Gravimetric Analysis of Laboratory Filter Paper

Gravimetric analysis is one of the most accurate and reliable quantitative analytical methods in classical chemistry. It is based on precise mass measurement and is widely used for determining the amount of a specific component in a sample. In this process, laboratory filter paper plays a critical role in separating, collecting, and retaining solid precipitates for accurate weighing.

Role of Filter Paper in Gravimetric Analysis

During gravimetric analysis, the analyte is converted into an insoluble compound through a chemical reaction. This solid precipitate must be completely separated from the liquid phase before weighing. Laboratory filter paper serves as the filtration medium, capturing the precipitate while allowing the filtrate to pass through efficiently.

The performance of the filter paper directly affects analytical accuracy, repeatability, and reliability.

Key Requirements for Filter Paper

Low Ash Content: Gravimetric analysis requires filter paper with extremely low ash content. Ashless or quantitative filter paper leaves minimal residue after ignition, preventing interference with final mass measurements.

High Particle Retention: The filter paper must effectively retain fine precipitates without particle loss, especially when dealing with small or gelatinous particles.

High Chemical Purity: Filter paper should be free from additives, binders, or contaminants that could react with the analyte or introduce weighing errors.

Adequate Wet Strength: Strong wet strength ensures the filter paper maintains integrity during filtration, washing, and handling.

Types of Filter Paper Used in Gravimetric Analysis

Ashless Filter Paper: Ashless filter paper is the most commonly used type in gravimetric analysis. Upon ignition, it decomposes almost completely, leaving negligible residue, typically below 0.01%.

Quantitative Filter Paper: Quantitative filter paper is manufactured under strict quality control to ensure consistent pore size, purity, and filtration performance, making it suitable for high-precision analytical work.

Typical Gravimetric Procedure Involving Filter Paper

Formation of an insoluble precipitate from the sample
Filtration of the precipitate using an appropriate laboratory filter paper
Washing the precipitate to remove soluble impurities
Drying or igniting the filter paper and precipitate
Cooling in a desiccator before weighing
Calculation of analyte content based on mass

Careful control of each step ensures accurate and reproducible results.

Applications of Gravimetric Analysis

Gravimetric analysis using laboratory filter paper is widely applied in:

Analytical and inorganic chemistry laboratories
Environmental and water quality analysis
Food and beverage testing
Pharmaceutical quality control
Academic teaching and research

Despite advances in instrumental analysis, gravimetric methods remain reference techniques due to their high accuracy and traceability.

The basic operation of gravimetric analysis includes steps such as sample dissolution, precipitation, filtration, washing, drying, and burning, which are introduced as follows.

a. Sample Dissolution, Precipitation

1. Dissolve the sample

Put the sample in a beaker, add solvent along the wall of the beaker, cover it with a watch glass, and shake it gently. If necessary, heat it to promote its dissolution, but the temperature should not be too high to prevent the solution from splashing.

If the sample needs to be dissolved with acid and gas is released, a small amount of water should be added to the sample to make it into a paste, cover it with a watch glass, and inject the solvent from the mouth of the beaker. After the action is completed, rinse the convex surface of the watch glass with a washing bottle and let it flow into the beaker.

2. Precipitation

Gravimetric analysis for precipitation requires it to be as complete and pure as possible. To achieve this requirement, different precipitation conditions should be selected according to other types of precipitation, such as the volume and temperature of the solution during rainfall, the concentration, quantity, addition speed, stirring speed, and placement time of the precipitant, etc. Therefore, it must be carried out by the prescribed operating procedures.

Generally, when performing a precipitation operation, hold the dropper in the left hand, add the precipitant, and hold the glass rod in the right hand to stir the solution continuously. When stirring, do not touch the wall or bottom of the beaker with the glass rod to avoid scratching the beaker. The solution needs to be heated, usually in a water bath or on a hot plate. After precipitation, check whether the precipitation is complete. The inspection method is: after the precipitation sinks, add 1 drop of precipitant along the wall of the cup in the upper clear liquid and observe whether there is turbidity at the dripping point. No turbidity indicates that the precipitation is complete. If turbidity occurs, add more precipitant until there is no turbidity in the upper clear liquid when checking again. Then, cover the table dish.

b. Filtration and Washing

(I) Filter with filter paper

1. Selection of filter paper

Filter paper is divided into qualitative filter paper and quantitative filter paper. Quantitative filter paper (or ash-free filter paper) is often used for filtration in gravimetric analysis. The ash content of quantitative filter paper is very small after burning, and its weight can be ignored. If the ash content is heavy, the blank should be deducted.

