Change in pH when changing the sequence of acidic solution

I have 4 acidic solutions: Barium nitrate, aluminium nitrate monohydrate, cobalt nitrate hexahydrate, and hexachloroplatinic acid. Everything is dissolved in water.


I mixed them in order to create a final acidic solution. Following is the mixing procedure


1)Platinic acid =>Cobalt Nitrate=>Alumnium Nitrate=>Barium Nitrate
(This means Aluminium Nitrate was added to Barium Nitrate , to the mixture of which cobalt nitrate was added, and finally, chloroplatinic acid was added.)


Now, when I add everything to barium nitrate, the final $\mathrm{pH}$ of the mixture is always around 1.5, 


2)Barium Nitrate =>Aluminium Nitrate =>Cobalt Nitrate =>Platinic acid


When I add everything to chloroplatinic acid, the $\mathrm{pH}$ of the mixture drops to around 0.3-0.5. Also if I do some other sequence, the $\mathrm{pH}$ is different.


The following are the details of the solutions-


Barium Nitrate- 2.1 g in 40 ml of water (0.008 moles) (0.2 M)


Aluminium Nitrate Nonahydrate-33.1 g in 40 ml of water (0.088 moles) (2.2 M)


Cobalt Nitrate-1.86 g in 40 ml of water (0.0064 moles) (0.16 M)


Hexachloroplatinic acid-0.83 g in 40 ml of water (0.0016 moles) (0.04 M)


I am trying to understand why this is happening because ultimately every time the constituents are the same, but why is the sequence of the addition causing this change?


Since always conceptualizing $\mathrm{pH}$ in terms of $\ce{H+}$ and $\ce{OH-}$, I am having a hard time trying to understand how the nitrates and the chloroplatinic acid are behaving with each other and with the water molecules and how exactly the final $\mathrm{pH}$ is dependent on them.


The $\mathrm{pH}$ of individual solutions when I measured them in our lab are:


* barium nitrate: 6.2;
* aluminium nitrate: 1.71;
* cobalt nitrate: 5.46; and
* hexachloroplatinic acid: less than 1.


Looking at these numbers, I can see why adding everything to hexachloroplatinic acid is giving less $\mathrm{pH}$ than adding everything to barium nitrate, but what mechanism is playing part here?


Any help would be greatly appreciated.


