In factory conditions, it is often necessary to dilute concentrated sulfuric acid with water or increase the concentration of diluted acid by adding concentrated acid to it. To do this, you must first establish or check the concentration of ORIGINAL ACIDS by determining the H2SO4 content in THEM.
By adding water to a concentrated acid (oleum or monohydrate), you can get an acid of any concentration, but when mixing it is concentrated. Sulfuric acid and water release a large amount of heat. The acid may heat up to a boil, a violent release of vapors will occur, and the solution may be ejected from the vessel. Therefore, acids are mixed in special apparatus - mixers, taking appropriate precautions.
Mixers for preparing low concentration acid are made of acid-resistant material, and for preparing concentrated acid - from cast iron. Mixers of various designs are used in sulfuric acid. In some cases, the mixer is made of cast iron, enameled on the inside, placed in a steel casing and closed with a lid. The mixed acids enter a cast iron cone enameled on both sides, in which they are mixed, after which they flow into the boiler. To remove the heat generated when mixing acids, a stream of water is continuously supplied into the space between the boiler and the casing, washing the walls of the apparatus.
In some cases, the acid, after mixing in a small tank, enters pipes irrigated with water from outside, where it is simultaneously cooled and further mixed.
When mixing concentrated sulfuric acid with water or more dilute sulfuric acid, it is necessary to calculate the amount of acids mixed. Calculations are carried out according to the so-called rule of the cross. Below are some examples of such calculations.
1. Determine the amount of 100% sulfuric acid and water that must be mixed to obtain 45% II2SO|.
On the left indicate the concentration of a more concentrated acid (in in this case 100%), and on the right - more diluted (in this case, 0% water). Below, between them, indicate the specified concentration (45%). Crossing lines are drawn through the number indicating this concentration, and the corresponding difference in numbers is indicated at their ends:
The numbers obtained using acids of initial concentrations show how many parts by mass of an acid of each of the indicated concentrations must be mixed to obtain an acid of a given concentration. In our example, to prepare 45% acid, you should mix 45 wt. including 100% acid n 55 wt. hours of water.
The same problem can be solved based on the overall balance of II2SO4 (or S03) in sulfuric acid:
0,45.
The numerator on the left side of the equation corresponds to the H2S04 content (in kg) in I kg of 100% sulfuric acid, the denominator corresponds to the total amount of a given solution (in kg). The right side of the equation corresponds to the concentration of sulfuric acid in fractions of unity. Solving the equation, we get x-1.221 kg. This means that 1.221 kg of water must be added to 1 kg of 100% sulfuric acid, resulting in 45% acid.
2. Determine the amount of 20% oleum that should be mixed with 10% nonsulfuric acid to obtain a 98% acid.
The problem is also solved using the cross rule, however, the concentration of oleum in this example must be expressed in % H2SO4 using equations (9) and (8):
A --= 81.63 + 0.1837-20 --= 85.304;
B 1.225-85.304 - 104.5.
According to the rule of the cross
Therefore, to obtain 98% sulfuric acid, it is necessary to mix 88 wt. including 20% oleum and 6.5 wt. including 10% sulfuric acid.
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How to mix two liquid substances? For example, some acid and water? It would seem that this problem is from the series “twice two is four.” What could be simpler: drain the two liquids together in some suitable container, and that’s it! Or pour one liquid into a container that already contains another. Alas, this is the same simplicity that, according to an apt popular expression, is worse than theft. Because things can end extremely sadly!
Instructions
There are two containers, one of them contains concentrated sulfuric acid, in the other - water. How to mix them correctly? Should we pour acid into water or, conversely, water into acid? The price of a wrong decision in theory can be a low score, but in practice - at best, a severe burn.
Why? But because concentrated sulfuric acid, firstly, is much denser than water, and secondly, it is extremely hygroscopic. In other words, it actively absorbs water. Thirdly, this absorption is accompanied by the release large quantity heat.
If water is poured into a container with concentrated sulfuric acid, the first portions of water will “spread” over the surface of the acid (since water is much less dense), and the acid will begin to greedily absorb it, releasing heat. And there will be so much heat that the water will literally “boil” and splashes will fly in all directions. Naturally, without avoiding the hapless experimenter. Getting burned with “clean” boiling water is not very pleasant, but considering that the water spray will probably still contain acid. The prospect is becoming completely gloomy!
That is why many generations of chemistry teachers forced their students to literally memorize the rule: “First water, then acid! Otherwise, big trouble will happen!” Concentrated sulfuric acid should be added to water in small portions with stirring. Then the unpleasant situation described above will not happen.