Quantitative filter paper is generally round, and there are several types according to diameter, such as 11cm, 9cm, and 7cm:

According to the pore size of the filter paper, there are three types: “fast”, “medium speed” and “slow speed”.

Choose appropriate filter paper according to the nature of the precipitate. For example, fine crystalline precipitates such as BaSO4 and CaC2O4·2H2O should be filtered with “slow speed” filter paper; Fe2O3·nH2O is a colloidal precipitate, which should be filtered with “fast” filter paper; coarse crystalline precipitates such as MgNH4PO4 should be filtered with “medium speed” filter paper.

Choose the size of the filter paper according to the amount of precipitation. Table 3-4 shows the ash content of commonly used domestic quantitative filter paper.

2. Choice of funnel

The funnel used for gravimetric analysis should be a long-necked funnel with a neck length of 15~20 cm, a funnel cone angle of 60°, a smaller neck diameter, generally 3~5 mm, so that the water column can be easily retained in the neck, and the outlet is ground into a 45° angle, as shown in the figure. The funnel should be cleaned before use.

3. Folding of filter paper

Wash and dry your hands when folding the filter paper. The folding of filter paper is shown in Figure 3-4. First, fold the filter paper in half and press it tightly, then fold it in half again but do not press it tightly, and put the folded filter paper into the funnel. The size of the filter paper should be about 0.5~1 cm lower than the edge of the funnel. If it is higher than the edge of the funnel, you can cut off a circle. Observe whether the folded filter paper can fit tightly with the inner wall of the funnel. If it does not fit tightly, you can change the folding angle of the filter paper appropriately until it fits tightly with the funnel, and then fold the second fold tightly. Take out the conical filter paper and tear off a piece of the outer corner of the three-layer filter paper on half of the side so that the inner filter paper can fit tightly on the inner wall of the funnel. The small piece of filter paper torn off is reserved for wiping the residual sediment in the beaker.

4. Make a water column

After putting the filter paper into the funnel, press it tightly with your hands to make it close, and then use a washing bottle to wet all the filter paper. Use your fingers to gently press the filter paper to drive away the bubbles between the filter paper and the funnel wall, and then add water to the edge of the filter paper. At this time, the funnel neck should be filled with water to form a water column. After all the water on the filter paper has flowed out, the water column in the funnel neck should still be retained. In this way, the gravity of the liquid can play a suction and filter role, speeding up the filtration speed.

If the water column cannot be made, you can block the lower mouth of the funnel with your fingers, slightly lift one side of the filter paper, and use a washing bottle to add water to the gap between the filter paper and the funnel until the funnel neck and part of the cone are filled with water, then press the filter paper tightly and slowly release the fingers blocking the outlet below, and the water column should be formed. If the water column still cannot be formed, or the water column cannot be maintained, and the funnel neck has been cleaned, it is because the funnel neck is too large.

The funnel with a water column should be placed on the funnel stand, and a clean beaker should be used to receive the filtrate. The filtrate can be used to determine other components. The filtrate is sometimes not needed, but considering that there may be precipitation infiltration or the filter paper may break accidentally during the filtration process, re-filtration is required, so a clean beaker should be used to receive the filtrate. In order to prevent the filtrate from splashing, the side with the long bevel of the funnel neck outlet is generally close to the inner wall of the beaker.

The height of the funnel position is determined by the fact that the outlet of the funnel neck does not contact the filtrate during the filtration process.

5. Pouring filtration and preliminary washing

First of all, it should be emphasized that filtration and washing must be completed at one time, so the time must be planned in advance and cannot be interrupted, especially for filtering colloidal precipitates.

Filtration is generally carried out in three stages. In the first stage, the pouring method is used to filter as much clear liquid as possible, and the precipitate in the beaker is preliminarily washed. In the second stage, the precipitate is transferred to the funnel. In the third stage, the beaker and the precipitate on the washing funnel are cleaned.

During filtration, to avoid the precipitate blocking the gaps in the filter paper and affecting the filtration speed, the pouring method is generally used for filtration, that is, the beaker is tilted and left to stand, and after the precipitate drops, the upper clear liquid is poured into the funnel first, rather than mixing the precipitate and the solution at the beginning of filtration.

