Concentration and density of sulfuric acid. Dependence of sulfuric acid density on concentration in the car battery

Dilute and concentrated sulfuric acid are such important chemical products that in the world they are produced more than any other substances. The economic wealth of a country can be estimated by the volume of sulfuric acid produced in it.

The process of dissociation

Sulfuric acid is used in the form of aqueous solutions of various concentrations. It undergoes a dissociation reaction in two stages, producing H ⁺ ions in solution.

H ₂ SO ₄ = H ⁺ + HSO ₄ ^- ;

HSO ₄ ^- = H ⁺ + SO ₄ ^-2 .

Sulfuric acid is strong, and the first stage of its dissociation is so intense that almost all the parent molecules break up into H ⁺ ions and HSO ₄ ^-1 ions (hydrogen sulfate) in the solution. The latter partially decompose further, releasing another H ⁺ ion and leaving the sulfate ion (SO ₄ ^-2 ) in solution. However, hydrogen sulfate, being a weak acid, still prevails in solution over H ⁺ and SO ₄ ^-2 . Complete dissociation of it occurs only when the density of the sulfuric acid solution approaches the density of water, i.e. under strong dilution.

Properties of sulfuric acid

It is special in the sense that it can act as a normal acid or as a strong oxidant - depending on its temperature and concentration. A cold dilute solution of sulfuric acid reacts with the active metals to produce a salt (sulfate) and the evolution of a hydrogen gas. For example, the reaction between cold dilute H ₂ SO ₄ (assuming its complete two-stage dissociation) and metallic zinc looks like this:

Zn + H ₂ SO ₄ = ZnSO ₄ + H ₂ .

Hot sulfuric acid is concentrated, the density of which is about 1.8 g / cm ³ , can act as an oxidizer, reacting with materials that are usually inert to acids, such as, for example, metallic copper. During the reaction, copper is oxidized and the acid mass decreases, a copper (II) sulfate solution in water and sulfur dioxide gas (SO ₂ ) are formed in place of hydrogen, which would be expected when the acid reacts with the metal.

Cu + 2H ₂ SO ₄ = CuSO ₄ + SO ₂ + 2H ₂ O.

How does the concentration of solutions

Actually, the concentration of any solution can be expressed in various ways, but the most widely used is the weight concentration. It shows the number of grams of solute in a certain mass or volume of a solution or solvent (typically 1000 g, 1000 cc, 100 cc and 1 dm ³ ). Instead of the mass of the substance in grams, one can take its quantity, expressed in moles, - then a molar concentration per 1000 g or 1 dm ^{3 of} solution is obtained.

If the molar concentration is determined in relation not to the amount of the solution, but only to the solvent, it is called molality of the solution. It is characterized by temperature independence.

Often, the weight concentration is indicated in grams per 100 g of solvent. Multiplying this figure by 100%, get it in weight percent (percentage concentration). It is this method that is most often used in application to solutions of sulfuric acid.

Each value of the concentration of the solution, determined at a given temperature, corresponds to a very specific density (for example, the density of a solution of sulfuric acid). Therefore, sometimes the solution is characterized by it. For example, a solution of H ₂ SO ₄ , characterized by a percentage concentration of 95.72%, has a density of 1.835 g / cm ³ at t = 20 ° C. How to determine the concentration of such a solution, if only the density of sulfuric acid is given? A table giving such a correspondence is an integral part of any textbook on general or analytical chemistry.

Conversion recalculation example

Let's try to move from one way of expressing the concentration of the solution to another. Suppose that we have a solution of H ₂ SO ₄ In water with a percentage concentration of 60%. First, determine the appropriate density of sulfuric acid. A table containing percentages (the first column) and the corresponding densities of an aqueous solution of H ₂ SO ₄ (fourth column) is shown below.

On it we determine the required value, which is equal to 1.4987 g / cm ³ . We now calculate the molarity of this solution. For this, it is necessary to determine the mass of H ₂ SO ₄ In 1 liter of solution and the corresponding number of moles of acid.