A reasonable question: it’s clear with sulfuric acid, but what about other acids? How to properly mix them with water? In what order? It is necessary to know the density of the acid. If it is denser than water, for example, concentrated nitrogen, it should be added to water, just like sulfur, observing the above conditions (little by little, with stirring). Well, if the density of the acid differs very slightly from the density of water, as is the case with acetic acid, it makes no difference.
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Approximate solutions. In most cases, the laboratory has to use hydrochloric, sulfuric and nitric acids. Acids are commercially available in the form of concentrated solutions, the percentage of which is determined by their density.
Acids used in the laboratory are technical and pure. Technical acids contain impurities, and therefore are not used in analytical work.
Concentrated hydrochloric acid smokes in air, so you need to work with it in a fume hood. The most concentrated hydrochloric acid has a density of 1.2 g/cm3 and contains 39.11% hydrogen chloride.
The dilution of the acid is carried out according to the calculation described above.
Example. You need to prepare 1 liter of a 5% solution of hydrochloric acid, using a solution with a density of 1.19 g/cm3. From the reference book we find out that a 5% solution has a density of 1.024 g/cm3; therefore, 1 liter of it will weigh 1.024 * 1000 = 1024 g. This amount should contain pure hydrogen chloride:
An acid with a density of 1.19 g/cm3 contains 37.23% HCl (we also find it from the reference book). To find out how much of this acid should be taken, make up the proportion:
or 137.5/1.19 = 115.5 acid with a density of 1.19 g/cm3. Having measured out 116 ml of acid solution, bring its volume to 1 liter.
Sulfuric acid is also diluted. When diluting it, remember that you need to add acid to the water, and not vice versa. When diluted, strong heating occurs, and if you add water to the acid, it may splash, which is dangerous, since sulfuric acid causes severe burns. If acid gets on clothes or shoes, you should quickly wash the doused area with plenty of water, and then neutralize the acid with sodium carbonate or ammonia solution. In case of contact with the skin of your hands or face, immediately wash the area with plenty of water.
Particular care is required when handling oleum, which is a sulfuric acid monohydrate saturated with sulfuric anhydride SO3. According to the content of the latter, oleum comes in several concentrations.
It should be remembered that with slight cooling, oleum crystallizes and is in a liquid state only at room temperature. In air, it smokes, releasing SO3, which forms sulfuric acid vapor when interacting with air moisture.
It is very difficult to transfer oleum from large to small containers. This operation should be carried out either under draft or in air, but where the resulting sulfuric acid and SO3 cannot have any harmful effect on people and surrounding objects.
If the oleum has hardened, it should first be heated by placing the container with it in a warm room. When the oleum melts and turns into an oily liquid, it must be taken out into the air and then poured into a smaller container, using the method of squeezing with air (dry) or an inert gas (nitrogen).
When nitric acid is mixed with water, heating also occurs (though not as strong as in the case of sulfuric acid), and therefore precautions must be taken when working with it.
Solid organic acids are used in laboratory practice. Handling them is much simpler and more convenient than liquid ones. In this case, care should only be taken to ensure that the acids are not contaminated with anything foreign. If necessary, solid organic acids are purified by recrystallization (see Chapter 15 “Crystallization”),
Precise solutions. Precise acid solutions They are prepared in the same way as approximate ones, with the only difference that at first they strive to obtain a solution of a slightly higher concentration, so that later it can be diluted precisely, according to calculations. For precise solutions, only chemically pure preparations are used.
The required amount of concentrated acids is usually taken by volume calculated based on density.
Example. You need to prepare 0.1 and. H2SO4 solution. This means that 1 liter of solution should contain:
An acid with a density of 1.84 g/cmg contains 95.6% H2SO4 n to prepare 1 liter of 0.1 n. of the solution you need to take the following amount (x) of it (in g):
The corresponding volume of acid will be:
Having measured exactly 2.8 ml of acid from the burette, dilute it to 1 liter in a volumetric flask and then titrate with an alkali solution to establish the normality of the resulting solution. If the solution turns out to be more concentrated), the calculated amount of water is added to it from a burette. For example, during titration it was found that 1 ml of 6.1 N. H2SO4 solution contains not 0.0049 g of H2SO4, but 0.0051 g. To calculate the amount of water needed to prepare exactly 0.1 N. solution, make up the proportion:
Calculation shows that this volume is 1041 ml; the solution needs to be added 1041 - 1000 = 41 ml of water. You should also take into account the amount of solution taken for titration. Let 20 ml be taken, which is 20/1000 = 0.02 of the available volume. Therefore, you need to add not 41 ml of water, but less: 41 - (41*0.02) = = 41 -0.8 = 40.2 ml.
