treated. No single laboratory result can describe all of these factors.

For this reason, Bluwat's PAC quality evaluation is organized into three layers:

• Physical inspection: appearance, solution condition and, for liquid PAC, density.
• Chemical analysis: aluminum oxide content, basicity, pH, insoluble matter, iron and trace elements.
• Application verification: jar testing with representative source water when coagulation performance must be confirmed.

1. Representative Sampling and Sample Preparation

Reliable testing starts before the sample reaches the instrument. A laboratory result represents only the material actually collected, so the sampling plan must account for possible variation within a tank, drum or bag.

Liquid PAC

Liquid material is sampled from different depths of the package or storage tank. The portions are combined and mixed to form a representative composite sample. The sample container must be clean, dry, chemically compatible and tightly sealed.

Solid PAC

For bagged powder or granules, samples are collected from selected packages using a suitable sampler inserted into the material. The portions are combined, mixed and reduced by an appropriate sample-splitting method without changing the composition.

Identification and retention

Each laboratory sample is labeled with the product name, batch number, sampling date and sampler information. One portion is used for testing and a separately sealed portion can be retained for traceability or repeat analysis.

2. Appearance Inspection

The sample is examined under suitable lighting for color, clarity, uniformity, visible contamination, caking or unusual sediment. Liquid PAC should be checked for separation or suspended foreign matter. Solid PAC should be inspected for uniform powder or granule condition and evidence of moisture absorption.

Appearance is a useful first screening step, but it is not a substitute for chemical testing. Differences in raw materials and production conditions can affect color without directly predicting coagulation performance.

3. Aluminum Oxide Content by EDTA Complexometric Back-Titration

Aluminum oxide content is one of the main composition measurements for PAC. The reference approach uses complexometric titration.

Method principle

The PAC sample is dissolved and acid-treated to break down polymerized aluminum species. A measured excess of EDTA is then added. EDTA forms a stable complex with aluminum ions. The EDTA remaining after the reaction is back-titrated with a standardized metal solution.

General procedure

1. Accurately weigh the liquid or solid PAC sample.
2. Dissolve it in laboratory water and transfer it quantitatively to a volumetric flask.
3. Filter the prepared solution if it is visibly turbid, using the filtration procedure specified by the controlled method.
4. Transfer a measured aliquot, add acid and heat briefly to depolymerize the aluminum species.
5. Add a known excess of EDTA and adjust the reaction conditions so that aluminum-EDTA complex formation is complete.
6. Add the required buffer and indicator.
7. Back-titrate the unreacted EDTA to the specified color endpoint.
8. Run a reagent blank in parallel.

The difference between the blank and sample titration volumes is used with the standardized titrant concentration, sample mass and dilution factor to calculate the aluminum oxide mass fraction.

GB 15892-2020 describes a zinc chloride back-titration as the arbitration method and also provides a copper sulfate back-titration option. The selected method, titrant standardization record and endpoint observation should be documented on the test sheet.

4. Basicity by Acid-Base Titration

Basicity reflects the degree of hydroxylation of PAC and is closely related to its hydrolysis behavior in water.

Method principle

A measured amount of standard hydrochloric acid is added to the prepared PAC solution. Potassium fluoride is then used to mask aluminum ions. The remaining acid is titrated with standardized sodium hydroxide using phenolphthalein to indicate the endpoint.

General procedure

1. Transfer a measured aliquot of the prepared PAC solution into a flask.
2. Add a known amount of standardized hydrochloric acid.
3. Heat the mixture as required by the method and cool it to room temperature.
4. Add the potassium fluoride masking solution and mix thoroughly.
5. Add the indicator and immediately titrate with standardized sodium hydroxide to a persistent faint-pink endpoint.
6. Perform a blank test with carbon-dioxide-free water under the same conditions.

The calculation uses the difference between the blank and sample titrations together with the measured aluminum oxide content. Because basicity depends on the aluminum result, both tests must be traceable and internally consistent.

