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Properties - Sulphuric Acid - Cloudiness
September 29, 2002

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There are a large number of producers and marketers of sulphuric acid which compete for customers and sales.  The quality of the acid is a major factor in its marketability.  Producers attempt to produce the best quality acid to ensure that it can compete in the open market.

Many contaminants can be introduced into the acid in the course of manufacture, storage and shipping.  Acid produced from bright sulphur in a sulphur burning plant is generally of the highest quality since very few contaminants are introduced to the process in the sulphur.

Acid produced in a metallurgical acid plant generally contains more contaminants then acid produced from bright sulphur.  A metallurgical plant must deal with a virtual periodic table of elements in the off-gas from a roaster or smelter.  The effectiveness of the gas cleaning system is the primary factor in controlling and maintaining the best quality acid.


Cloudiness is an appearance property of the acid.  Cloudiness results from the presence of very finely divided suspended solids.  The concentration of these suspended solids does not need to be very high in order to impart a cloudy appearance to the acid. 

Cloudiness is measured quantitatively by measuring the percent transmittance using a spectrometer.   A sample is placed in a glass cell and light at a specific wavelength is passed through the cell.  The amount of light that is transmitted through the sample is compared against a similar glass cell filled with deionized water.  The percent transmittance is the amount of light transmitted through the sample versus the amount of light transmitted through the reference cell containing the deionized water.

The suspended solids that cause cloudiness are generally cause by the presence of iron in the acid.   Iron will be present mostly as Fe(III) in the form of anhydrous Fe2(SO4)3, or acid sulphates Fe2(SO4)3•H2SO4 or Fe2(SO4)3•H2SO4•H2O.  Iron will enter the acid due to corrosion in piping systems and storage tanks.

Factors affecting cloudiness are:

Acid containing less than 5 ppm Fe(III) will generally not turn cloudy.  Fe(III) in concentrations greater than 10 ppm will turn the acid cloudy.  Acid containing 50 ppm Fe(III) will turn cloudy within 3 days.  In general, the higher the initial Fe(III) concentration the faster the acid will turn cloudy.

Higher strength acids are more prone turning cloudy than acids of lower concentration.  This is due to the fact that Fe(III) is less soluble in acid of higher concentration.  The solubility of Fe(III) decreases from 30 ppm in 98% H2SO4 to a minimum of 5 ppm in 99.1 to 99.4% H2SO4.  There is slight increase in solubility at acid concentrations greater than 99.4% H2SO4.

There are limited ways to treat acid to remove cloudiness.  Conversion of Fe(III) to Fe(II), which is more soluble in strong sulphuric acid would reduce the cloudy appearance of the acid.  However, chemical reducing agents such as hydrazine sulphate or hydroxylamine sulphate are ineffective in reducing Fe(III) to Fe(II) in strong sulphuric acid although they are quite effective in weak acid.

Since Fe(III) is more soluble in weaker acid concentrations, dilution of stronger acid in theory should cause Fe(III) to go back into solution.  In practice, however, diluting strong acid (say 98% to 93%) is ineffective in reversing the cloudy appearance of the acid.

Reducing the concentration of Fe(III) by blending cloudy acid with iron free acid should reduce the cloudy appearance of the acid.  The theory is to reduce the concentration of Fe(III) to less than 5 ppm at which point the acid should be clear.  In practice, this technique has met with limited success.  Large quantities of iron free acid are required and the reversal of the cloudy appearance is very slow.

Experimental evidence shows that precipitated ferric sulphate is very difficult to redissolve in concentrated sulphuric acid.  This is the reason why the above methods failed to clarify the acid even though in theory all the Fe(III) should be soluble and in solution.

The most effective means of controlling cloudiness is to produce an acid product containing minimum Fe(III).  This can be achieve by the extensive use of stainless steel in acid piping systems, stainless steel storage tanks, anodically protected carbon steel storage tanks and lined tank cars and tank trucks.