* To measure the acid, use a thoroughly dried burette with a ground stopcock. .
The corrected solution should be checked again for the content of the substance taken for dissolution. Accurate solutions of hydrochloric acid are also prepared using the ion exchange method, based on an accurately calculated sample of sodium chloride. The sample calculated and weighed on an analytical balance is dissolved in distilled or demineralized water, and the resulting solution is passed through a chromatographic column filled with a cation exchanger in the H-form. The solution flowing from the column will contain an equivalent amount of HCl.
As a rule, accurate (or titrated) solutions should be stored in tightly closed flasks. A calcium chloride tube must be inserted into the stopper of the vessel, filled with soda lime or ascarite in the case of an alkali solution, and with calcium chloride or simply cotton wool in the case of an acid.
To check the normality of acids, calcined sodium carbonate Na2COs is often used. However, it is hygroscopic and therefore does not fully satisfy the requirements of analysts. It is much more convenient to use acidic potassium carbonate KHCO3 for these purposes, dried in a desiccator over CaCl2.
When titrating, it is useful to use a “witness”, for the preparation of which one drop of acid (if an alkali is being titrated) or alkali (if an acid is being titrated) and as many drops of an indicator solution as added to the titrated solution are added to distilled or demineralized water.
The preparation of empirical, according to the substance being determined, and standard solutions of acids is carried out by calculation using the formulas given for these and the cases described above.
For safety and ease of use, it is recommended to buy the acid as diluted as possible, but sometimes you have to dilute it even more at home. Don't forget to wear protective equipment for your body and face, as concentrated acids cause severe chemical burns. To calculate the required amount of acid and water, you will need to know the molarity (M) of the acid and the molarity of the solution you need to obtain.
Steps
How to calculate the formula
- The formula will also use the value V 1. This is the volume of acid we will add to the water. We likely won't need the entire bottle of acid, although we don't know the exact amount yet.
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Decide what the result should be. The required concentration and volume of acid are usually indicated in the text of the chemistry problem. For example, we need to dilute the acid to 2M, and we will need 0.5 liters of water. Let us denote the required concentration as C 2, and the required volume is as V 2.
- If you are given other units, first convert them to molarity units (moles per liter) and liters.
- If you don't know what concentration or volume of acid is needed, ask a teacher or someone knowledgeable about chemistry.
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Write a formula to calculate the concentration. Each time you dilute an acid, you will use the following formula: C 1 V 1 = C 2 V 2. This means that the original concentration of a solution multiplied by its volume equals the concentration of the diluted solution multiplied by its volume. We know that this is true because the concentration times the volume equals the total amount of acid, and the total amount of acid will remain the same.
- Using the data from the example, we write this formula as (6M)(V 1)=(2M)(0.5L).
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Solve equation V 1. The V 1 value will tell us how much concentrated acid we need to get the desired concentration and volume. Let's rewrite the formula as V 1 =(C 2 V 2)/(C 1), then substitute the known numbers.
- In our example, we get V 1 =((2M)(0.5L))/(6M). This equals approximately 167 milliliters.
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Calculate the required amount of water. Knowing V 1, that is, the available volume of acid, and V 2, that is, the amount of solution that you will get, you can easily calculate how much water you will need. V 2 - V 1 = required volume of water.
- In our case, we want to get 0.167 liters of acid per 0.5 liter of water. We need 0.5 liters - 0.167 liters = 0.333 liters, that is, 333 milliliters.
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Wear safety glasses, gloves and a gown. You will need special glasses that will cover the sides of your eyes as well. To avoid burning your skin or burning through your clothing, wear gloves and a robe or apron.
Work in a well-ventilated area. If possible, work under a switched-on hood - this will prevent acid vapors from harming you and surrounding objects. If you don't have a hood, open all windows and doors or turn on a fan.
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Find out where the source of running water is. If the acid gets into your eyes or skin, you will need to rinse the affected area under cool running water for 15-20 minutes. Don't start work until you know where the nearest sink is.
- When rinsing your eyes, keep them open. Look up, down, to the sides so that your eyes are washed from all sides.
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Know what to do if you spill acid. Can buy special set for collecting spilled acid, which will include everything you need, or purchase neutralizers and absorbents separately. The process described below is applicable to hydrochloric, sulfuric, nitric and phosphoric acids. Other acids may require different handling.