5. Density of Liquid PAC

Density is measured only for liquid PAC. A clean, dry measuring cylinder is filled carefully so that no air bubbles remain. The sample is brought to the controlled reference temperature in a constant-temperature bath. A calibrated hydrometer is lowered slowly into the liquid and allowed to stabilize without touching the wall.

The density is read at the correct meniscus according to the hydrometer design. The sample temperature, instrument identification and observed density are recorded together because temperature directly affects the result.

6. Water-Insoluble Matter by Filtration and Gravimetry

This test measures the portion of the product that remains undissolved under defined acidic dissolution conditions.

Method principle

A known mass of PAC is dissolved in acidified water. The remaining solids are collected on a pre-weighed quantitative filter, washed, dried to constant mass and weighed.

General procedure

1. Condition and weigh the quantitative filter medium.
2. Accurately weigh the PAC sample and dissolve it in prepared acidic water with thorough mixing.
3. Filter the solution under vacuum through the pre-weighed filter.
4. Wash the residue until chloride is no longer detected in the washings using the prescribed silver nitrate check.
5. Dry the filter and residue in a controlled oven, cool in a desiccator and weigh.
6. Repeat the drying, cooling and weighing cycle until constant mass is obtained.

The increase in filter mass, divided by the original sample mass, gives the water-insoluble matter result. Careful washing is essential because retained soluble salts would create a falsely high result.

7. pH Measurement of a Standardized PAC Solution

The pH test is performed on a PAC solution prepared at the concentration specified by the applicable method, rather than by placing the electrode directly into concentrated liquid PAC or an arbitrarily prepared powder solution.

1. Prepare the test solution gravimetrically and dilute it to volume with suitable laboratory water.
2. Calibrate the pH meter with appropriate buffer solutions that bracket the expected sample response.
3. Rinse and blot the electrode according to laboratory procedure.
4. Immerse the electrode in the stirred PAC solution without allowing it to contact the vessel wall or stir bar.
5. Record the stable reading, sample temperature and instrument identification.

Using a consistent solution concentration, water quality, temperature and equilibration time is necessary for meaningful batch-to-batch comparison.

8. Iron Determination

Iron is determined according to the applicable water-treatment-agent method, such as GB/T 22596 referenced by GB 15892-2020. Depending on the selected validated procedure and laboratory equipment, the analysis may use a calibrated spectrometric method after suitable sample preparation.

The analyst prepares standards and a reagent blank, processes the PAC sample under the same conditions, measures the instrument response and calculates the iron content using the calibration relationship and all dilution factors.

9. Arsenic by Atomic Fluorescence Spectrometry

The reference method for arsenic uses hydride-generation atomic fluorescence spectrometry.

Method principle

The acid-treated sample is pre-reduced so that arsenic is converted into the required chemical state. A borohydride reagent then generates volatile arsine. Argon transports the arsine into the atomizer, where the fluorescence signal is measured and compared with a calibration series.

Critical controls

• Use high-purity reagents and laboratory water suitable for trace analysis.
• Pre-clean glassware with acid and rinse thoroughly to minimize contamination.
• Prepare a reagent blank and a multi-point calibration series.
• Allow sufficient pre-reduction time before measurement.
• Confirm calibration performance and check for sample turbidity or matrix interference.

An alternative spectrophotometric procedure may also be used where permitted by the standard and the laboratory's controlled method. The arbitration method should be used when formally resolving disputed results.

10. Lead and Cadmium by Atomic Absorption Spectrometry

Lead and cadmium can be measured using electrothermal atomic absorption spectrometry. The PAC sample is acid-treated, diluted quantitatively and introduced into a graphite or other suitable electrothermal atomizer. The instrument carries out controlled drying, ashing and atomization stages before measuring element-specific absorbance.

A blank and a calibration series are analyzed under the same instrumental conditions. The concentration obtained from the calibration curve is corrected for sample mass, aliquot volume and dilution.