The volume that occupies 100 g of the stock solution:

100 / 1.4987 = 66.7 ml.

Since in 66.7 milliliters of 60% solution contains 60 g of acid, in 1 liter it will contain:

(60 / 66.7) x 1000 = 899, 55 g.

The molar weight of sulfuric acid is 98. Hence the number of moles contained in 899.55 grams of grams will be:

899.55 / 98 = 9.18 mol.

The dependence of the sulfuric acid density on the concentration is shown in Fig. below.

Use of sulfuric acid

It is used in various industries. In the production of iron and steel, it is used to clean the metal surface before it is coated with another substance, it participates in the creation of synthetic dyes, as well as other types of acids, such as hydrochloric and nitric. It is also used in the manufacture of pharmaceuticals, fertilizers and explosives, and it is also an important reagent when removing impurities from oil in the oil refining industry.

This chemical is incredibly useful in everyday life, and is readily available as a sulfuric acid solution used in lead-acid batteries (for example, those that stand in cars). Such an acid generally has a concentration of about 30% to 35% H ₂ SO ₄ by weight, the balance is water.

For many household applications, 30% H ₂ SO ₄ will be more than enough to meet their needs. However, a much higher concentration of sulfuric acid is required in industry. Usually in the production process, it is first obtained sufficiently diluted and contaminated with organic inclusions. Concentrated acid is obtained in two stages: first it is brought up to 70%, and then - in the second stage - is raised to 96-98%, which is the limiting indicator for economically profitable production.

The density of sulfuric acid and its varieties

Although almost 99% sulfuric acid can be obtained briefly on boiling, but the subsequent loss of SO ₃ at the boiling point leads to a decrease in the concentration to 98.3%. In general, the variety with an indicator of 98% is more stable in storage.

Commodity grades of acid differ in its percentage concentration, and for them are selected those values for which the crystallization temperatures are minimal. This is done to reduce the precipitation of crystals of sulfuric acid in the sediment during transport and storage. The main varieties are:

• Tower (nitrosis) - 75%. The density of sulfuric acid in this class is 1670 kg / m ³ . Get it so-called. Nitroses method, in which the burning gas, containing sulfur dioxide SO ₂ , obtained in the firing of the primary raw material, is treated with nitroses (this is also H ₂ SO ₄ , but with nitrogen oxides dissolved in it) in the lined towers (hence the name of the variety). As a result, acid and nitrogen oxides are released, which are not consumed in the process, but return to the production cycle.
• Contact - 92.5-98.0%. The sulfuric acid density of 98% of this variety is 1836.5 kg / m3. It is also obtained from a calcined gas containing SO ₂ , the process comprising oxidizing the dioxide to an SO ₃ anhydride upon contact (hence the name of the variety) with several layers of a solid vanadium catalyst.
• Oleum - 104.5%. Its density is equal to 1896.8 kg / m ³ . This solution of SO ₃ in H ₂ SO ₄ , in which the first component contains 20%, and the acids - it is 104.5%.
• High-interest oleum - 114.6% . Its density is 2002 kg / m ³ .
• Rechargeable - 92-94%.

How is the car battery

The work of this one of the most massive electrotechnical devices is completely based on electrochemical processes occurring in the presence of an aqueous solution of sulfuric acid.

The car battery contains dilute sulfuric acid electrolyte, as well as positive and negative electrodes in the form of several plates. Positive plates are made of reddish-brown material - lead dioxide (PbO ₂ ), while negative ones are made from grayish "spongy" lead (Pb).

Because the electrodes are made of lead or lead-containing material, this type of battery is often called a lead-acid battery. Its efficiency, that is, the magnitude of the output voltage, is directly determined by the current density of sulfuric acid (kg / m3 or g / cm3) filled in the battery as an electrolyte.

What happens to the electrolyte when the battery is discharged?