The filtration operation is shown in the figure. Move the beaker above the funnel, gently extract the glass rod, and lightly touch the lower end of the glass rod to the beaker wall to make the hanging droplets flow back into the beaker. Put the beaker mouth and the glass rod close together, and the glass rod is upright, with the lower end close to one side of the three-layer filter paper. Slowly tilt the beaker to make the upper clear liquid flow into the funnel along the glass rod. The liquid level in the funnel should not exceed 2/3 of the height of the filter paper, or the liquid level should be about 5mm away from the upper edge of the filter paper, to avoid a small amount of precipitation from passing over the upper edge of the filter paper due to capillary action, causing loss.

When pouring is paused, the beaker’s mouth should be lifted up along the glass rod, and the beaker should be gradually made upright. When the glass rod and the beaker change from being perpendicular to each other to being almost parallel, the glass rod should be moved away from the beaker’s mouth and into the beaker. This can prevent the liquid left on the rod end and the beaker mouth from flowing onto the outer wall of the beaker. When the glass rod is returned to the original beaker, do not stir the clear liquid, and do not lean against the beaker’s mouth because there is a small amount of precipitation on the mouth. Repeat this operation until the upper clear liquid is poured out. When the liquid in the beaker is less and it is inconvenient to pour out, the glass rod can be tilted slightly to the left to make the beaker tilt at a larger angle. After the upper clear liquid is poured out, do a preliminary wash in the beaker.

The choice of washing liquid to wash the precipitate should be determined according to the type of precipitate.

① Crystalline precipitate: It can be washed with a cold and dilute precipitant. Due to the common ion effect, the dissolution loss of the precipitate can be reduced. However, if the precipitant is a non-volatile substance, it cannot be used as a washing liquid. In this case, distilled water or other suitable solutions can be used to wash the precipitate.

② Amorphous precipitate: Use hot electrolyte solution as a washing liquid to prevent the occurrence of peptization. Most volatile ammonium salt solutions are used as washing liquids.

③ For precipitates with high solubility, washing the precipitate with a precipitant and an organic solvent can reduce its solubility.

When washing, inject a small amount of washing liquid around the inner wall of the beaker, about 20 mL each time, stir thoroughly, let it stand, and after the precipitate settles, pour and filter according to the above method. Wash the precipitate 4 to 5 times in this way. Each time, pour out as much washing liquid as possible and then add the second washing liquid. Check whether the filtrate is transparent and does not contain precipitate particles at any time. Otherwise, filter it again or repeat the experiment.

6. Transfer of precipitate

After the precipitate is washed by pouring, add a small amount of washing liquid to the beaker containing the precipitate, stir and mix, and pour it all into the funnel. Repeat this 2-3 times, then place the glass rod horizontally on the mouth of the beaker, with the lower end of the glass rod 2-3 cm longer than the mouth of the beaker. Press the glass rod with the index finger of your left hand, with the thumb in front and the other fingers behind. Pick up the beaker and place it above the funnel. Tilt the beaker so that the glass rod still points to one side of the three-layer filter paper. Use a wash bottle to rinse the precipitate attached to the wall of the beaker and transfer it all into the funnel, as shown in the figure below.

Finally, wipe the glass rod with a small piece of filter paper that you have saved, then put it in the beaker, and use the glass rod to press the filter paper to wipe it. After wiping, use a glass rod to push the filter paper block into the funnel, and rinse the beaker with detergent to transfer all the remaining precipitate into the funnel. Sometimes, you can also use a dust broom, as shown in the figure, to scrub the precipitate on the beaker, and then wash the dust broom. The dust broom can usually be made by yourself. Cut a piece of latex tube, put one end on the glass rod, and glue the other end with rubber glue. Use a clip to flatten it and dry it.

7. Washing

After the precipitate is completely transferred to the filter paper, the final washing is performed on the filter paper. At this time, use a washing bottle to move downward in a spiral from a little below the edge of the filter paper to wash the precipitate as shown in the figure. This allows the precipitate to be concentrated at the bottom of the filter paper cone. Do not flush the washing liquid directly onto the central precipitate of the filter paper to avoid splashing the precipitate.

Use the “small amount and multiple times” method to wash the precipitate; that is, add a small amount of washing liquid each time, drain as much as possible after washing, and then add the second washing liquid, which can improve the washing efficiency.

c. Drying and Calcining

The drying and calcining of the precipitate are carried out in a crucible that has been pre-calcined to a constant mass. Therefore, before drying and calcining the precipitate, the crucible must be prepared in advance.