- Ventilate the room by opening windows and doors and turning on the hood and fan.
- Apply A little sodium carbonate (soda), sodium bicarbonate, or calcium carbonate onto the outer edges of the puddle, ensuring that the acid does not splash.
- Gradually pour the entire puddle towards the center until you cover it entirely with the neutralizing substance.
- Mix thoroughly with a plastic stick. Check the pH value of the puddle with litmus paper. Add more neutralizing agent if the reading is greater than 6-8, then rinse the area with plenty of water.
Explore what you already have. Look for the acid concentration designation on the packaging or in the task description. This value is usually indicated as molarity, or molar concentration (M for short). For example, 6M acid contains 6 moles of acid molecules per liter. Let's call this initial concentration C 1.
How to dilute acid
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Cool the water with luda. This should only be done if you will be working with high concentration acids, for example, 18M sulfuric acid or 12M hydrochloric acid. Pour water into a container and place the container on ice for at least 20 minutes.
- Most often, water at room temperature is sufficient.
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Pour distilled water into a large flask. For applications requiring extreme precision (such as titrimetric analysis), use a volumetric flask. For all other purposes, a regular conical flask will do. The container must fit the entire required volume of liquid, and there must also be room so that the liquid does not spill.
- If the capacity of the container is known, there is no need to accurately measure the amount of water.
The percentage concentration of a solution expresses the ratio of the mass of the solute to the mass of the solution as a whole. If we dilute a solution by adding a solvent to it, the mass of the solute will remain unchanged, but the mass of the solution will increase. The ratio of these masses (concentration of the solution) will decrease by as many times as the mass of the solution increases. If we begin to concentrate the solution by evaporating the solvent, the mass of the solution will decrease, but the mass of the solute will remain unchanged. The mass ratio (concentration of the solution) will increase as many times as the mass of the solution decreases. It follows that the mass of the solution and the percentage concentration are inversely proportional to each other, which can be expressed in mathematical form as follows: l. This pattern underlies calculations when diluting and concentrating solutions. Example 1. There is a 90% solution. How much of it should be taken to prepare 500 kg of a 20 percent solution? Solution. According to the relationship between the mass and the percentage concentration of the solution. Hence, it is necessary to take 111 kg of a 90% solution and add enough solvent to it so that the mass of the solution becomes equal to 500 kg. Example 2. There is a 15% solution. To what mass should 8.50 tons of this solution be evaporated to obtain a 60% solution? Solution. If the quantities of solutions are given in volumetric units, they must be transferred to masses. In the future, calculations should be carried out according to the method outlined above. Example 3. There is a 40% solution of sodium hydroxide with a density of 1.43 kg/l. What volume of this solution must be taken to prepare 10 liters of a 15% solution with a density of 1.16 kg/l? Wound" We calculate the mass of a 15% solution: kg n the mass of a 40% solution: Determine the volume of a 40% solution: Example 4. There is 1 liter of a 50% solution of sulfuric acid with a density of 1.399 kg/l. To what volume must this solution be diluted to obtain an 8% solution with a density of 1.055 kg/l? Solution. Find the mass of the 50% solution: kg and the mass of the 8% solution: Calculate the volume of the 8% solution: V - - 8.288 -. = 8 l 288 ml Example 5. 1 l of a 50% nitric acid solution, the density of which is 1.310 g/lm, was diluted with 690 ml of water. Determine the concentration of the resulting solution *. Solution. We find the mass of a 50% solution: your = g and the mass of a dilute solution: We calculate the concentration of a dilute solution: 1 Examples No. 5,6,7 are taken from the book Ya L. Goldfarb, Yu. V. Kho-lakova “Collection of problems and exercises in chemistry.” M., “Enlightenment”, 1968 Example c. There is a 93.6% acid solution with a density of 1.830 g/ml. How much of this solution is required to prepare 1,000 liters of a 20% solution with a density of 1,140 g/ml, and how much water is required for this? Solution. We determine the mass of a 20 percent solution and the mass of a 93.6 percent solution required to prepare a 20 percent solution: We calculate the mass of water required to prepare a dilute solution: We find the volume of a 93.6 percent solution: Example 7. How many milliliters of sulfuric acid with a density of 1 .84 g/ml is required to prepare 1,000 liters of battery acid with a density of 1.18 g/ml) The percentage concentration of the solution and its density are in a certain relationship, recorded in special reference tables. Using them, you can determine the concentration of the solution by its density. According to these tables, sulfuric acid with a density of 1.84 g/ml is 98.72 percent, and with a density of 1.18 g/ml - 24.76-