The electrolyte of the lead-acid battery is a solution of storage sulfuric acid in chemically pure distilled water with a percentage concentration of 30% when fully charged. The pure acid has a density of 1.835 g / cm ³ , the electrolyte is about 1.300 g / cm ³ . When the battery is discharged, electrochemical reactions occur in it, as a result of which sulfuric acid is taken from the electrolyte. The density of the solution concentration depends almost proportionally, so it should decrease due to a decrease in the electrolyte concentration.

As long as the discharge current flows through the battery, the acid near its electrodes is actively used, and the electrolyte becomes more diluted. Diffusion of acid from the volume of the entire electrolyte and to the electrode plates maintains approximately constant intensity of chemical reactions and, as a consequence, output voltage.

At the beginning of the discharge process, the acid diffusion from the electrolyte to the plates occurs quickly because the sulfate formed thus does not yet block the pores in the active material of the electrodes. When sulfate begins to form and fill the pores of the electrodes, diffusion occurs more slowly.

Theoretically, you can continue the discharge until all the acid is used, and the electrolyte will consist of pure water. However, experience shows that discharges should not continue after the density of the electrolyte has fallen to 1,150 g / cm ³ .

When the density falls from 1,300 to 1,150, this means that so much sulphate has been formed during the reactions, and it fills all the pores in the active materials on the plates, that is, almost all the sulfuric acid has already been removed from the solution. The density depends on the concentration, and the charge depends on the density. In Fig. The dependence of the charge of the battery on the density of the electrolyte is shown below.

Changing the density of electrolyte is the best way to determine the state of the discharge of the battery, provided that it is used properly.

Degrees of discharge of a car battery depending on the density of the electrolyte

Its density should be measured every two weeks and a record of readings should be kept for future use.

The denser the electrolyte, the more acid it contains, and the more charged the battery. The density of 1,300-1,280 g / cm ³ indicates the total charge. As a rule, the following degrees of discharge of the battery differ depending on the density of the electrolyte:

• 1,300-1,280 - fully charged:
• 1,280-1,200 - more than half discharged;
• 1,200-1,150 - less than half charged;
• 1,150 - practically discharged.

For a fully charged battery, before connecting its car network, the voltage of each cell is 2.5 to 2.7 V. Once the load is connected, the voltage drops rapidly to about 2.1 V for three or four minutes. This is due to the formation of a thin layer of lead sulfate on the surface of the negative electrode plates and between the layer of lead peroxide and the metal of the positive plates. The final value of the cell voltage after connection to the automotive network is about 2.15-2.18 volts.

When the current begins to flow through the battery during the first hour of operation, a voltage drop of up to 2 V occurs, due to the increase in the internal resistance of the cells due to the formation of a larger amount of sulphate that fills the pores of the plates and the removal of acid from the electrolyte. Shortly before the current flows , the density of the electrolyte is maximal and equal to 1,300 g / cm ³ . Initially, its rarefaction occurs quickly, but then a balanced state is established between the acid density near the plates and in the main volume of the electrolyte, the acid extraction by electrodes is supported by the arrival of new acid parts from the main part of the electrolyte. At the same time, the average density of the electrolyte continues to decrease steadily according to the dependence shown in Fig. higher. After the initial drop, the voltage decreases more slowly, the rate of its decrease depends on the load of the battery. The time graph of the discharge process is shown in Fig. below.

Monitoring the electrolyte in the battery

A densityometer is used to determine the density. It consists of a small sealed glass tube with an extension at the lower end, filled with shot or mercury, and a graduated scale at the upper end. This scale is marked from 1,100 to 1,300 with different intermediate values, as shown in Fig. below. If this hydrometer is placed in the electrolyte, it will fall to a certain depth. In this case, it will displace a certain volume of electrolyte, and when the equilibrium position is reached, the weight of the displaced volume will simply be equal to the weight of the hydrometer. Since the density of the electrolyte is equal to the ratio of its weight to volume, and the weight of the hydrometer is known, each level of its immersion in the solution corresponds to a certain density. Some areometers do not have a scale with density values, but are labeled "Charged", "Half discharge", "Full discharge" or similar.