I cannot give you any reference, but my experience is that pH cannot be preserved. You need to measure pH in the field or very soon after the sample is collected.

If you leave the sample untreated processes like biodegradation to remove organic acids or form CO2 will occur as well as precipiation and dissolution of inorganic ions that can influence pH. If you freeze the sample the CO2 and NH3 tend to be lost and precipiation and dissolution of inorganic ions can change as the ice forms.

If the soils were transported under frozen condition there is very less chance for Soil pH to change.

Yes my Dear Eveline Pinseel you can do pH determination accurately  in your sample even after long transportation, pH of these samples have not been changed. freezing of pH samples is not necessary. For reference you can consult any soil analysis reference book. May be book by M l Jackson, Black and Piper anyone. Thanks

Normally pH won't change under the conditions you have described. If the soil samples were already dried before transport, there will be no change in pH from the original level. 

sorry for switching to another soil parameter - SOC

are there any noticable changes in soil organic carbon after store soil samples  > 1 year - assuming they were air dried before packing?

Apparently, (soil) pH of (pore) water can stay the same or change between the moment of sampling in the field and analysis in the laboratory. You need to measure pH both times to know which is the case. Even if the pH is the same, it doesn't mean the sample has not changed with respect to hydrogen ion mass balance. You can have a buffer.

It may also be helpful. We investigated the change of pHH2O suspension during 20 days in relation to pH-buffering of sod-podzolic soil with different land use.

Thanks! Zenon Hamkalo, how did you treat your samples during incubation? Did you dry them?

I prepare suspension : fresh soil in the water (1: 2,5) and then the pH was measured during 20 days (each day). Controls the weight of the suspension, adding distilled water. This makes to evaluate the  acid buffering capacity of the soil. As seen from the fig. 1 and 2, the different acidification takes place during incubation suspension, especially hayfield variant (5-10 cm).

Best Regards

ISO 10390:2005 Soil quality — Determination of pH

ISO 10390 was prepared by Technical Committee ISO/TC 190, Soil quality, Subcommittee SC 3, Chemical methods and soil characteristics.

This International Standard specifies an instrumental method for the routine determination of pH using a glass electrode in a 1:5 (volume fraction) suspension of soil in water (pH in H2O), in 1 mol/l potassium chloride solution (pH in KCl) or in 0,01 mol/l calcium chloride solution (pH in CaCl2).

This International Standard is applicable to all types of air-dried soil samples, for example pretreated in accordance with ISO 11464.

ISO 11464:1994, Soil quality — Pretreatment of samples for physico-chemical analyses

Yes, if you are transported the sample from anywhere than you should have to do test with in a week. if you want to do test later then keep the soil sample at 4 degree celsius..without freeze...then do in 1 mol/l potassium chloride solution (pH in KCl) or in 0.01 mol/l calcium chloride solution (pH in CaCl2) ..it will give more accurate value as compare to soil and water (1:2, 1:5).

It depends whether samples are kept wet/ or dry. If dry then no problem of time with in a month, but if it is wet then pH may differ with time of taking pH from the date of sampling.