(1) Preparation of the crucible

First, wash the porcelain crucible, dry it or dry it on a low fire, number it, and then heat and calcine it at the required temperature. The calcination can be carried out in a high-temperature electric furnace. Since the temperature rises or falls suddenly, the crucible often breaks. It is best to put the crucible in a cold furnace to gradually increase the temperature or preheat the crucible at the mouth of the furnace that has been raised to a higher temperature, and then put it into the furnace. Generally, it is burned at 800~950 C for half an hour (a new crucible needs to be burned for 1 hour). When taking the crucible out of the high-temperature furnace, the high-temperature furnace should be cooled first, and then the crucible should be moved to the dryer. The dryer and the crucible should be moved to the balance room together, cooled to room temperature (about 30 minutes), and then taken out and weighed. Then, carry out the second burning, about 15~20 minutes, cooling and weighing. If the difference between the two weighing results is not greater than 0.2 mg, it can be considered that the crucible has reached a constant mass, otherwise, it needs to be burned again until the mass is constant. The temperature of burning the empty crucible must be consistent with the temperature of burning the precipitate later.

(2) Drying and burning of the precipitate

After the crucible is ready, the drying and burning of the precipitate can be started. Use a glass rod to take the filter paper and the precipitate out of the funnel, roll them into a small package as shown in the figure, and wrap the precipitate inside. At this time, special attention should be paid to avoid any loss of the precipitate. If there is a little precipitate on the funnel, you can wipe it off with a piece of filter paper and wrap it together with the precipitate.

Pack the filter paper into a crucible with a constant mass so that the side with more filter paper layers faces upward, which can make the filter paper easier to ash.

As shown in the figure, place the crucible on the clay triangle at an angle, cover the crucible cover, and then dry and carbonize the filter paper. During this process, the filter paper must be prevented from catching fire, otherwise, the precipitate will be scattered and lost. If it has caught fire, the gas lamp should be moved up immediately, and the crucible cover should be covered to let the flame extinguish itself. After the filter paper is carbonized, the temperature can be gradually increased, and the crucible can be rotated with crucible tongs at any time to completely burn off the black charcoal on the inner wall of the crucible. The process of burning charcoal into CO2 and removing the ash is called ash. After the filter paper is ashed, place the crucible vertically on the clay triangle, cover the crucible with the lid (leaving a small gap), burn the precipitate at the specified temperature, or place the crucible in a high-temperature electric furnace for burning. Generally, the first burning time is 30~45 min, and the second burning time is 15~20 min. After each burning is completed and taken out of the furnace, it needs to be cooled slightly in the air and then moved into the dryer. Weigh the precipitate after cooling to room temperature, and then burn, cool, and weigh again until the mass is constant.

Microporous glass crucibles (or funnels) only need to be dried to be weighed. Generally, the microporous glass crucible (or funnel) is placed on a watch glass together with the precipitate and then placed in an oven. The drying temperature is determined according to the nature of the precipitate. Generally, the first drying time is longer, about 2h, and the second drying time can be shorter, about 45 min to 1h, depending on the nature of the precipitate. After the precipitate is dried, take out the crucible (or funnel), cool it to room temperature in a dryer, and then weigh it. Repeat drying and weighing until the mass is constant.

(3) How to use the dryer

The dryer is a sealed thick-walled glass container with a ground lid, which is often used to store dry crucibles, weighing bottles, samples, etc. A thin layer of Vaseline is applied to the edge of its ground edge so that it can fit tightly with the lid, as shown in the figure.

The bottom of the dryer contains desiccant. The most commonly used desiccant is color-changing silica gel and anhydrous calcium chloride, and a clean perforated porcelain plate is placed on it. Crucibles and the like can be placed in the holes of the porcelain plate. The ability of desiccant to absorb moisture is limited. The air in the dryer is not dry, but the humidity is low.

The following matters should be noted when using the dryer:

① Do not put too much desiccant to avoid contaminating the bottom of the crucible.
② When moving the dryer, hold it with both hands and press the lid tightly with your thumb.
③ When opening the dryer, do not lift the lid. Use your left hand to hold the dryer and carefully push the lid slightly open with your right hand. Wait until the cold air slowly enters before you can completely push it open. The lid must be placed on the table with its back facing up.
④ Do not put objects that are too hot into the dryer.
⑤ Sometimes, when a hot object is placed in the dryer, the air will expand due to heat and push the lid up. To prevent the lid from being overturned, you should hold it down with your hand and push the lid slightly open from time to time (less than 1 second) to release the hot air.
⑥ The crucible and precipitate after burning or drying should not be placed in the dryer for too long, otherwise, the mass will increase slightly due to the absorption of some moisture.
⑦ The color-changing silica gel is blue when dry and turns pink when damp. You can bake the damp silica gel at 120°C until it turns blue and reuse it repeatedly until it breaks and cannot be used.