GB 15892-2020 also describes flame atomic absorption alternatives. In those methods, the target element is chelated, extracted into an organic phase and then measured by flame atomic absorption. Extraction time, phase separation, pH control and blank correction are important sources of analytical quality.

11. Mercury by Atomic Fluorescence or Cold-Vapor Atomic Absorption

The reference mercury method uses atomic fluorescence spectrometry after acid digestion. In an acidic medium, a borohydride reagent reduces mercury to elemental vapor. An inert carrier gas transfers the vapor to the detection system, and the fluorescence signal is compared with freshly prepared mercury standards.

A cold-vapor atomic absorption method may also be used. In this procedure, mercury is converted to the divalent form and then reduced with stannous chloride to elemental mercury vapor for measurement.

Mercury analysis requires particularly strict contamination control, freshly prepared standards where specified, clean glassware, blank monitoring and prompt analysis to minimize loss or memory effects.

12. Chromium by Electrothermal Atomic Absorption Spectrometry

For chromium determination, the acid-prepared PAC solution is analyzed by electrothermal atomic absorption. Chromium standards and a reagent blank are used to establish the calibration relationship. The sample aliquot is introduced into the atomizer and measured under validated drying, ashing and atomization conditions.

The reported result is calculated from the measured concentration together with the sample mass and dilution factors. Calibration verification and matrix-interference checks are important when working near trace levels.

13. Coagulation Performance by Jar Test

Chemical analysis confirms composition and impurity control, but a jar test shows how PAC behaves in actual water. GB 15892-2020 includes coagulation performance testing as an informative method rather than a replacement for the chemical tests.

General jar-test workflow

1. Use representative natural raw water or the customer's actual water sample.
2. Prepare a fresh PAC stock solution with a known aluminum basis.
3. Fill identical beakers with equal volumes of raw water.
4. Add different PAC dosages to the beakers.
5. Apply rapid mixing to disperse the coagulant, followed by slower mixing to promote floc growth.
6. Allow the flocs to settle under the same conditions.
7. Measure residual turbidity and any other relevant water-quality parameters in the clarified water.
8. Record floc formation time, floc size, structure, settling behavior and clarity.

The results are compared by dosage-response curves or a structured observation table. Because raw-water quality changes with source and season, jar-test conclusions apply to the tested water and conditions rather than serving as a universal dosage recommendation.

Laboratory Quality-Control Practices

Reliable PAC testing depends as much on quality control as on the analytical technique itself. A controlled laboratory workflow should include:

• Calibrated balances, volumetric equipment, pH meters and analytical instruments.
• Documented reagent preparation and titrant standardization.
• Reagent blanks, calibration standards and verification samples.
• Parallel determinations and review of the difference between replicate results.
• Traceable sample identification and controlled calculation sheets.
• Appropriate high-purity reagents and water for trace-element analysis.
• Retention of raw data, instrument records and batch test results.

If a result is unusual, the laboratory should first review sampling, preparation, blank response, calibration, endpoint observation and dilution calculations before deciding whether repeat sampling or repeat analysis is required.

How Bluwat Reports PAC Quality

For export orders, the Certificate of Analysis identifies the product and batch and reports the agreed inspection items. Supporting product documentation can include the Technical Data Sheet, Safety Data Sheet, packing list, invoice, certificate of origin and other documents required for the shipment or destination market.

Acceptance criteria depend on the PAC grade, intended application, contract specification and applicable regulation. The absence of numerical limits in this article is intentional: customers should refer to the current product specification and batch Certificate of Analysis for the values relevant to their order.

Conclusion

A complete PAC quality evaluation connects representative sampling, composition testing, impurity analysis and application verification. By controlling each step from sample collection to result review, Bluwat can provide customers with traceable batch data and more useful technical support for drinking-water and industrial water-treatment applications.

For a grade-specific specification, batch Certificate of Analysis or PAC jar-test recommendation, contact Bluwat Chemicals with your raw-water information and treatment objective.